All the current (no pun intended) talk about electric cars etc got me thinking - modern NiMh AA size batteries are commonly available as 2500mAh now at less than £1 each (2700 are available, but a bit dearer at present), now please correct me if I'm wrong, but as these are 1.2v, if I put 10 in series I have a 12v, 2.4Ah battery (can you see where this is going?) if I put 10 of these groups in parallel then I have a 12v 24Ah battery. Ok I hear you say but your 24Ah battery has cost you £100 & some work to get them all together! Yes but a typical lead acid battery (say 32Ah) weighs about 10 kg, each AA battery weighs 28g so 100 weigh 2.8kg & you could probably buy the NiMh's for half the above price (maybe less) if buying in bulk.
So you now have a battery that has cost perhaps half as much again, that weighs a little over a quarter of a similar existing lead acid one, it also can be recharged at least 1000 times against the lead acid 500 times,what's more it can be made virtually any shape you want, but I would have thought preferably flat - AA batteries are 15mm dia & 50mm long, so 100 would go into a space 0.5m long, 15mm high & 150mm wide - that's around the same "footprint" as a 100Ah lead acid battery, so taking up less space you could have a 240Ah 12v battery (admittedly costing around £500).

So am I absolutely barking mad (ok we all know the answer to that one ), have I got the maths wrong? Why aren't there more 12v NiMh batteries around? Presumably they could make a 12v battery that was effectively 10 (or 100) cells without all the individual "wrappers" or am I missing something colossal??? (I do realise they might well not make cranking current, but that's not the sort of application I'm referring to).

Will be interested to hear your views!

I think cranking current might be an issue and just think of the fun you'ld have when one of the cells dies and goes to heaven! Oh the endless fun of taking apart a 100 NiMH cell pack and testing each one

^thats what i thought, i'd imagine if you tried cranking, somewhere between 1 and 1000 AA batteries would melt.

i think in theory it works, but the thickness of the materials inside each cell can't handle the current?

i don't know...

I did say I'm not looking at cranking current as this would be electric car type application, lower current draw deep discharge cycle. Wasn't suggesting we all throw away our car batteries!!!
Thepoint re one duff cell is certainly valid, how often do cells go duff if tested b4 fitting?? Hmmmmmmmmmm ..........

putting cells in parallel is the problem, as they're never the same some will have to work harder then others and will fail very fast.

Though the high currents are not that much of a problem, been racing rc model cars and a friend of mine flies rc planes and they've measured current draws of over 150Amps

Mainly to get the plane airborne

I think the issue you would have is matching and balancing the batteries to work as one unit. I used to make up r/c car battery packs from individual cells and I used to spend a bit of effort in matching cells to each other to get them to complement each other as opposed to working against each other. I think it made a reasonable difference to how well they charged/discharged and delivered the current. So to my point, you would probably need 1000 cells to match 100 close enough to get a workable 'matched' pack together.

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Originally posted by carpmart
I think the issue you would have is matching and balancing the batteries to work as one unit. I used to make up r/c car battery packs from individual cells and I used to spend a bit of effort in matching cells to each other to get them to complement each other as opposed to working against each other. I think it made a reasonable difference to how well they charged/discharged and delivered the current. So to my point, you would probably need 1000 cells to match 100 close enough to get a workable 'matched' pack together.

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^^^ Indeed - i'd at least start with a bunch of D cells rather than AAs (which is basically what cordless power tool batteries are if you've ever happened to take one apart). But see above for why this isn't common practice for such high power applications as cars.

Liam

as above, pack balance is absolutely key to achieving any sort of performance. In my larger leccy RC heli I was running ~14v and just keeping one of those packs in balance was a mare in itself. The high current demand kills them pretty quickly and charging requires full discharge to give any sort of cell life - obviously with an RC heli I can just run it on the ground down to min voltage, wouldn't fancy pushing a leccie car back from where it expired though...

If you could get 100s of AA size batteries for reasonable money, just think what a large battery of the same capacity would cost. Specialist battery companies can supply all kinds of things, you just have to look further than your local B&Q.

Surely you would use sub-c sized NiMH's as rc cars do?
- The can do about 30amps without too much harm. Ten together is 300amps, which is fine for cranking i would suggest.


Daniel

^^^ as they say, the problem arises as batteries do not like working in parallel.

it should be possible to make some electronics to over come this.

This is probably a dumb question as I dont know a lot about electronics, but does putting all the batteries in series and then using a transformer to get the correct voltage not overcome the parallel probem? Aren't transformers meant to be very efficient?

Forgive me if I'm wrong but isnt this the principle Tesla use in their very nice (expensive) roadster???

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Originally posted by Liam
^^^ Indeed - i'd at least start with a bunch of D cells rather than AAs (which is basically what cordless power tool batteries are if you've ever happened to take one apart).

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Originally posted by sammy
This is probably a dumb question as I dont know a lot about electronics, but does putting all the batteries in series and then using a transformer to get the correct voltage not overcome the parallel probem? Aren't transformers meant to be very efficient?

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I'll dare to put my head above the parapet....

But, I was always of the understanding, that putting batteries in series, adds up the voltage but doesn't similarly add up the amp hrs.

There's some leccy engineer types on here should be able to put things right. One of the Mikes is electrical or electronics???

Should have added, The electric car project I worked on for a number of years was based on 36volt low speed and 72 volt high speed. The car started on 36v, then switched to 72v to get higher speed.

Cheers,
Syd.



[Edited on 13/7/08 by Syd Bridge]

If you actually read the initial maths he's well aware of that Hence the scheme of series to give the desired voltage and then parallelisation to get capacity.

Liam

I think you've gone wrong in failing to understand 2500mAh = 2.5Ah.

His 10 series 1.2V 2500mAh (2.5Ah) cells gives 12V 2.5Ah.

10 of these groups in parallel will give 12V 25Ah. Etc etc

Liam

[Edited on 13/7/08 by Liam]

That I have, 2500mA is 2.5Ah. So, his initial maths is indeed correct.

The research in the project I was doing, came up with the conclusion that in £/KWh stored, the good old lead acid is still hard to beat.

Hence, six big deep discharge gel cells. Total cost of about £400, weight about 380lb, gave a full charge to 30% range of 90miles at 45mph.

Cheers,
Syd.


[Edited on 13/7/08 by Syd Bridge]

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Originally posted by Syd Bridge
I'll dare to put my head above the parapet....

But, I was always of the understanding, that putting batteries in series, adds up the voltage but doesn't similarly add up the amp hrs.

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quote:

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Originally posted by coozer
Forgive me if I'm wrong but isnt this the principle Tesla use in their very nice (expensive) roadster???

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