Paralleling Packs With Different Capacity

I got this customer who wants a 40Ah pack with an external 12Ah pack.

Both of course brand new, same cells (40T) and each has it's own BMS.

He really insists and I always avoided that (even avoided paralleling same capacity).
So here I am asking what are potential issues of such a thing?

My main concerns are that if pulling 100A from both at the same time, how will that current be shared over different capacity packs?
Also, it's obvious that the packs wouldn't deplete at the same rate so the large pack will be feeding the small pack after each touch of the throttle to balance the lower voltage the small pack has and that concerns me as it may cause a shorter lifespan for the smaller pack (and maybe even the large pack) due to charging and discharging themselves into each other in order to constantly balance their voltage since they paralleled.

The last issue I'm concerned about is the charging.
If charging them paralleled, how would each BMS behave? that seems like a big one meaning the customer will have to charge them separately and only connect them back together when their voltage is very close.

What do you guys think?

No problem, as long as the voltage vs SoC correspondence is close, ideally identical.

The bigger bank will feed the greater share discharging, and absorb faster charging.

Different amp rates, similar C-rates and voltage / SoC% level

But the voltage will be pretty much the same, any temporary imbalance will get restored as they "rest".

Dunno about the BMSs, but guessing also NP.

john61ct said:

No problem, as long as the voltage vs SoC correspondence is close, ideally identical.

The bigger bank will feed the greater share discharging, and absorb faster charging.

Different amp rates, similar C-rates and voltage / SoC% level

But the voltage will be pretty much the same, any temporary imbalance will get restored as they "rest".

Dunno about the BMSs, but guessing also NP.

Click to expand...

The voltage vs. soc isn't always the same since they are both lithium ion.

Doesn't it shorten the lifespan having to always close imbalances? (charging/discharging between one another)

What about the charging issue?

dont think theres any issue, do the math for amp draw put a fuse in between the two packs, hit it with some amps and see when fuse blows, i did it with different resistance cells and have used daily in my trike
https://endless-sphere.com/forums/viewtopic.php?f=6&t=104027&p=1556192&hilit=marin#p1527424

as far as doing charge/discharge between the packs while balancing i consider it to be like using regen but more gentle

i did a test to simulate a situation where an electric boat powered by 30q and solar panels, motor into a bay with solar and battery, drop anchor. the 30q pack would go from discharging to charging with no break
30Q didnt suffer any extra capacity loss/cycle

https://endless-sphere.com/forums/viewtopic.php?f=14&t=108547#p1592108

sony vtc6 did
https://endless-sphere.com/forums/viewtopic.php?f=14&t=108661#p1591576

did you get youre tester yet? if not i can put a 40T on same test to see

rg12 said:

The voltage vs. soc isn't always the same since they are both lithium ion.

Click to expand...

Wut?

You stated:

> brand new, same cells

> Doesn't it shorten the lifespan having to always close imbalances? (charging/discharging between one another)

I doubt significantly, is it a high C-rate use case? High DoD% ?

Then life will be short anyway. . .

_____
What about the charging issue?

See if you can explain what that issue might be, in detail

john61ct said:

rg12 said:

The voltage vs. soc isn't always the same since they are both lithium ion.

Click to expand...

Wut?

You stated:

> brand new, same cells

> Doesn't it shorten the lifespan having to always close imbalances? (charging/discharging between one another)

I doubt significantly, is it a high C-rate use case? High DoD% ?

Then life will be short anyway. . .

_____
What about the charging issue?

See if you can explain what that issue might be, in detail

Click to expand...

Sorry, meant to say IS instead of isn't

It is a high discharge pack, all 8C running at half the discharge rate max.

rg12 said:

I got this customer who wants a 40Ah pack with an external 12Ah pack.

Click to expand...

No worries - as long as they are the same S number. (i.e. they both have to be 14S - 14 cells in series - or whatever.) Number in parallel doesn't matter much.

rg12 said:

It is a high discharge pack, all 8C running at half the discharge rate max.

Click to expand...

Then the longevity impact of "self balancing flows" will be statistically insignificant.

Great thanks for the help guys