oivey...
You CAN do it and not have issues IF you are ready to deal with the quirks. Obviously, it bis far better to have battery packs of identical or near-identical capacities as that really keeps things simpler but once you diverge more than 10-15% things start to get complicated. I run 2x280's & 2x175's side x side in a parallel bank and quirks are there to be handled.
When all the batteries are within their working voltage range (3.000-3.400V per Cell) they are generally content and will charge/discharge relative to their capacity.
- The smaller packs will output slightly less Amps than the larger packs during discharge.
- The smaller packs will reach both Empty & Full before the larger packs.
-- Depending on BMS type & setup, the smaller packs may either begin to pull from the bigger ones to compensate (reduces run time due to losses) once they drop out of Working Range. During Charge they will reach full before the bigger packs and the Amps they take will reduce and drop quite low as the bigger batteries take in the balance. THIS IS TRICKY because it is possible to charge to a point that triggers HVD (Hi Volt Disconnect) which starts to cycle on the other BMS' and some Inverter/Chargers & AIO's can't deal with rapid changes from those cycles, some external chargers will actually shut down with trouble codes. Ther result is to play with the settings and tweakinng to avoid these troubles, it is doable and easier again if there is less difference between packs.
Something NORMAL that freaks newbs out... an example of a real-time situation.
If I push 100A Charge at my bank( I have Passthrough power to house as well). It is not unusual to see the 280AH packs taking 30A Each while the 174's are taking between 12-18A at the same time (it varies relative to resistance). Again a similar thing occurs during discharge where the higher capacity cells will discharge more amps than the smaller packs.
A real TRICK to get it all working without overunning the system is to get the EndAmps / Tailcurrent just right and by being SMART !
LFP can take full charge at Constant Current to about 90% SOC, after that it need Constant Voltage Variable Current to top off to 100% SOC (which is NOT 3.650V per cell, but the top of the working range which is 3.400V per cell).
NB with BattleBorn using premium A+ Cells are good for a working range of 2.900-3.450 Vpc. SOK are basic A Grade so 3.000-3.400 is their working range. NOTE that between 2.500-2.950 and from 3.450-3.650 only represents about 5-8% of total GROSS Capacity ! This is generally NOT calculated into the full battery capacity. Example, Properly Matched Batched Grade A Tested 280AH cells on average will report above 288AH Capacity and typically just over 290AH. Grade B cells "may" just reach 280AH GROSS providing about 270AH workable.
The Inverter/AIO/SCC will see the voltage on the common DC Bus and will have no idea what each battery pack is doing internally. It is honestly best to parallel batteries to a common DCBus using a (+)&(-) Covered Busbar and using the exact same length of wire to each pack, with identical connectors etc. Remember to fuse each battery.
Hope it helps, Good Luck.
