HELP with racing car nstrumentation


Senior Member
Now I have your attention <img src="smile.gif" width=15 height=15 align=middle>

I have enormous respect for the inventiveness of the members of this forum. End of smarm.

i have a problem that I would like you all to help with.

As a school we have entered a competition to build an electric racing car. I need to develop some instrumentation for the car and would like to elicit the help of the biggest pool of picaxe intellegence on the planet (no BS honest)

<A href='http://' Target=_Blank>External Web Link</a> is the web site to tell you all about it.

The question is what and how. - The good news anyone who contributes to the effort will get a credit on the car. Big deal, but Mclaren were small once!

Come on guys this is for kids and the future of engineering in the UK <img src="smile.gif" width=15 height=15 align=middle>

I have little time as we have to race on the 14 Sep and we have only just finished the car as a running entity. Any advantage has to be worth its weight.

I hope with the combined experience and problem solving expertise we have we can really make a difference. join the Team - BOSTON BULLETS.



Senior Member
So... do you accept any help from any place in the planet?
Because I'm from a VERY very far away place Overseas from you :eek:)


New Member
Oooh...this sounds like fun :) I used to both Rally and road race on a track, and have built successful cars for both...just so you dont think I'm talkin out of my bum <img src="smile.gif" width=15 height=15 align=middle>

Do you want telemetry or cockpit info? I'll address them separately.

In the car: Keep it simple. If you drown the driver with information one of two things (or even both!) will happen. Either the driver will ignore the displays and concentrate on driving, or will pay attention to the displays and ignore the driving. There is no happy medium, despite what a lot of people would have you believe.

Look at a full blown professional race have a tach, sometimes a gear indicator, temperature measurement for various parts of the powertrain, and sometimes electrical system monitoring. That's really it. Everything but the tach and gear indicator (if you have one) should be off to the side, and only referenced if there seems to be a problem. If you're on a track, skip a speedo. They're generally useless and are nothing but distracting.

In an electric car, I would have the following:

In front of the driver:

1) RPM of the motor. This can be an encoder wheel using pulsin to measure the RPM of the motor.

2) Current sensor. This should measure the current from the battery/power source tot he motor, as this is your key indication of power. There are plenty of solid state sensors that you could ADC to get this. A high power resistor would probably be ridiculously expensive for this application, unless you can get a &quot;sponsor&quot;.

Notice there is no voltage. This will swing wildly on acceleration (and possibly braking) and will pretty much never be at a steady state so is pointless in the cockpit.

There also is no RPM monitoring for the wheels. If something goes int he gearbox this will be useless, and if that happens, the driver will know and doesn't need a gauge to tell them so.

Off to the side of the driver:

1) temperature of the power source. You can probably get away with a DS18B20 here to make it simple.

2) Temp. of the motor. This one you may need to use a thermocouple. Those temps can get really high and would probably melt the trusty DS18B20.

3) Current from each battery pack (if there is more than one cluster). These can be used to tell the driver what's going on if there is a loss or surge of power. Either one can be bad...a surge of power can cause the batteries to explode. They can also indicate a short or open from a pack. Either way, they will know to let up before they get hurt.

And that is all &quot;I&quot; would put int he car.

Telemetry: The sky is the limit, as long as you can move the data. Even amateur racers are now using quite a bit of telemetry, as someone dedicated to looking at a computer screen doesn't have to concentrate on driving, and can look at graphs and bars rather than gauges, and can track small but potentially catastrophic changes in real time.

The problem with advanced telemetry is that unless you are very clever, it can get very expensive.

If you can swing it, I would have everything that the driver has, plus more temperature and current sensing. You can check a non driven wheel for RPM and compare it to a driven wheel's RPM to see if theres slippage on acceleration. You can check the motor and transmission (if there is one) for matching RPM's to pick up a problem that may be too small or quick for the driver to notice. Imagination is all that's holding you back in this regard <img src="wink.gif" width=15 height=15 align=middle>

If you have any more in depth questions, or something that I said doesn't make sense, feel free to email me at &quot;andy at evolvingnerd dot com&quot; and I'll be happy to respond.

--Andy P


Ex-Staff (retired)
Sound excellent and I'm impressed by the number of teams and schools participating.

What sort of instrumentation projects are you looking at ? Any specs or ideas as to how much work there would be, and what sort of commitment would need to be given ? Are you looking for people who can take on a whole project, act as mentors or do various parts; project planning, designs, spec writing, coding, circuit designs ?

I'm sure people would be interested in helping out but will be a bit cautious on over committing themselves or not being able to give the time you might like or need, or worse still, not delivering. I wouldn't commit without knowing more about what I was letting myself in for :)


Senior Member
Hippy and others

I - or at least the team are looking for any technical input we can - I figure through the verious technical forums I take part in including this one I can access some if the best ideas in the world, should give us a head start.

It realy is ideas and concepts as I don't expect anyone to have to give physical or hands on support so it's just brain power and a bit of time really.

I don't by any means claim to be the owner of anything but the most essential ideas but can impliment the genius of others (hey I sound like Bill Gates)
Although we will enter this year I dod not expect to do well as we have little time to practice or refine anything - BUT next year - Look out.

So all ideas welcome

Thanks to those so far

Perhaps Rev ed would consider sponsoring us with some at cost hardware?? Lots of schools in ther to benefit form the advertising.

Edited by - rickharris on 09/07/2007 09:07:22


Ex-Staff (retired)
I think the best thing then is to throw your ideas and &quot;needs&quot; out to Joe 'PICAXE' Public, see what comes back and see what people are prepared to pursue.

I personally don't see a problem with doing that via this forum ( and it 'open sources' any advantages there may ), but they are not my forums.


Senior Member
Unless anyone else complains I would prefer it to be open source through the forum as it may attract other end users.

Like all technology it is the lead that is important. I am a teacher anyway so keeping secrets isn't my style.

Technical details:

250 watt 24volt motor simple drive at present on or off. The motor draws 20 amps at its most effecient point and heats up rapidly over 20 amps. It runs at 2000 RPM so the car is geared to do about 30 MPH.

The motor drives one of the rear wheels of a 4 wheel car.

The body will be as aerodynamic as we can manage.

The race is over 4 hours and is maximum distance on 4 x 12 volt batteries no recharge allowed. The drivers will be 11 year old students.

The car can be push started but after that depends on the motor, during the race the drivers must change at least 5 times.

The winners earlier in the year did 110 miles.

Instruments minimum we are interested in motor current, speed, distance, motor temp (it will go as high as 200 deg C but is normally cooled to run around 100 deg C), Communications and telemetry may be useful.


New Member
A global effort - I like that!

I'd keep things simple and modular. In a way, I'm wondering how much instrumentation is needed at all. My 100W scooter doesn't have any instruments. It just runs. Then it goes flat. Then you charge it up.

Thinking of displays, LEDs use power and LCDs might be a bit hard to read when bumping along. I wonder if good old fashioned panel meters might be easiest? Standardise all sensors to convert to volts - 0-5V, and use 5V panel meters. Then later, when you add telemetry, it is easy to sense all these voltages and send them off via a picaxe to an RF Tx.

I see the rules are specific - you must use a particular motor and a particular battery. They also limit the RPMs to 2000 and you get disqualified if you go over that?! So may as well speed limit it automatically? Also, rather than tell the driver the temp of the motor, maybe just put lots of cooling on the motor so it always is happy. This can be automatic.

It would be good to get the power curves for the motor and run it at the max efficiency. The rules seem so tight but maybe there can be clever things with the gearing. Bike gears probably are the most practical. Aerodynamics and bearings and tyres will be very important. I presume the motor will be PWM controlled, and if so, the mosfet will need to have a very low on impedence.

The one display that could be really useful for the driver is the most efficient gear to be using for that particular speed. That might be a bit of a complex calculation, though it would be easier if the track has no hills. You don't want the driver starting off in a high gear (maybe that is why motors get hot?)

Can you give us some specs on your car - eg does it have gears, PWM control etc.

I see the design tips are on the website you gave in the 'education' tab. The efficiency, speed and torque graphs are interesting. Max efficiency is at 2Nm but this corresponds to a speed of 2700 RPM. 2000 RPM is 6Nm which lops 10% off the efficiency. Something doesn't add up with the rules if they won't let you run at max efficiency. Gearing is going to be critical.

Edited by - Dr_Acula on 09/07/2007 14:37:26


Senior Member

All vehicles entered for any event organised by Greenpower under Formula 24 rules will comply with the following regulations. All cars will be subject to pre-event and possible post-event scrutineering to ensure compliance.

These regulations have now been posted to all new entrants, and will shortly be dispatched to all veteran entrants.

Regulations for existing fully enclosed or 'streamliner' cars can be obtained from Greenpower - ask for details.

1. Power Source:

1.1 The vehicle will be powered by one 24-volt, 240 W, Fracmo dc electric motor manufactured by Denis Ferranti Meters Ltd and supplied by Greenpower. Forms of additional free natural power can be used but must only provide motive force to the wheels through this motor.
1.2. The motor is sealed and any tampering with the motor without clearance from Greenpower will result in disqualification. The motor is designed to run at 2000 rpm and any motor found to be running faster than this will result in the user being disqualified. 1.3. Cooling of the motor is allowed but this must be in the form of either fans powered by the main batteries, extended surfaces or heat sinks, dripped or pumped water (again using power from the main batteries), or ducted air. No form of cooling that has had prior energy input is allowed; that is ice, liquid gases etc.
1.4. Once fitted the motor must be accessible for inspection by scrutineers.

2. Batteries:

2.1. Two 12-volt dc batteries of the type below will be used on each vehicle. Two further batteries of the same type are allowed as spares.
2.2. All new vehicles must use Yuasa &#8216;TIP TILT&#8217; type &#8216;100T&#8217; Elite batteries supplied by Greenpower. These are of the same dimensions as the previously specified 096 type batteries.
2.3. Existing vehicles may continue to use conventional lead/acid 096 type batteries until the 2008 season. Teams wishing to change over to the new specification batteries may do so if they wish.
2.4. The batteries in the car must be fitted with an isolator switch and the Health and Safety Regulations pertaining to the batteries and their handling and use must be adhered to.
2.5. The two batteries must be firmly secured using rigid fixings to the chassis of the vehicle and be located within the bodywork of the vehicle.
2.6. It is recommended that the centre of gravity of the batteries should not be above the rear axle line.

3. Drive System:

3.1. The motor may be coupled to all or any of the wheels, directly or indirectly, by whatever means is deemed appropriate. Do not attempt to drive through both back wheels without a differential. There is no restriction on ratios or gearing.
3.2. The drive must be fully guarded with respect to the driver and pit crew.

4. Wheels:

4.1. The wheels must be pneumatic and capable of sustaining the loads imposed on them with respect to both vehicle weight and cornering loads.
4.2. They must comply with the following wheel sizes (front and rear):

Max 500mm (20in.) Min 200mm (8in.)

4.3. There must be FOUR wheels located as a matching front pair and a matching rear pair. Any other layout of wheels is not acceptable.
4.4. We recommend that the track of both the front and the rear should not be less than 1.5 times the diameter of the wheels on that axle - centreline to centreline an absolute minimum of 500mm.
4.5. It is recommended that wheel stub axles are not less than 9.53mm or 3/8&quot; in diameter.

5. Size and Configuration:

5.1. The vehicle must not exceed 2500mm in length and 1200mm in width overall. The vehicle must be able to pass under a 1200mm high horizontal pole.
5.2. It must have a minimum ground clearance of no less than 40mm. A low ground clearance and centre of gravity is advisable.

6. Weight:

There is no weight restriction on the vehicle from either a maximum or minimum point of view. This is left up to the entrant. However, we strongly advise that the centre of gravity of the car is on or below the axle lines.

7. Driver and Seating:

7.1. The vehicle will have one seat only for the driver who will be in the vehicle at all times while racing. The seat must be securely fixed to the chassis of the vehicle when running but can be removable or able to be tilted.
7.2. The drive must be seated in a conventional feet forward head to the back position. Drivers may not kneel, sit astride as seat, or lie down, in any way, such that their chests and heads are forward of their waist.
7.3. The driver must be able to demonstrate an ability to get out of the car in ten seconds unaided.
7.4. All cars must have a solid floor under the driver.

8. Chassis and Bodywork:

8.1. The chassis of the vehicle will be of the entrant's design and fabrication. It may contain a number of elements from other proprietary chassis but these must constitute parts of a unique chassis and not be the wholes of it. It might also include the whole or parts of another item that was not intended to be a vehicle chassis.
8.2. The vehicle will have bodywork reaching back to at least the back of the driver of the vehicle and should cover up to at least elbow height on the sides. This must incorporate an element of 'side-impact protection' and may have provision for removal if deemed necessary. Bodywork must not prevent the driver from giving hand signals but may cover the wheels - All enveloping bodywork is not allowed.
8.3. Drivers must be able to exit from the cockpit opening unaided. This must be a permanent opening, large enough for the driver to exit the car through. However, during normal pitstop use, parts of the bodywork may be removed to aid both driver and battery access.
8.31. The head of the driver must be visible above the surrounding bodywork to the sides, and there must be no bodywork over the top of the driver's helmet.
8.4. Energy absorbing foam (closed cell polyethylene) of approximately 25mm thickness and of a reasonable area must be securely attached to the inside of the vehicle body at the front and along either side of the driver. (Greenpower car help with supply).
8.5. Particular attention must be paid to sharp edges in and around the car. All hard edges must be protected or removed. The outside of the car must be free of sharp protrusions and care must be taken that the driver is not liable to be injured by chassis parts or bodywork in the event of a collision. In particular details of where knees and feet might go in the event of rapid deceleration should be examined for safety. If bumpers are fitted these must not protrude past the wheels in a straight-ahead position. All cockpit edges around the driver must be fully padded.
8.6. The shape of the bodywork should be as aerodynamic as possible.

9. Mechanical Parts:

Mechanical components can be of a proprietary design salvaged from other vehicles or equipment, or purchased as components. The intelligent incorporation of proprietary sub-assemblies or parts is acceptable but should be limited to areas where safety is a prime consideration. The innovative use of re-cycled components is encouraged and may be rewarded.

10. Brakes:

10.1 The vehicle must have a fully operational and effective braking system.
10.2. The brakes should be capable of stopping the vehicle to a dead stop when travelling at its full speed within 25 metres.
10.3. The vehicle should not be able to be pushed from a standstill with the brakes applied and a driver in the car.
10.4. There must be a mechanical brake in addition to any electrical braking system that might be incorporated on the vehicle.
10.5. The vehicle will be fitted with a clearly visible red brake light at the rear of the vehicle that will illuminate upon application of the mechanical brake/s. This light will not be of the flashing type. This light can incorporate its own dry-cell battery and should be at least 30mm x 30mm in size.
10.6. The driver must be able to operate the braking system without removing either hand from the steering mechanism.

11. Safety Equipment:

11.1. The vehicle must be equipped with suitanly sized functioning rear view mirrors. A clearly audible warning device will be fitted remembering that other drivers will have helmets on.
11.2. A 24-volt rated battery isolation switch must be fitted and will be used when changing batteries and at all times when the car is not in use. It should be clearly visible and accessible from outside of the car as well as accessible by the driver. Two such switches must be fitted if needed.
11.3. The vehicle will be fitted with a minimum four-point 50mm width racing safety harness, with secure fixing points on the roll bar or chassis. Care should be taken when designing the belt system for prone drivers to prevent submarining. Inertia reel type belts are not acceptable.
11.4. Roll bars:
a) The vehicle must have roll bars offering roll protection in two places generally in accordance with the diagram below (available in hard copy from Greenpower or pdf download on previous page). The roll bars must be demonstrably strong enough to support the car and driver if inverted.
b) Roll bars should ideally be manufactured from steel or aluminium tube with a minimum outside diameter of 25mm, and minimum wall thickness of 1.25mm.
c) There must be at least one brace from the top of the roll bar rearwards to the chassis.
d) Composite roll bars are permitted.
e) The top of the driver's helmet must be under the line shown in the diagram below.
f) The front wheels must not be regarded as part of the roll protection.
g) If in doubt about the design of your roll bar please contact Greenpower now. If you wish to manufacture a roll bar from materials other than aluminium, steel, or composites you must first obtain clearance from Greenpower.

12. Steering and Turning Circle:

The car must be able to complete a 180-degree turn between two &#8216;walls&#8217; 10 metres apart as tracks could include a hairpin to these dimensions. Any steering system that, in the opinion of the scrutineers, can have a dangerous tendency to &#8216;over-steer&#8217; and cause loss of control may be banned.

13. Electrical Specification:

13.1. The car must be electrically safe and use established 'good practise'.
13.2. The motor can be controlled by an 'on/off' method or by a speed controller of proprietary or unique design. However the switching must be spring loaded to OFF. No form of permanent ON switching is allowed.
13.3. The vehicle must have a fused link in the main power circuit to protect both the motor and the wiring. This should be between 60 and 100 amps and be of a re-settable or 'burn-out' type. This will be in addition to any current limiting devices fitted.
13.4. All wiring must be rated correctly and used in accordance with the manufacturer's recommendations.
13.5. All wires on the vehicle must be neatly run and fully and safely secured.
13.6. All connections must be capable of resisting vibration for lengthy periods.
13.7. The chassis may be used as the earth return if so desired.
13.8. There must be a battery isolation switch fitted as described in item 11.2.
13.9. Batteries must be fitted such that their terminals are not close together or near any earthing points so as to avoid any chance of accidental short-circuiting.
13.10. If batteries are joined as one 24 volt unit this must have suitable carrying handles to allow two people to carry them.
13.11. Any battery trolleys used must be safe and stable with no danger of the batteries falling off.

14. Other Features:

14.1 The vehicle must carry three clearly visible numbers on the vehicle. These must be on a white circle of minimum 250mm diameter on both sides of the car midway down and on the front of the car.
14.2 The car must be smartly decorated and can carry advertising or sponsors promotional material provided it does not obscure the numbers.
14.3 Lifting Points. As cars frequently break down during races and have to be returned to the paddock on pick up vehicles it is important that clearly marked lifting points are shown on the cars in order that the marshals can lift the vehicles safely.
14.4 Fixing Points. Please ensure that the vehicle has marked points for attachment of securing devices when being transported by race recovery vehicles. Greenpower can take no responsibility for the safety of vehicles being recovered unless these are provided.

15. Drivers Information:

15.1. All drivers must wear a suitable crash helmet when driving. This must be of the motorcycle or motor racing type, and preferably be full-face. It should have a full visor or, if not, goggles will be worn. The fitting of windshields will not replace the use of the eye protection described above.
15.2. Drivers must wear full overalls or clothing and must have their hands protected by suitable leather gloves in all vehicles.
15.3. Drivers must wear shoes.
15.4. All drivers must not have any hair showing from beneath their helmets when driving. Hoods must be removed or hidden.
15.5. Drivers must be able to prove a good level of driving competency to the scrutineers if required to do so.
15.6. Drivers will be fully conversant with the Drivers Regulations for the event.
15.7. Drivers will always have with them their completed Driver Log Book.
15.6 Drivers must be in secondary school at the time of the race and from years 7-11 only. Team members who start the project in year 11 and then qualify for the final in October can continue with the teams to the final should they qualify. New year 7 pupils joining school in the September before the October final are not permitted to drive.

16. Team Regulations:

16.1. These regulations are applicable in particular to endurance events. For special F24 events separate Team Regulations may be issued.
16.2. Each team should comprise a maximum of 12 pupils from years 7 to 11 in the current academic year.
16.3. Six pupils will be declared as drivers on race day. Five drivers must drive, and one may act as a reserve.
16.4. There will be a maximum of six further pupils acting as mechanics and pit crew.
16.5. Older pupils can help with the team effort but only act in an advisory role on race day.
16.6. If battery charging is allowed in the race, two members of the team must be designated as battery handlers, and no other members will be allowed into the battery charging facility.
16.7. Adults will not be allowed to assist in pit stops unless specific by specific arrangement beforehand with the race organisers. All members of the team must be clearly identifiable through their clothing.
16.8. Each team must have a name. If not on the Confirmation of Entry Form, the team name must be notified to Greenpower at least one month prior to the event.

17. Logbooks:

17.1. Each team will receive a car logbook upon Confirmation of Entry. On this must be fixed a photo of the finished car, and any other details completed. The logbook must be brought to scrutineering at every event.
17.2.All drivers will have their own Log Books issued by Greenpower with a passport type photograph in it that they will be responsible to keep up to date. They must be brought to all race meetings in which they wish to drive. Do not lose these, as drivers will not be allowed to race without them.

From time to time further regulations might be required and these will be forwarded to the schools involved and posted on the website &#8211; keep a watch out for these.

It is understood that all persons participating in events under these rules are doing so at their own risk and all competitors will have disclaimers signed by their parents or guardians prior to competing. The organisers will ensure that full third party liability and accident insurance is always in place for events under their control.

In the event of any queries about these regulations or other related matters please contact Greenpower for assistance. Do not assume things or decide to do anything the way &#8216;you think is best&#8217;.


Senior Member
Pictures of what we have at present (without the body which is in development) can be seen <A href='' Target=_Blank>External Web Link</a> here


Senior Member
The motor data sheets and some battery data are on the greenpower,co,uk web site, if your connection is slow I can extract them and make it easier to get to.

Essentially it is on/off control at present and the motor is 250 watt and draws 20 amps at 3000RPM. The car has been geared to go at 30 MPH at that RPM.

The gearing is fixed at present and drives only one of the back wheels.


New Member
The pictures and rules are very helpful. I presume the aim is to get up to 30MPH as quickly as possible and then go continuously at that speed for most of the time? Is rule 1.2 going to be a problem re the 2000RPM limit, and how do they enforce it anyway?

With the motor getting hot, I see it is mounted on a plate and that will provide a bit of a bigger heatsink. Sticking CPU heatsinks on the motor may work, though the curve will be a problem. I found a curved heatsink once for a motor about half this size - it just clipped on.

I see rule 1.1 might let you put an 80W solar panel on the car, but that might blow the budget (and it would be annoying if it was cloudy on the race day).


Senior Member
My english isn't good enough to read (and understand) the whole rules list, but look like &quot;last car running&quot; instead of &quot;1st to reach finish line&quot; and can use any of free natural power to support the motor ON...
If you need some other place to put pics and talk about PicAxes and related hardware or software feel free to come here <A href=' ' Target=_Blank>External Web Link</a>to discuss those.



Interesting to see that solar power is allowed.
From my sums with the dimensions given it should be possible to get as much as 300W of solar energy in direct sunlight using highgrade cells. It would however cost about &#163;1500 to buy the cells new.

Regenerative braking would only be worth while if there is a lot of breaking to be done. In an endurance race I can't see it being worth while.
As a driver, I would like to see remaining battery capacity. Not too hard for a PICAXE and a few op-amps.
Don't fit a special sense resistor. They are very expensive. Simply measure the voltdrop from one of the battery leads using a differential amp. Feed that into an integrator. The output of the integrator then feeds into a PICAXE adc input.
In addition to the current sense feeding the integrator, also have a feed into the virtual earth via a resistor from a PICAXE digital output. When the integrator gets above X volts, the PICAXE sends a pulse of known duration to 'reset' it. Each pulse is counted and the accumulated count is the used capacity.
If solar charging is used, then the integrator will require a 'charge' pulse as well whenever the integrator gets below X volts.

If want a speed control that does not require any electronic components, then consider one of the old mechanical charge regulators used in cars. They are good for around 30A and can be fairly easily modified to be a variable current limit. The principle is similar to an electric door bell. When the current flowing through a coil becomes high enough, the contacts open switching off the current. Adjustment can either be mechanical or via a secondary (voltage) coil.

What county are you in?
Hands on help offered if not too far from Sussex.


Senior Member
Lincolnshire - Stamford to be precise. thanks for the offer it's not that far away.

I think Solar whilst interesting would be too costly and to reliant on the weather <img src="smile.gif" width=15 height=15 align=middle> not too good at present.

I had considered charging via peddles but the weight would probably iron out the advantage.
I don't have any precise data but most people seem to go for simple on off, even those who have tried speed control go back to on/off.

The contest is to see who can go the furthest in 4 hours - you don't actually have to finish the 4 hours if you go further. this brings up the question do batteries recover capacity if you rest them. Personally I can't see how unless there are physical changes internally that recover when they are unloaded.

I have contacted the battery supplier to see if they have any information.


Senior Member
<BLOCKQUOTE><font size=1 face=arial>quote:<hr height=1 noshade>Looks like you could cool the motor with a heat pipe structure legally... <hr height=1 noshade></BLOCKQUOTE></font><font face='Verdana, Arial, Helvetica' size=2>

Can and will cool the motor - intending to use a total loss water system of microbore copper pipe wrapped round the motor and run water through it, should limit temp to about 100 deg C (until water runs out!)

I looked at heatsinks but getting good contact is difficult and copper expensive.


New Member
The farthest distance part gives me a few thoughts.

I read the rules when you first posted this, and dont recall seeing anyhitng about using a flywheel. either that or I jsut missed it.

Why not put a nice big (within reason, obviously) flywheel on the motor, then pulse the motor rather than running it continuously. It's not necessarily speed, but a balance between speed and distance.

If you can get an average speed of, say, 25 MPH for 4 hours, you're going to go farther than people going 30mph for only 3 hours.

In a sense very long on time PWM. The motor will run much cooler, the average current will be less so the batteries will last longer. All in all not a bad idea.

--Andy P


New Member
If the aim is to go the farthest distance in 4 hours, then the aim is for the batteries to go flat at exactly 4 hours. If they go flat at 3 hours, then the car was going too fast and wind resistance goes up as the square of speed. If the batteries still have some capacity at 4 hours then the car could have used this to go faster.

So the aim might be to do some very careful experiments to determine the battery capacity under these load conditions, and then use PWM to adjust the power.

One problem is that every time the battery is discharged it will lose a bit of capacity. So, do they give you new batteries for the race? And how do cars usually finish - do they all finish by the clock stopping at 4 hours, or do most of them go flat before 4 hours? Or do they fall to bits?!

I think I read somewhere on the website that most do 'finish the race', which presumably means they finish when the clock stops, which implies the batteries still had some charge left.

Edited by - Dr_Acula on 11/07/2007 07:55:04


Senior Member
"- intending to use a total loss water system"

So... a BIG weight in water at start...

And... I understand (or I thing so) you have 4 batteries, 2 for mains and 2 for spares
So if you can drop the water weight when use it... so you can drop 2 of the batteries when flat and reduce weight too... right?

Last edited:


Senior Member
I look at your photos and see front bumper, but read at the rules
"If bumpers are fitted these must not protrude past the wheels in a straight-ahead position."

So what that means?

Last edited:


Dr_Acula has it spot on for strategy which makes a capacity meter even more crucial.
True that the battery will lose capacity each time but even new ones can have +20% -10% variation so testing is essential to get figures for YOUR specific batteries and what the degredation is per run.
Don't forget that discharging below about 10.5v is likely to cause very rapid permanent damage, so only let it go that low in the final run.

Lincolnshire - Stamford is a bit far for me to make a drop in visit but I'll not rule it out. Meanwile, here will have to do.


Senior Member
I am still groping for a definition of what &quot;flat&quot; means for a lead acid battery.

The batteries are only used in pairs you have to pit to changes them.

I agree with the battery dead at the end of the race (see above) Also like the capacity remaining indication just not sure how to assess this figure.

To some degree it has to be a compromise I guess, more speed go further in a given time, BUT also more speed higher current, The flywheel ised has some merit - or rather the Pulsing the motor, I have a feeling the fly wheel would add a lot of weight and that has to be bad. But perhaps some kind of automatic pulse rate whilst the motor is on, have to think that through.

Thanks for the ideas - keep them coming this is just what I wanted LOTS of thought even if it may seem a bit bizarre often they spur on other ideas that are a little less bizarre.

As far as weight is concerned we have a minimum commitment to the motor, chassis, battery and driver load. The body will be very light weight. and is just for aerodynamics.

I know the rear is critical to reduce drag but when does that become significant and can you measure it on the fly?


&quot;I am still groping for a definition of what &quot;flat&quot; means for a lead acid battery.&quot;

A very valid point and one with many answers. The real distinction between the miriad of answers is determined by what your purpose is.
In your case it is also the most important issue to understand and get right.
In &quot;normal&quot; use, it is not recommended to discharge lead acid more than about 20% of its capacity in the interest of battery longevity. However, for a competition race lasting 4 hours, if the battery is totally useless and not even fit for scrap after 4 hours, then it doesn't matter. This will indeed be the case if you get things right.

&quot;Flat&quot;, for your purpose, is the point at which the battery can no longer supply any power that is of advantage to your mission.

Amazingly, this is about the only time I would ever recommend trying to get out of a battery the number of amp hours that is actually written on the side of it. Quite simply because doing so is pretty much a once only thing.

You should be able to get very close to the number on side in practice.
Predicting &quot;flat&quot; is quite tricky until it is about to happen and then it's too late. Open circuit voltage is a good indicator but it's only valid after an hour or so rest, so not very practical for your application. Loaded voltage is not too bad for a constant load. This is because like many rechargeable batteries, the lead acid one drops off fairly linearly before making a very sudden rapid drop right at the end. The actual numbers however, are specific to load and the ACTUAL battery being used. Anything below about 10.5v will be VERY close to dead.

As already mentioned, the optimum speed to run at for the full four hours is the one that will &quot;flatten&quot; the battery in exactly four hours. This could be done by simply running at constant current of C/4 but that would be too simple ;)
A crucial part of such a race is getting to &quot;speed&quot; quickly and then holding the &quot;optimum&quot; speed for the duration. That involves using much more of the available capacity very early on. How much more will require experimentation and, if you have the patience, a lot of maths based on factors determined by your experiments.

In similar situations I have resorted to producing mathematical models of the vehical in VB. This is not too frightening a task if you have the time, patience and ability to obtain good test results.
All you need are set of reliable results for torgue vs current and speed vs drag. A couple of good data points can then be used to plot out the appropriate curves. These are then extrapolated for any value within your model.
You can now use your model to try out different gear ratios and starting accelleration to get the optimum values for YOUR vehicle. You can run a virtual race as often as you like in just a few seconds and see how changing a few things alters the outcome. Don't forget to try a few out on the real thing periodically just in case your model has some floors.
It all comes down to Ohm's law and Newton's laws of motion with a little bit of magnetism and whole lot of itterations.


Senior Member
sedeap the bumper issue means that any bodywork you put on to protect the front or back in a crash must not be wider than the wheels.


Thanks for the info on battery life:

Our problem may well be that we want to reuse the battry packs either for practice or another race: for a school replacing 4 car batteries every race would be too expensive so that will limit out options i guess to something that can recover and recharge.

What is it that kills the battery?

Do they recover after standing for a while, if so why?

I gather that the manufacturers charge them to 16.65 volts to make sure they are fully charged when they reach the customer but a &quot;normal&quot; battery charger will only charge them to 14.5 volts to prevent/minimize gassing.

Is charging to 16.5 going to be an advantage? if so i can charge them outside to ensure safety, Also assuming I can get hold of a charger that will do this.


New Member
What would be really helpful is a data logger curve of the battery going flat under real world conditions. That probably means going out on the track and running round till the battery goes flat, and measuring volts and amps. A picaxe + an eeprom would be really helpful here. Measure every minute or so. Probably need low pass filters of 1-2sec time constant on the current and volts. Current sensor could be a few cm of copper wire and an op amp, and calibrate with a 1A source and a multimeter. Maybe build two identical datalogger circuits and run them side by side in case data is all over the place or in case one dies.

The battery will drop off quickly at the end and you don't want that to happen on the other side of the track from the pitstop. On the other hand, you don't want to come in too early. Once you have your data set with voltage under load (and the battery running warm etc), it may end up that volts do give an accurate indication of when to change batteries, and it might be a simple matter of a yellow light that comes on at 11V and a red light at 10.5V and then the driver knows it is pitstop time.

I would hope it ends up being simple like that, because calculating remaining power using watts in and watts out and battery temp is bound to end up with errors.

I hope the batteries they are using are deep cycle. Lead acid batteries are very good for standby when they hardly ever are discharged. Eg alarm backup and cars, where the alternator keeps up with the load and the battery really only is there for starting. This application isn't very kind to lead acid batteries and certainly it isn't very 'green' (do we tell the sponsors that?). NiMH batteries would be much better and also have a better power to weight ratio.

Re aerodynamics at the back, the best shape is a teardrop, or symmetrical airfoil where the air that went round the car is gently brought back together again.

Re 'other natural energy sources', does that include the driver? I have just been watching the 'tour de france' where they average 41KPH up and down hills, with an average power output of 200W and outputs of 400W in the alps. That could be via a charger, but a better and simpler answer might be to couple the non driven wheel to some pedals at the right ratio and the driver can pedal as well (if that is allowed). Get those kids in training!

Edited by - Dr_Acula on 12/07/2007 01:27:32


Senior Member
"...the bumper issue means that any bodywork you put on to protect the front or back in a crash must not be wider than the wheels. "

Rick this photo shows the bumper wider and larger than wheels...

<External Web Link>

But WTF if you understand that is allowed...
Step back and I concentrate myself in you primarily question... PicAxed "racing car nstrumentation " .. So... What I can do for you?
I assumes that you're advanced Builder of modules than me, so the communication part of telemetry I let it in your hands.

So as jack the Ripper use to say....
Come piece to piece... (yeah... black humour)

"...Instruments minimum we are interested in motor current, speed, distance, motor temp (it will go as high as 200 deg C but is normally cooled to run around 100 deg C), Communications and telemetry may be useful."

For speed and distance, putting one magnet on the ring of one wheel, you can count the revolutions and the frecuency to calculate it (read by a coil or one reed switch attached to bodywork), for motor temp, you must attach one of this ones <External Web Link> closely to motor chassis, and feed the PicAxe too, then you can use one kind of VU-meter to "read" in the cockpit by driver too.

One friend of mine are researching about how some materials (for bodywork and covers) "catch" more electrostatic power when the air pass over them, and how can convert this to useful extra power like solar-panels do.
But i don't know, right now, if he agree to tell that in public forum... or open source info.

Keep in touch

Edited by - sedeap on 12/07/2007 09:44:41
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Whoever suggested 16.5v must sell batteries for a living!

Even then, only if your race result depends on rapid charge should you take them over 13.8v.

Regarding 'other natural energy sources', the rules are quite clear. Anything where the energy is pre-stored is not allowed. That would include the driver, so NO pedals.

So what kills the battery?
In the longer term (eg 'normal' use) it is taking the chemical reaction beyond the reversal point. This is slightly forgiving in that emmediate recharge will limit the damage. Taking the battery down to no lower than 10.5v (on load) should be safe enough for a dozen or so cycles before any appreciable drop in capacity is noticed.
The danger is if one of the cells is slightly weaker than the rest. This is a very common reality of any battery made from more than one cell.
The weak cell will become reverse charged. This then causes massive outgassing and loss of electrolyte. Loss of electrolyte causes loss of capacity and increased internal resistance which in turn makes it even more likely to suffer further loss of electrolyte. It happens VERY quickly below 10.5v and is not reversible.

I've recently cycled some 38Ahr batteries that have been in a UPS for 2 years.
Discharging to 10.5v at C/5 gave 33Ahr first cycle, dropping to 28Ahr after the 4th cycle.

You said there is no re-charge during the race, so charge your batteries slowly the day before and leave them on float over night at 13.8v. (do not leave at 13.8v for weeks on end).
That way, they will have the maximum possible charge. Fast charging to 14.2v is only to get charge in quickly. The battery does not like it and it does not give a full charge. You can get 80% of the capacity into a battery over just a few hours but at the price of damage. It will still a further 10-14 hours to get the remaining 20% in no matter what rate you use, so be kind to them, there's no rush.

Batteries are Amp-Hour devices, not power devices so there is no need to measure power in or power out. Only current out.
You know the capacity, an integrator lets you know what's taken out, hence you can quite accurately predict empty.
By keeping an eye on remaining capacity, drain rate can be adjusted to keep usage/speed at the optimum for the competition.

&quot;Do they recover after standing for a while, if so why?&quot;
Not if they are good quality designed for high discharge.
Tests I made on the 38Ahr units indicate that you can squeeze out an extra Ahr after discharging to 10.5v at C/5 when they are left to stand for 2 hours.
The reason for this is because when discharged at high rates, the plate material close to the terminals discharges faster than the material at the furthest point due to simple IR effects. In addition to that, with thick plates normally found on deep-cycle batteries, the chemicals have further to migrate and deep penetrated ions take longer to surface. Hence, during a rest period after rapid discharge, there is a very small amount of recovery.
This should not be confused with the effects noticed with zinc carbon dry cells. When they come to the end of their life, the electrolyte dries out and an insulation barrier develops between the two electrodes. Resting (or even putting in an oven) allows electrolyte migration which decreases internal resistance and gives the illusion of increased capacity.

Discharging at lower rates certainly gives greater capacity but the reason is based on the deffinition of flat. Normally, &quot;flat&quot; is determined by a specified terminal voltage. When the battery is close to empty, the internal resistance goes up. This means that the voltage that determines &quot;flat&quot; will be reached earlier at higher currents. Hence, &quot;flat&quot; will occur after the same capacity has been extracted even if the AVERAGE current is lower. It is determined by the peak current. Reducing the average current will only have a very marginal effect due to the marginal recovery effects described earlier.

Edited by - beaniebots on 12/07/2007 08:59:50

Bob Elton

New Member
BeanieBots, I gotta ask a question re: weak cells / very common / reverse charged.

I'm not saying you're wrong, but how 'very common' is this in reality? This failure would also apply to standard car batteries too wouldn't it? In my 20 years of driving I've never seen premature battery failure like this.
I've also seen a similar story of doom being peddled on Wkipeadia for Ni-Cd and Ni-Mh, but I've charged multi-packs hundreds of times over years and never had this problem.

So, what really is the likeliehood of this happening?

P.S. Whats happened to the Instrumentation bit of this thread?

Edited by - bobelton on 12/07/2007 08:41:21


BobElton, it is VERY common except in electric car racing where packs use hand picked cells.
In 20 years of driving, how often have you fully discharged the battery and how long did it last after you did that? Remember, your car floats the battery during normal use which applies an equalisation charge to prevent that very thing from happening.

As for the instrumentation part of this thread, the current discussions hopefully will indicate what type of instrumention is required and why. I'd also go as far to suggest the use of a PICAXE to control the throttle. This could be overridden by the driver at any time and it would then compensate by increasing or decreasing the average level in order to ensure overall optimum discharge rate.

Bob Elton

New Member
Oh, I see. Handy to know.

So this unwanted property is when you are deep cycling mainly? Is this more common with particular types of Lead Acid? Is this more common with heavy discharge/charge apps vs light-duty like burglar alarms?

(On my Ni-Mh packs used in radios I run til abs flat but never seen this, though I appreciate this a different kettle of fish).

Anyway, useful stuff. Back to the plot...


It's only applicable when discharging to &quot;flat&quot; where &quot;flat&quot; means that the pack voltage is less than (N-1)*Vcellnom where N = number of cells in pack and Vcellnom is the nominal single cell voltage. Obviously, not relevant to 'standby' use such as alarms.

Running your radio till absolutely flat is not a good thing to do. This WILL do irreversible damage to the cells. I would guess that you get away with it because the radio draws less than C/20 but even at that rate it's not a good thing to do.

Please note, the specific comments I have made about batteries in earlier replies on this thread are aimed specifically at the use of lead acid used in this application. In no way whatsover are these comments to be taken out of context and applied as 'general' principles of battery care. That is exactly how battery myths get propogated.

Bob Elton

New Member
Agreed, I try not to run them flat BeanieBot , but sometimes it happens and a couple of Ni-MH cells conked after about 6 years = ~100 recharges. But they weren't left flat for long before recharge. And , yes, very light duty - so they don't get much stick.

I reckon I can put up with irreversible damage after 6 years :) but I'm not suggesting it's good practice by the way.

Well, enough of my OT.


Senior Member
whats your budget?

if you have the cash then you could always use a high powered zigbee and a picaxe net sever to get the data back to the pit for someone to pour over. obviuosly you could cut out the pic-axe net server quite easily, if you have a picaxe and a serial terminial which formats the data for display on the screen (prehaps using the serial terminal from hippy!)

as for the in car instruments - as many people have said, the best way seems to be to keep it simple. only monitor sruff like motor speed and maybe temprature. A very simple way of displaying, with the use of a tri-colour LED's would be handy to make it easy to distinguish in all light conditions.



New Member
If the car is very simple with just on/off speed control, I would presume the only thing the driver does is turn it on and go as fast as possible. The only decision the driver really makes is when to make a pitstop to change batteries, so the only instrumentation that is really needed it a way of telling the driver when to do this. A few leds indicating low batt voltage should be sufficient.

There are lots of other instruments that are interesting, but they will all take time to make. Of course, if you have an army of willing students keen to build instruments...


Ex-Staff (retired)
With my Project Manager hat on ( it's also a Wizard hat, a Dunce's hat and a traffic cone depending on rotation ), and we're getting close to four pages, I'd suggest a sensible step is to decide -

1) What you want for this year
2) What you'd like this year, but can do without
3) What can wait until next year

Things can be bumped up ( and shunted down ) according to progress.

Of what you want this year, determine what are the priorities -

1) Need
2) Want
3) Would like

Choose one or maybe two projects and throw them to the wolves to attack. I'd consider each instrument as its own project even if they do use the same inputs or control the same outputs. Get them working then integrate later.

It seems to me that 'battery power left' and/or more simple 'current drain' are the two primary indicators, but I stand to be corrected. Those would also be key to 'you're pushing to hard / can push harder' information for the driver.

On the information display front, my preference is for bar graphs of LED's not colour changing LED's, much easier to notice and comprehend in peripheral vision.

Audible warnings have to be carefully done because their sudden activation can cause a change of trouser colour when really concentrating on driving at the limit, any warning sound can be very distracting and cause a loss of performance.


Senior Member

Continuing from my last post...

I'm working around Speed and distance instrumentation, so, please tell me which size're the wheels in one full turn. (very external diameter)
and if your's budget don't allow lcd modules inside cockpit, then I try to make one 2x7segments watch for speed status, and for distance one 3x7segments watch.
If you want the motor RPM too, so then tell me.
BTW the safe lights allow their own dry-Pack battery power, but the instrumentation allow it too?

If you really want telemetry, i can provide serial interface with Wireless module ( or one datalogger perhaps...)

Another thing... the race is the type "cross country" ( Urban route/road) or inside Karting circuit stadium ( circular shape road) is flat terrain or not at all.
This maybe helps me (inclination, breaking shake, and quakes can be used to recover electric power)
I build one gadget like this to provide power at my SailBoat. (Cinetic usine)

My friend can't reveal the exact procediment to recover electric power from electrostatic charges, but tell me about the materials to do, so Glass fiber coated in strips with Polivinyl doped with Cupric oxide make enough "net catcher" for electrostatic power which can be converted to useable electric power, but the size of the cover be more than 2 square meters, and provide 100mAh at 12V with air speed above 50Km/h. ;o(

Ok... thats all by now... i'm back to work...

Keep in touch

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