Different size Lifepo4 battery banks in parallel

[ericfx1984]

So I've been thinking about this and I realize if I build some now and add more batteries later I may not necessarily get the same size.. now I realize if I end up getting something a little smaller later that I simply programmed the BMS to use less capacity on the larger battery Bank

But I got to thinking if the batteries are in parallel, and the BMS has a low voltage cut off then it doesn't really matter if they're matched

I mean I guess what I'm saying is from my perspective I would think that if I had four battery Banks in parallel, let's say one is 302ah, 16s,

the next is 280ah 16s

Then a pair of 230ah, 16s batteries...

Now I would think if each of these batteries were put in parallel with each other on a common load, that the pair of 230 amp hours batteries would hit low voltage disconnect followed by the 280, followed by the 302, of course this is assuming that everything else is equal which who knows

So one way I can think to take care of this issue would be to simply change the maximum discharge rate on the larger batteries to match the lower

But I suppose there's also advantage to having the two smaller ones run out sooner so you would know once they disconnected that you had a short period of time to get everything recharged before you had some issues I don't know just curious something that's rattling around in my brain

 

[brb58]

if they are all in parallel, their voltage would always be the same. A 100AH battery would half half the current being discharged as a 200AH battery so the SOC would be nearly identical between the two batteries assuming the exact type of cells.

 

[Cdkipp]

 

[rmaddy]

There are a lot of existing discussions on this that you may find helpful:

Search results for query: different batteries in parallel

diysolarforum.com

 

[Blackwind]

We wanted to add an additional "like-kind" battery (batteries) to our current battery set up but they are unavailable at present. So we began to research using completely different battery banks etc. We of course would keep it within the appropriate voltage of 48V and close as possible, the same size. (example three battery sets equalling 200Ah ea at 48V) This thread piqued my interest.

 

[Number21]

It took me a while to wrap my head around this and be OK with it...you're always told not to do that with other batteries...but it is fine. Think about it like this - many new cordless power tools use two different 18v batteries for more power. They don't have to be identical batteries, as per the manual. Each battery just has it's own BMS.

 

[Steve_S]

I run Diff Capacity packs within one LFP Bank. It is not a BIG problem but does add Quirks !
My Production Bank:
2x 24V/175AH Used EV Grade LFP (recovered from ShunBin)
1x 24V/280AH Matched & Batched EVE-280 ESS cells. from Luyuan
1x 24V/280AH Bulk EVE-280 ESS cells. from Luyuan
1x 24V/280AH Bulk EVE-280 ESS cells. from XUBA

- During Charge the different sized packs will take Varying Charge Amps relative to the packs capacity.
- Smaller packs will of course reach FULL before the larger ones. As this happens, the Amps taken by them decreases "as it should" but the extra amps go to the bigger packs within the bank so they'll increase their charging proportionately.
- The smaller packs will continue decreasing the amps taken which is good as that is saturating the cells.
- Now as the larger packs start to reach full, their amps taken will decrease as the cells saturate. This will continue until the EndAmps/Tail Current is reached.

During discharge all packs will discharge Amps proportionately relative to their AH capacity. This is expected & normal. This will continue quite naturally while all packs are within the Working Voltage Range (3.000-3.400 Vpc).
- Once reaching the low end of the Working Voltage Range things get WEIRD. The larger packs of course still have more left to give and the smaller packs Amps out will decrease until the cells reach a parity and then the bigger packs will "backfill" and attempt to charge / boost the smaller packs in the bank. EXPERIENCE with my bank shows this happens just below 2.900Vpc.

EndAmps/TailCurrent is calculated on the Largest AH Pack within the pack as that is the one to ensure reaches full. The Charger, Inverter/Charger or AIO will see the "EndAmps" from the entire bank as one number. This means the smaller packs will more then likely be taking <1A while the largest will be taking a bit more. When the entire bank reaches the total of amps taken to be less than EndAmps that is when charging will stop.

Saturate is when cells reach "Target Voltage" which is a "surface voltage" but the amps decrease as they saturate & increase resistance. Cells are "actually full" when they reach the "EndAmps / TailCurrent" which is AH-Capacity * 0.05. (100AH*0.05=5A). *LFP always settles post charge, the deeper the saturation of the cells, the less they will settle.

NOTES:
Charge / Discharge Voltage differences.
Charge Devices like SCC or external Charger MUST be corrected so that whatever voltage / amperage drop there is between Device & Battery Terminals is compensated for. If you intend to have the Batteries charged to 27.0V AT the Battery Terminal you do not want line loss affecting those readings. If there is 0.3V Loss and SCC thinks it sees 27.0 but at batt terminals it is 26.7, then you have a problem ! BEWARE because Under/Over Charge is too easy, so compensation is important.
ON DISCHARGE Devices like Inverter, it is CRITICAL that you are also Voltage Corrected. Again if there is a 0.3V Line Loss and you want to cut off at 22.0V (2.750Vpc) but it has to drop to 21.7 @ BATT to cutoff you could end up with troubles.

SCC's, Inverter/Chargers & AIO's all handle things differently. Some have settings for Charge & Discharge Compensation or Correction, while many do not. Those that do not, have to have their SETTINGS corrected to compensate for Voltage / Amperage drops acxcross the wires, terminals, fuses etc. Ultimately the Voltage & Amperage at the Battery Terminals is the MOST IMPORTANT THING as everything ties into that ! When to operate, when to cutoff and what state the batts are in.

BMS Trickery:
There are so many different BMS' and they handle things their own way. When it comes to Battery Banks with multiple packs within, you really want identical BMS's and the battery packs setup as close to identical as possible. I'm now replacing $2500 of BMS, Relays & Contactors & more... U don't want to get there ! Some BMS' are better / friendlier to Banks such as when the smaller packs get lower, rather than take charge off the bank, they can be put to "sleep or storage" mode by the BMS, other BMS may just turn off those batteries while others do nothing.

A FINAL WARNING
Yep, gotta put it here. The REALLY KEY THING is to NOT have too much of a difference between packs within a bank. 280 & 302 next to each other is FINE but 175 & 280 is NOT, the 105AH difference DOES CAUSE ISSUES and mostly during charge as the small packs start to drop Amps Taken they also start to deviate and that gets wide when batteries are full. THIS IS IMPORTANT TO NOTE, Once Cells cross past Working Range (3.425Vpc) they will deviate & "RUN". The ONLY Exception is the Matched Cells which don't deviate much at all until they reach 3.500. RUNNER Cells are prudominant with Bulk & lower-grade cells, including those which are only Voltage & IR Matched (That is not REAL Matching & Batching, if they say different RUN they are lying to you) ALSO Note that Unmatched/Unbatched cells will also start to run from Lower Voltage Ranges from 3.375 on up quite frequently and may also present Low Runners which once below the Working Range of 2.950Vpc will run to low volt well ahead of the other cells.

Hope it helps, Good Luck