Guide to battery pack sizing?

[Greenbeast]

I really need a guide that describes what happens when you series and parallel cells/modules.k
Bonus points for various example modules/packs.

So I can easily figure out how to get a given voltage and kwh numbers out of given OEM battery modules /packs

So for example I was just looking at i3 batteries on ebay.
They are nom 28.8V, they use 35Ah cells.
How do figure out how to make a, say 200V, pack that has 30kWh?

Many thanks

[johu]

First you start looking how many series strings you need to approach your target voltage: 200/28.8 = 7. One module is 28.8*35=1 kWh, 7 in series is thus 7 kWh. Now you see how many you need in parallel for your target capacity: 30/7=4,3. So 4P7S would get you 7*4=28 kWh or 7*5=35 kWh.

[Greenbeast]

Many thanks!

So 35kWh of those particular modules (£150 ea) would be £5250

The listing says they are 8s2p internally, does that affect your example calculations?

[johu]

It doesn't affect my calculations.

That said, usually cells are paralleled at cell level to minimize monitoring effort (not really relevant if that is already built in).

If you want to use the modules as is you can only parallel them on a module level like in the example calculation. If you plan to extract the cells from the module anyway, you would end up with 2P8S*5P7S=10P56S

[Greenbeast]

Ok i'm trying to get my mind straight, i need to series modules to get my Voltage up, Parallel to get the amps up, but total capacity in kWh is more a product of these
i.e. in leaf gen 1 modules, i need 13s to get 200V, each string would then be 0.488*13= 6.34kWh
So to get a pack of ~35kWh i need, 35/6.34=5.5 (6) strings in parallel?
The pack would be 13s6p (78 modules) and in fact total 38kWh, which at the price of £250/ea, which i can see them at individually sold on ebay would be nearly £20k? Hence why people look for entire packs or try to get gen2/3 modules?

[New Electric Ireland]

i.e. in leaf gen 1 modules, i need 13s to get 200V, each string would then be 0.488*13= 6.34kWh

Each module is nominally 7.6V and you'll need 26 in series for 198V.

Each module is nominally 500Wh and 26 in series will give you 13kWh at 198V.

If you construct a battery with 52 modules (26 series x 2 parallel) you'll have 26kWh at 198V.

If you construct a battery with 78 modules (26 series x 3 parallel) you'll have 39kWh at 198V.

Note capacity is very dependent on module age, climate, and use pattern. We see many second hand Gen 1 Leafs with 50-60% capacity remaining.

Some useful info here;

https://evbatterycenter.com/HAC4/index. ... Itemid=196

So to get a pack of ~35kWh i need, 35/6.34=5.5 (6) strings in parallel?

No, you'll need 26S3P if using Gen 1 or Gen 2 modules.

The pack would be 13s6p (78 modules) and in fact total 38kWh, which at the price of £250/ea, which i can see them at individually sold on ebay would be nearly £20k
? Hence why people look for entire packs or try to get gen2/3 modules?

The most cost effective way of buying a battery is to buy a complete car and sell unwanted parts.

Here's a 48kWh battery constructed from 96 Leaf Gen 2 modules... note the weight without battery box was 400kg;

https://www.diyelectriccar.com/threads/ ... ost-805713

[Greenbeast]

Many thanks for your reply, it helps me get straight