Battery charging advice please?

[inglewoodpete]

My two bob's worth.

Get a grossly oversized panel Eg 25 or 40 watts. Mount it vertically so that is gets very poor illumination in summer. I know that the sun is pretty low at noon in winter, so it should just about maximise the winter energy capture. If it over charges the battery in summer, put some shade cloth (or your wife's lace undies) over it to reduce the exposure to the sun.

 

[AllyCat]

Hi Paul,

Well, a vertical panel might reduce the snow building up on it. But at Yorkshire latitudes I think it will still generate about 5 times more power in Summer than in Winter, so I wouldn't omit the regulator (but where does the power go when the battery is fully charged?). Or maybe even face a larger panel downwards slightly?

The listings from the ebay seller in #32 for their 20W and 30W panels do specifically state that a regulator IS included, whilst those for their other panels (up to over 100W) specifically say that a regulator is NOT included. So you'd be on quite strong ground if they didn't supply one with that 20W panel.

In view of your specific location (high wind, low Winter light levels) I think a "windmill" might just be worth a punt, but only with absolutely minimum investment (both financial and of time/effort). Personally, I'd get a scrap electric car radiator cooling fan and mount it firmly facing the prevailing wind (probably west). I've no idea at what windspeed it will start generating useful power (or even turning) but they're designed to tolerate 100mph airflow so probably no overspeed protection required! Connect to an "available" battery (perhaps your current 1.2Ah, or just a few AA NiMHs) and monitor the voltage/current/time that it actually generates electricity, using a simple PICaxe circuit.

Cheers, Alan.

 

[geoff07]

The power doesn't go anywhere. It isn't generated. Power = volts x current. No current, no power regardless of voltage. The job of the reg is to cut the current when it isn't needed any more to prevent overcharge.

You can get UK wind speeds from the internet by county. Sadly (or not), in my location the average speed is 6mph. Most commercial wind generators aren't specified to generate until the speed is higher than that. Which just leaves the high wind situations, which might be enough. It would be interesting to know what speed it takes to turn an electric fan as a generator.

 

[SAborn]

It would be interesting to know what speed it takes to turn an electric fan as a generator.

On average you need to drive the motor at a greater rpm then it runs at as a fan to produce current out to a battery at the same voltage, so a 2000 rpm fan will need to run above 2000 rpm to produce a equal or greater voltage than the battery.

Not very practical for a wind generator.
In most case you want to be looking to produce power between 100 to 600 rpm from wind.

 

[boriz]

I'd prolly use a hub-motor, something like this : http://www.youtube.com/watch?feature=player_embedded&v=OD1gSx92DuM

If it comes built into a bicycle wheel, all the better for a VAWT

 

[SAborn]

I'd prolly use a hub-motor, something like this : http://www.youtube.com/watch?feature...&v=OD1gSx92DuM

You would need to change the motor wiring from Star to Delta for 12v operation, or it will load the generator too heavy in lower rpm.
If the generator is overloaded in low rpm it wont allow the mill to get out of stall, the mill will need to pick up enough speed to allow it to reach its power curve, other wise its like trying to start off with the handbrake on.

On average these motors will cutin to a 12v battery about 60-70rpm in Star, and around 120-150rpm in Delta, Star is perfect for a 24v battery not 12v.
You can expect around 500 watts at about 300 rpm from these type of motors.

 

[PaulRB]

Fascinating as this discussion is, remember I need enough power to light 4 x 3W light strips for around an hour a day!

 

[SAborn]

Perhaps i missed it earlier, but now we have some values to work with.

Ok, so 4 x 3w = 12w
12w / 12v = 1 amp
1hour x 1amp = 1 amp/hour per day. (1Ah or 12Wh )

Now you said you have a 2.4w solar panel, so to add that in.

12w / 2.4w = 5 hours of full output from the panel to replace the power used for 1 day.
Given the great English winters i doubt you would get enough sun to give a total of 12w in a day, even here in Australia mid winter it would be a big expectation.

I would suggest a 10W panel or larger and a new 7Ah SLA battery, this should allow a daily recharge of 1.2 hours of full sun or a slower combined total of solar hours for the full winters day.
It would also permit 3-4 days storage for poor solar days, in theory you have 7 days storage with a 7Ah battery and drawing 1Ah per day, but if you draw the battery down that low it will quickly start to fail and not recover on each recharge.

In mid winter with days of poor light you might want to do a battery recharge from the mains power every 3 to 4 days, or twice a week.

We all would like to have a perfect system but with seasonal changes its not 100% practical, unless you want to use a far over rated system to cope with the bad solar days, then you have a excessive system in summer.

Its a matter of balance and what you are prepared to do during the short period of bad power days.

 

[BeanieBots]

Perhaps i missed it earlier, but now we have some values to work with.

Ok, so 4 x 3w = 12w
12w / 12v = 1 amp
1hour x 1amp = 1 amp/hour per day. (1Ah or 12Wh )

I think that might be a bit optomistically over simplified.
To get 1A in to a 12v battery will require ~14v at the knee point of a panel which would actually be a panel of around 17v. Hence the 12Wh required needs to be bumped up to ~17W.
There is also the efficiency of the battery. 1Ahr in will not give 1Ahr out AND the charge voltage is considerably higher than the discharge voltage. If you consider drain protection diode losses as well it can all get as bad as 50%.
So, consider something nearer 25 or even 30W.

Also (as mentioned) I would not suggest taking more than a few Ahr out of a 7Ahr Pb battery.

 

[AllyCat]

In mid winter with days of poor light you might want to do a battery recharge from the mains power every 3 to 4 days, or twice a week.

Hi,

The best suggestion so far (except I said much the same about three pages ago).

The calculations are not too difficult, there are some "known unknowns" and even some "unknown unknowns" but you should be able to get a ballpark figure.

The Met Office quote typical sunshine hours for Yorkshire in December of about 45 hours, so 1.5 hours per day. The manufacturers do claim that "some" electricity is generated in cloudy conditions, but don't seem preapared to put a figure on it! Average solar intensity in December (during potential sunshine hours) is about 120-150 watts/m2, and PV efficiency 15%, so expect perhaps 30 watt-hours/day/m2. So IMHO you may be looking for a 0.5 square metre panel, or more. That will be "rated" at something towards 100 watts (at 1kW/m2 illumination).

The power doesn't go anywhere. It isn't generated. Power = volts x current. No current, no power regardless of voltage.

Yes, my original comment was somewhat rhetorical. The point is that the energy stays in the panel and heats it up. That's not good for semiconductors and is one of the reasons why the panels are not mounted "on" a roof but about 3 inches above the roof (to allow convection cooling behind). And IMHO it makes little sense to buy expensive PV panels and then waste most of the energy available (in Summer).

Cheers, Alan.

 

[manuka]

A immense PV benefit relates to scalability. If one small panel doesn't deliver the goods,then further can simply be added (usually in parallel) or a larger one installed. Initial trials can usually be quickly made at ground level too. After a 150 million km journey a few metres is incidental - the sun is just as strong on your back lawn as at the roof!

Such cut & try approaches are not a feature of even small wind turbines,which demand suitable prior design,on-going maintenance and serious installation- the later often costly, unsightly, manpower intensive & even possibly requiring permits. Exploiting a local wind resource really only becomes feasible with an elevated turbine too, otherwise the little darling may well end up just chasing it's tail with low level turbulence. (Urban mounting is considered a folly with the likes of the micro-turbine Scottish "Swift").

The sort of sunny & sheltered locations us humans prefer are not usually those with tempting wind resources either ! If you do find a great wind spot it may well be on a ridge top 100s of metres away, which then may "blow" your budget purchasing,ducting & trenching the costly copper cabling.

Aside from niche applications, wind energy harvesting really only comes into it's own with LARGE (MW level) structures in exposed sites with smooth laminar wind flows. Even 5 years ago the economics of small scale domestic wind turbines were marginal, but since then the price of PVs has so dramatically collapsed that it's really a no contest for needs under ~a few kW.

Stan- in "windy" (but sunny) Wellington,NZ

 

[PaulRB]

All, thanks all for the advice. Here is the latest update on this project:

Replacement 20W rated mono-crystalline panel, mounted vertically: http://www.ebay.co.uk/itm/200825776705?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Leisure battery: minor brand, capacity 85AHr

Charge controller: http://www.kemo-electronic.de/datasheets/m149.pdf

Additional to the 4 x 3W led light strips (for about 1 hr per day), the setup now runs a Hotline Gemini 40 electric fence energiser 24/7: http://www.hotline-fencing.co.uk/pdf/HLC.pdf

The energiser has a low battery indicator, so I should get warning when/if I need to recharge the battery. Early days yet - We'll see how long before I have to re-charge.

Paul

 

[Michael 2727]

Recently I was playing with 5W PV Mono Panels rated @ 21V, 0.56A Max Output, full sunlight.
I found that on Dull days the output was as Little as 20mA, quite a surprise.

 

[srnet]

Recently I was playing with 5W PV Mono Panels rated @ 21V, 0.56A Max Output, full sunlight.
I found that on Dull days the output was as Little as 20mA, quite a surprise.

Its an issue I have seen quite a lot in the UK, where its maybe not as sunny as down under.

A lot of campers and caravaners over here buy these panels, say a 100W, and dont realise that if they are to be used as a replacement for mains connection, you need to significantly down rate the panels to cope with average UK conditions. Full days of a full 100W are rare and have been almost non-existent in the last 2 months.