x2 7S Li-ion 18650 different total capacities cells in parallel

[paulmurdoch]

Hello,
I have built two 7S Li-ion batteries consisting of 182 (18650) cells and each with its own Daly BMS installed (24V). The first battery consists of 182 2900mAh cells and the other battery consists of 182 1500mAh cells. My question is, considering each battery is protected by it's own BMS, what would/could be the issues if I parallel these batteries to increase the Ah?

 

[snoobler]

Did you measure all the cells to confirm capacity before you build each battery, thus confirming that each "cell" within the 7S battery had the same capacity? If so, great. If not, I'd start over.

The expectation is that the two batteries in parallel would see the 78Ah battery handle 2/3 the load while the 39Ah battery handled 1/3 the load, charge and discharge.

Connect them using best practices ("wiring" in link #2 in my signature) and confirm current flow through each battery under high charge and discharge currents and insure they are operating within limits, hopefully at the same C rating.

Issues would include:

1. Uneven load sharing that might cause one of the BMS to shut down, overloading the other and then shutting it down.
2. Uneven battery wear/reduced life due to uneven load sharing.

 

[paulmurdoch]

Thank you for the quick reply!
When I built the first battery I didn't plan on a second, (same as my kids) so that's why I used two different battery type's; it was just what was available at the time. Each 7S battery is balanced in capacity, it's just the two 7S have different capacities. The load on the cells will be minimal, they will be used for a remote gate/internet/camera doorbell system, but Id like to a add a dusk to dawn light, so the extra capacity would be very nice, to get through the dark days.

 

[snoobler]

Probably not an issue as those loads sound pretty low.