How to put 2 different battery pack in parallel

Hello guys,

I'm working on a new ebike build EEB frame, QS205, 72200 mobipus controller.

I currently have this battery pack for the build :

- ANT BMS
- 20s5p
- 21700 format
- 12A burst per cell
- 3800mah per cell
- 3.65V nominal
- 1.387KW total
- 1C max charging current

I would like to add another one in parallel with 18650 :

- ANT BMS
- 20s6p
- 18650 format
- 3500mah per cell
- 10A burst per cell
- 3.65V nominal
- 1.533KW total
- 1C max charging current


My worries are the following :

- As the 2 battery packs does not have the same amount of power, these two will be unbalanced. When this will arrive, the big one will drive current into the small one to refill it. My fear is that the current flow may be too strong and will be far superior as the 1C max charging current of the batteries.

Are these worries are founded ?

How can I limit the max charging current between the two ? DC-DC converter with max-current limiting ?

Thanks for your help guys!

Cheers

Not an expert so others correct of any of this is wrong.

It is Volts that matters with paralleling. Ensure the voltages are close before connecting, ideally within 0.1V, and very little current will flow between them.

Charging separately should not be necessary, but may be useful, especially if the spec'd finish voltage is not exactly the same.

If the voltage sag on both packs is about the same, they will both do about half the work and stay happy. But you can't really tell from the specifications given. I'm running a pair of similar parallel packs on my Sur-ron. So far I have not noticed any issues. If one pack sags a lot more than the other, it will contribute less during high drains and pump charge into the other pack during low drain. You want to avoid having much charge transfer between packs.

You could measure the sag at a given current for each pack individually to see how close they are.

Always make sure the packs are within about 0.5v of each other before joining.

How to put 2 different battery pack in parallel

Click to expand...

Short anwser: you don't!!

You only can do it when the 2 battery's are iddentical to another for voltage, w/h and charge/discharge cycles.

Lol... my battery is actually x14 individual 36v 10s 1p packs. Each have their own BMS and I routinely pull up to 120a out of this battery during rides.

7 in parallel, then series the 2 parallel packs of 7.

Once every 2 months I pull the series/parallel XT connector harness I made, charge each pack seperately to full.

Over the last 2 years I haven't seen any one of the 14 packs with more than 0.3v difference.

Yes in other contexts all kinds of unmatched capacity batteries work in parallel no problem.

Yes there is greater efficiency if little to no energy transfer between banks.

As long as each pack has a well-operating BMS, should prevent any harm.

It's just at that initial joining point that care needs to be taken so there isn't a big inrush from a higher voltage to lower voltage, decent AWG thick wires between so they don't heat up too much.

And if the cells have different charge profiles spec'd, best to charge them separately.

But if the same chemistry even that will be marginal, use the lower less aggressive profile.

The same-age resistance capacity everything matching yes " best case " but really more for series connections, where the units have no natural communications and need to be balanced.

Thanks guys for all your answers.

So it's seams still unclear if my through are founded or not... Here a schematic about what i'm not sure could happen.


Also if i'm refering about this graph, the difference must be wider when the battery is almost fully discharged :

Once the two banks have been joined, they are merged into a single circuit, thus there will no longer be any voltage difference.

The initial inrush is the only potentially "acute" issue, prevented by ensuring both are at the same voltage when connected.

After you disconnect them, let both rest no load no charge, then measure.

If you see their "bounceback" is to different voltages, or otherwise determine they are at different SoC, that indicates there was uneven usage.

But as long as neither is lower than you'd let them go on their own, I don't think that means more than uneven wear, likely NBD.

As others have said, be sure that the newly charged packs are at or near the same final resting Voltage before electrically connecting them. My packs are 51.2 nominal and I allow up to .4 Volts differential when connecting. My packs are usually something like 55.2 and 55.5 Volts when I connect them. One pack always contributes a fraction of an AH more over each discharge. No issues at all. If your controller is programmable, program its' limits based on the lowest performing pack. There are no issues running different AH capacity, same Voltage packs in parallel. Each pack will contribute its proportionate share to the load automatically. I charge my packs separately due to potential BMS issues.

i have 2 30Q packs, 22s12p and 22s18p, id like to run in paralele for 30p, they are not exactly same age but both still fairly new. i was considdering removing the shrink to then make connections between the series groups, so that essentially they become one large battery, and can balance charge 30p blocks . is this necessary/helpful or not? would just putting the 2 pack output cables in paralele be good enough?