russbost
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| posted on 8/7/08 at 06:41 PM |
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Locost NiMh battery alternative
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 no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
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BenB
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| posted on 8/7/08 at 06:49 PM |
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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 
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blakep82
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| posted on 8/7/08 at 06:51 PM |
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^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...
________________________
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russbost
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| posted on 8/7/08 at 07:04 PM |
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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 ..........
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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Werner Van Loock
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| posted on 8/7/08 at 07:09 PM |
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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
http://www.clubstylus.be
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carpmart
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| posted on 8/7/08 at 07:55 PM |
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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.
You only live once - make the most of it!
Radical Clubsport, Kwaker motor
'94 MX5 MK1, 1.8
F10 M5 - 600bhp Daily Hack
Range Rover Sport - Wife's Car
Mercedes A class - Son's Car
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tks
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| posted on 8/7/08 at 09:21 PM |
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quote:
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.
It indeed has to do with the matching caracteristics of 100cells how constant is the production...
you could be better (i think) joining the really big batteries they have allot more amps...
and your maths will be better
The above comments are always meant to be from the above persons perspective.
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Liam
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| posted on 8/7/08 at 09:33 PM |
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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
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Delinquent
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| posted on 8/7/08 at 09:55 PM |
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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...
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smart51
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| posted on 8/7/08 at 10:06 PM |
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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.
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dhutch
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| posted on 8/7/08 at 10:34 PM |
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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
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02GF74
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| posted on 9/7/08 at 08:52 AM |
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^^^ 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.
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sammy
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| posted on 9/7/08 at 11:51 AM |
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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?
Build diaries...
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coozer
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| posted on 12/7/08 at 10:56 AM |
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Forgive me if I'm wrong but isnt this the principle Tesla use in their very nice (expensive) roadster???
1972 V8 Jago
1980 Z750
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MikeRJ
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| posted on 12/7/08 at 09:07 PM |
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quote:
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).
They've been sub-C cells in all the power tools I've taken apart?
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MikeRJ
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| posted on 12/7/08 at 09:15 PM |
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quote:
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?
You can't use a transformer with DC, it would have to be an inverter.
Higher voltage/lower current is always better from the point of view of losses anyway, and no serious modern EV motor would use a voltage as low as 12
or 24v. Even the baby 7hp motors use 48v, and the big ones use 150v or more.
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Syd Bridge
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| posted on 13/7/08 at 07:14 PM |
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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]
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Liam
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| posted on 13/7/08 at 07:45 PM |
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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
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Liam
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| posted on 13/7/08 at 08:41 PM |
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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]
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Syd Bridge
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| posted on 13/7/08 at 08:51 PM |
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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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MikeRJ
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| posted on 13/7/08 at 10:19 PM |
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quote:
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.
This is very true, a hundred 2.5 Ah cells in series gives you a 2.5 Ah battery.
However, power = voltage * current, so to get equivalent power from a high voltage battery you draw less current. In terms of actual energy content,
a series or parallel connection are the same. The advantage of high voltage is that the power loss through the cables and any switching elements (e.g
MOSFETS) is lower. The power loss is proportional to square of the current, so moving to higher voltages can give considerable benefits.
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Delinquent
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| posted on 16/7/08 at 12:00 PM |
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quote:
Originally posted by coozer
Forgive me if I'm wrong but isnt this the principle Tesla use in their very nice (expensive) roadster???
which bit? If you are referring to just having them all in series, then no. Approx 6600 lith cells in a tesla, in 11 banks wired 600 in series, then
paralleled up.
The amazing thing with the Tesla that everyone wants to get their hands on is exactly how they've managed to get 6,600 odd lith cells to stay in
perfect balance and not explode... From the brief chat I had with the Tesla bloke, there is some serious kit in there to ensure it doesn't all
go very pear shaped.
What was very surprising (to me) was that they have gone for air cooled packs an motor as opposed to liquid cooled. Chap admitted that the tesla would
not, at present, stand up to a track day but could more than adequately cope with "excessively hard road use". They are working to get the
cooling up to trackable levels.
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