watt hours / amp hours

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suzukishoes
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watt hours / amp hours

Postby suzukishoes » Mon Apr 11, 2011 4:04 pm

I'm almost scared to write this! BUT

Can anyone explain to me what the net result of these battery configurations are.

If I was to string together (for example) in series 4 x 12volt 200amp batteries I would have a 48volt 200amp pack.................... Right?

If I strung together in series 8 x 6volt 200amp batteries together I would have a 48volt 200amp pack but with double the watt hour capacity.................. is that right?

If so whats the net result? :oops: is watt hours good?

Its a basic question I know but if you don't know the basics your never gona build one are you!

Lee

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Jeremy
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Re: watt hours / amp hours

Postby Jeremy » Mon Apr 11, 2011 5:34 pm

Generally it's always best to compare battery packs in terms of watt hours, rather than amp hours, as this allows a fairer comparison of the actual stored power in the pack.

Four 12V 200 Ah batteries connected in series will give you 48V at 200Ah, so 9,600Wh (4 x 12V x 200Ah = 9,600Wh)

Eight 6V 200Ah batteries will also give you 9,600Wh, as it's still a 48V, 200Ah pack (8 x 6V x 200Ah = 9,600Wh).

Hope this makes it a bit clearer.

Jeremy

suzukishoes
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Re: watt hours / amp hours

Postby suzukishoes » Mon Apr 11, 2011 6:17 pm

So basically they will deliver the same, meaning it comes down to cost & dimensions to establish the preferred layout?

Lee

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Jeremy
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Re: watt hours / amp hours

Postby Jeremy » Mon Apr 11, 2011 7:00 pm

Pretty much, yes, but bear in mind that there may be some difference in usable power from different battery configurations. The key thing is to try and get hold of the discharge data for each battery type you're considering and see which will have the greatest usable capacity under the average discharge current that you're looking at.

For example, some lead acid batteries will have a capacity rated at 1/20th of their Ah capacity (so 10A for a 200Ah battery), whilst others may be rated at 1/10th C or even 1/5th C. Most reputable manufacturers provide discharge rate/capacity curves so that you can see how discharge current affects capacity. It's not unusual to find that a battery rated at 200Ah may only deliver 100Ah in practice, even the very best lead acid battery will probably only deliver 70 - 80% of its rated capacity in an EV application.

Jeremy

suzukishoes
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Re: watt hours / amp hours

Postby suzukishoes » Mon Apr 11, 2011 7:24 pm

Thanks for that Jeremy. I feel into this trap on some batteries in my G-Wiz, changed them from Trojans to US battery (which offered higher Amphour rating) & millage, whilst enough for my needs is rubbish in comparison to the Trojans I'll have to see if I can find the discharge rate for them.

So moving this on just a little, a chap in Italy installed 16 Thunder sky LFP 3.2V 200Ah Cells into a G-Wiz wired in series which would give 200ah but with the benefits of lithium which presumably also includes a greater usable capacity?

Lee

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Jeremy
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Re: watt hours / amp hours

Postby Jeremy » Mon Apr 11, 2011 8:42 pm

Lithium chemistry cells pretty much deliver all of their capacity as usable capacity, although their life is extended if discharge is limited to around 80% of maximum capacity. In general, the usable capacity of lithium cells is around 30% or so more than lead acid cells of the same capacity, plus they have around 2 to 3 times the life (in terms of charge/discharge cycles).

The big snag is that it's hard to tell which lithium cells are of decent quality and which are rubbish. Thundersky cells are a bit variable - some are great, some less so. In general it's best to stick to cells that have a good track record from other users, but even then it's possible that you might get some duff cells.

In the long run, good quality lithium cells are almost certainly a better investment than lead acid cells, but they are more expensive and do need more care and attention to charge and discharge management.

Jeremy

suzukishoes
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Re: watt hours / amp hours

Postby suzukishoes » Mon Apr 11, 2011 8:56 pm

Thanks Jeremy, you didn't treat me like an idiot! much appreciated.

Lee

guymark
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Re: watt hours / amp hours

Postby guymark » Sat Apr 16, 2011 11:24 pm

Just a quicky Lee

4 x 12v batteries might weigh a little less than 8 x 6v batteries - as if you imagine a 12v battery being effectively two 6 v batteries side by side, you can dump two "end walls" and just have a thin cell separator in place - hence a 12v sometimes being a little lighter and a little smalle than two similar AH 6v units.

On the other hand of course, with 8 x 6v batteries, if one DOES die (or you want a spare), you will only need to spend an extra 1/8th of you budget for a spare 6v battery but an extra 1/4 of your budget to have a spare 12v battery.

Other than that, I guess it is just personal preference as to which type takes your fancy - that and which batteries happen to fit the space you have for them best perhaps.

Blue Skies

Mark

suzukishoes
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Re: watt hours / amp hours

Postby suzukishoes » Sun Apr 17, 2011 6:52 am

Thanks for that Mark. I'm in two minds at the moment, I'm undecided either to build my next EV out of something classic, probably using lead or convert my G-Wiz (which I do actually quite like!) to Lithium. Either was I'm in the mood for a project!

Lee

guymark
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Re: watt hours / amp hours

Postby guymark » Sun Apr 17, 2011 3:04 pm

Oh wow.

I wish I had the facilities (and skills) to build an EV from an existing car.

The electrical side wouldn't worry me too much but everything else would faze me - I can remove things easily enough given time and tools - but adapting *different* things to fit instead is where it all falls down a bit with me :) You know that thing learned when little - taking apart is fun and easy - but putting back together is not as much fun - and usually results in a small dish full of "spares".

Pity as I have a humping great 3 tonne lump of a classic car which drinks about 2 pint per mile when cold and at best offers 12mpg when warm. With the engine and turbo removed (and sold) it might even provide not only the space but also some funds towards motorising it.

I think I would be up for a ldea acid refit to an EV but I don't think I would be brave enough to tackle a dino>electro conversion.

Please take loads of photographs if you do decide to d the classic conversion Lee, I would guess it could be a fascinating feature as well as offering invaluable insight for people wanting to do it themselves.

LOL - take enough photos and notes and you would probably end up only a few hours away from making it into a "How to" book. Ebook of course :)

Kind Regards

Mark


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