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Can I use different capacity cells

Posted: Fri, 03 May 2019, 18:59
by soyachips
Hi everyone,

I didn’t plan it this way but I’ve ended up with 22 x 10Ah Headways and 22 x 15Ah Headways.

Can I put all the 10Ah cells together in series to make 72V and the same with the 15Ah cells, then put these together in parallel to make s bigger capacity 72V pack?

Thanks!

Re: Can I use different capacity cells

Posted: Fri, 03 May 2019, 20:42
by antiscab
Yep, that'll work fine. If you want to save on bms, you can parrallel individuallly 10Ah and 15Ah to make 25Ah then put in series. Might be a little awkward in layout though

Re: Can I use different capacity cells

Posted: Fri, 03 May 2019, 21:40
by soyachips
Thanks that’s good news but I’m curious why the 10Ah cells don’t run out of energy first and hit low voltage before the 15Ah cells?

Re: Can I use different capacity cells

Posted: Fri, 03 May 2019, 22:25
by antiscab
Currents flows proportional to the inverse of resistance. The smaller cells generally have higher resistance, and don't put out as many amps in parrallel as the larger cells. It all evens out

Re: Can I use different capacity cells

Posted: Fri, 03 May 2019, 23:43
by soyachips
Got it, thanks.

Also by paralleling each 10Ah and 15Ah together first they can be monitored as one “cell” and I only need 1/2 the number of BMS?

Re: Can I use different capacity cells

Posted: Sat, 04 May 2019, 08:40
by coulomb
soyachips wrote:
Fri, 03 May 2019, 23:43
by paralleling each 10Ah and 15Ah together first they can be monitored as one “cell” and I only need 1/2 the number of BMS?
Yes.

Re: Can I use different capacity cells

Posted: Sun, 05 May 2019, 16:09
by soyachips
Excellent, thanks for the help ... now have to work out how to arrange different size cells together into a pack!

Re: Can I use different capacity cells

Posted: Thu, 08 Aug 2019, 04:52
by KDRYAN
Hello Everyone,
A slight variation to the above post. I also want to use cells I already have.
I want to assemble a 700ah @ 12 volt module using 3 x 700ah and 7 x 100ah cells. My question is, can I arrange the 7 x 100ah cells in parallel and place these in series with the 700ah cells to complete the module. I will also be using a BMS unit also. This module will be used in a campervan with current draw less than 10amp continuous.
Thanks in advance.

Re: Can I use different capacity cells

Posted: Thu, 08 Aug 2019, 06:07
by brendon_m
The cells won't be a perfect match and heat, internal resistance etc will be different throughout the pack so it will have a tendency to slide out of balance. But with a bms it should be OK. Its only a 12v pack so there isn't a lot of room for an individual cell to move before the overall voltage trips out so a lot of lithium 12v packs have no bms and just get away with the pack being balanced/matched at the start. You won't be properly balanced/matched so a bms is a wise investment but otherwise it should work

Re: Can I use different capacity cells

Posted: Thu, 08 Aug 2019, 09:54
by jonescg
KDRYAN wrote:
Thu, 08 Aug 2019, 04:52
Hello Everyone,
A slight variation to the above post. I also want to use cells I already have.
I want to assemble a 700ah @ 12 volt module using 3 x 700ah and 7 x 100ah cells. My question is, can I arrange the 7 x 100ah cells in parallel and place these in series with the 700ah cells to complete the module. I will also be using a BMS unit also. This module will be used in a campervan with current draw less than 10amp continuous.
Thanks in advance.
Welcome back Kim! Haven't seen you around for a while.

Re: Can I use different capacity cells

Posted: Thu, 08 Aug 2019, 10:49
by KDRYAN
Thanks Brendon,
I was thinking along those lines, so I will give it a go.

Jonescg,
Still here, you know, once bitten by the ev/lithium bug .....

Re: Can I use different capacity cells

Posted: Fri, 09 Aug 2019, 12:03
by T1 Terry
You will find the 700Ah cells will wander more than the 7 x 100Ah cell group. The 7 x 100Ah cells tend to balance each others strong and weak points out within the parallel pack, the single high capacity cells can't do that because they can not shift any differences in resistance or capacity to the other high capacity cells because they are connected in series.
An 8 x 100Ah cell 2P4S 12v battery will stay in balance much better than a 4 x 200Ah cells connected in series, this has been evident in multiple installs in RV's over the last 8 yrs.
If you chose to attempt active balancing to keep the pack within a suitable voltage differential range, you will need balancers that can move a lot of current. The cell runaway doesn't occur until the top end of the charge and at 12v nom., the charging current is high so the balancing must be able to move that high current from one cell to the next rapidly to prevent cell voltage run away. Stopping all charging if a cell hits 3.6v and not resuming until the cell drops below the 3.6v is probably the easiest method, simply dropping the charge rate to 1/2 amp so the balancer can cope will result in the battery not reaching fully charged if you are relying on solar to do the recharging.

T1 Terry