Power recommendation question (12VDC)

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IronJungle

Senior Member
I am building an outdoor robot that has two 12/24VDC motors (see pic). When completed the robot will wander around the back yard to do the evil bidding of my choosing. The motors are geared down pretty good and very very strong. Forward motion is a slow ~33 feet/minute; which is about the speed I want.

What 12VDC power source is best? Car battery? Marine battery? Motorcycle battery? I am not very familiar with powering motors of this size. The design objective would be motor run time.

TIA.
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Roman505

Member
What are specifications on current draw for the motor? Was 33 ft/min at 12V or 24V? How heavy will the robot be, or was at that speed? These things affect your average and peak current demands and hence battery sizing, which has also to be balanced against the load capacity of your motors. With what power source were you doing the test? Could you insert a current measurement in the line, and perhaps monitor it over time?

In general, Pb batteries (all of those you mentioned) have large nominal capacities and fairly low leakage but are very heavy and usually intolerant of deep discharge. I try to run anything on one of the other chemistries, load willing, unless I have heavy start demands and can otherwise leave the Pb on float.
 
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Circuit

Senior Member
IronJungle said:
What 12VDC power source is best? Car battery? Marine battery? Motorcycle battery? I am not very familiar with powering motors of this size. The design objective would be motor run time.

TIA.
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Look at power supplies for similar technology - powered wheelchairs and mobility scooters. Google up suppliers for replacement batteries for wheelchairs and mobility scooters in your area and you should find something suitable. The design objectives for wheelchairs and mobility scooters will, I think, very much match your parameters. Lots of batteries in various shapes and sizes.
 
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AllyCat

Senior Member
Hi,

Yes, we really need more information about overall size, weight, current consumption, expected running time and acceptable cost, etc..

IronJungle said:
The design objective would be motor run time.
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If (maximum) run time is really the only objective, then almost certainly use Lithium technology, which has a high energy/weight ratio. In particular, for "traction" purposes, Lithium IRON phosphate (LiFePO4) are recommended, which are safer and more tolerant of high currents, excessive charge/discharge and general "abuse" than "normal" Lithium Ion batteries (as used in phones, laptops, etc.).

You'll find several threads on this forum concerning such cells, and for potential availability see for example this website.

Cheers, Alan.
 
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Puuhaaja

Senior Member
I think that in this kind of project it doesn't matter if batteries weight are 0.7kg with li-ion or 3-10kg with motorcycle/car batteries. I said this because I suppose you are making really big robot.

http://www.hobbyking.com/hobbyking/store/__14074__ZIPPY_Flightmax_8400mAh_4S2P_30C_LiFePo4_Pack.html
That battery has 13.2V x 8,4Ah = about 110 watthour and it's price is 82$.
If you buy lead acid battery(12-14V x 7.4Ah= 100Watthour) it's price is about 10-25$
So...car and motorcycle batterie are much cheaper than lion and I suppose that cheapest solution would buy a normal car battery.
If price is not so important then I would recommend to use lion or lifepo4 batteries. They are not so heavy and you can charge them fast.

Edit:
If you are planning 12v battery system then I would recommend to use optima batteries: http://www.optimabatteries.com/us/en/
Normal car batteries can lost even 30% of it's capacity when deep charced but with optima batterie you don't have that problem and also it doesn't matter if your optima batterie are held upside down for a short time.
 
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bluejets

Senior Member
Car battery usually around 40Ah and cost is about $120.00 last time I bought one.
CCA is what you pay for and above is about 320A. (cold cranking amps)
 
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MPep

Senior Member
LiFePO4 are very light weight. And have a very good Power/Weight ratio. Charging is a bit more precise, but plenty of good chargers are now available.
I picked one up, and thought that it was a demo casing, only to find out that it was the whole battery pack!
 
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IronJungle

Senior Member
All good suggestions. Thanks. Looks like a car/MC Pb battery would not be up to the task. That is why I was considering a marine type battery that is designed to run a trolling motor. The suggestion about wheel chair applications was also spot on.

Thanks and stay tuned. I welcome any other comments.
 
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inglewoodpete

Senior Member
IJ: If you get it going soon, you are welcome to test it in my back yard. It will need a light/dak sensor to detect (white) "royal" ibises. The "royal" is in quotes because I don't find them very regal. Most of the year they land in the back yard and wreak havoc with their long beaks: bulbs, seedlings (as well as snails and grubs, of course). I need a robot that will tirelessly chase them away.
 
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IronJungle

Senior Member
I'm back with the data.

Both motors running at 12VDC
0.5 Amps with motors free wheeling (ie, no load, just spinning)
1.0 Amps with 50 pounds on the robot and driving up a descent hill grade.

I turned my power supply up to 24VDC and the motors sounded much happier. Current was about the same.

Big motors remind me of my experiments with rocketry....
More power means more weight and the extra weight means more power, which means more weight {repeat loop} :eek:
 
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Paix

Senior Member
It's only a matter of time before hobbyists are forming orderly lines outside car breaker yards looking for second hand motors from wrecked electric/hybrid vehicles.

Over the years it seems that most things have a motor of some sort in them. Before very long they will have a big motor of some sort in them, aided and abetted by guys like the forum members here.

I can remember a time when a lot of electronics supply houses shunned the hobbyist and lone engineer, but it seems these days it is a market worth courting, if only because they don't know where the next small project will turn big and it's seen as the education and low level pre-recruitment to the industry path. As well as being exceedingly satisfying for a few of the more mature tinkerers. Technically we do live in exciting times. Will we have to look back on this being a golden era in a decade or two, in the same way that the Tottenham Court Road in London used to be full of electronics component shops, before the video recorders, porn and whatever happens to be there now. Time for a look at Google Street View I guess.
 
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rossko57

Senior Member
7.5Ah lead gel batteries are cheap and plentiful (used in UPS, alarms), not unreasonably heavy for a ground vehicle 2.5kg
 
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Circuit

Senior Member
Paix said:
It's only a matter of time before hobbyists are forming orderly lines outside car breaker yards looking for second hand motors from wrecked electric/hybrid vehicles.
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My Toyota Auris Hybrid has a 60 kilowatt, 650 volt three-phase traction motor in addition to the petrol engine... it is run from a 200 volt NiMH power pack through a water-cooled inverter that ups the voltage to 650 volt three-phase AC. If Paix is seeking one of these in the breakers' yard then I just wonder what sort of robot he is imagining to build! The film "Transformers" comes to mind...:)
 
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IronJungle

Senior Member
Just a quick follow up to my current draw post. Here is the rig in 'action'. No PICAXE control here. I am prototyping the rig with simple 3VDC motors until I get things coded 'somewhat' correctly, then scale up.

 
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Goeytex

Senior Member
Battery selection is many times a compromise between cost and performance. If you can afford a good LiFePO pack and the required charger system then great. Otherwise I would suggest a pair of 15ah AGM (Deep Cycle) used in mobility scooters, etc. Ive seen these for about $40 each.
 
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Paix

Senior Member
@Circuit, :) Given the level of understanding that I have displayed on the subject, you will be deliriously happy to know that I have no robot building in mind, now or in the foreseeable future.

The motor size that you indicated was rather more than my imagination stretched to. Best not to hang around the breakers yard on the off chance for at least a few years yet then!
 
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