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Parallel p42a 12p and 3p batteries, how terrible an idea is this?

I am finishing up building a monster 20s12p P42a pack for my ONYX ebike.
At the same time I am moving the controller and opening up a good amount of internal space that can be used.
A possible idea would be to build a smaller 20s3p P42a pack and wire it in parallel with the large one.
I would run a harness between the packs to also parallel all the balance leads to help keep them together.
They would share a bms for charging.
But they would both be bypassing the bms for discharge.

I know paralleling batteries is done frequently and would probably be fine.
But would the significant capacity difference between packs cause any issues?
Or would it largely be just fine.
I would probably have all of the balance lines between packs fused too just incase something happens one pack would have a harder time killing the other.

I wouldn’t parallel the balance leads, I would just put a small BMS on the other battery.

Connecting them in parallel is fine AS LONG AS they are at the same voltage when you connect them together, and then leave them connected together. Don’t connect and disconnect them.


Gonna repeat this one as you can fuck up the batteries pretty badly and make a big spark if you don’t do this.


So just give each their own bms for charging then just parallel the main outputs.
That sounds like a plan to me and a lot simpler than having a whole bundle of wires connecting them.

And yeah having them charged to the same voltage before any connection for sure.
I have had a spark or two from my last 20s battery build and they are certainly not great.
I don’t want to see what a pack of this capacity and output would do if hooked up wrong.

Now off to inventor to see how I can actually cram in another 60 cells.
Bringing this up to a whopping 20s15p 62ah 300 cell pack.
Range for days!!! maybe literally…


Yeah it’s worth repeating that if you ignore that, you can make a giant fireball that spews toxic smoke and can’t be put out with any kind of extinguisher.

They can be within 0.1V or so of each other (preferably not in the middle of a charge, but on the low or high end) and it’s okay but I wouldn’t go much past that.

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Yes, exactly this :+1:

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If I didn’t have so much money tied up in this battery a big plasma fireball sounds like fun.
But yeah I imagine it would be a little rough on the battery cell health.


I’m not sure that connecting one charger to 2 different bms is a great idea…

You just connect the charger to one of the BMS.

The two battery packs should be separated for charging. Otherwise, if you charge them via BMS A (which is connected to the battery pack A) and the p-groups on the battery pack B get out of balance, the BMS B has no way of cutting the charge and you risk overcharge on any of the high p-groups.

If you connected the balance leads in parallel and only used a single BMS, you wouldn’t have this problem. As long as you make sure all p-groups (of both packs) are roughly at the same voltage level, this would actually be fine and maybe a better solution overall.

Or just break the parallel connection for charging and charge the pack A through BMS A and the pack B through BMS B as you should.


This is what I was thinking of doing from the beginning.
It is a bit more work for the extra wiring.
But has the benefit of only having to buy a single smart bms for monitoring.
Probably using something a little thicker like silicone 20ga for the balance parallel wires.
Just incase there is ever any appreciable current flow through them.
I just need to find a relatively small connector with 20+ pins on it.

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There is an argument to be made to keep the balancing leads as small as possible (but still meeting any voltage drop preferences/limits).

If any appreciable current flows through any balancing lead then something terrible has gone wrong. Having a small gauge lead can help it to act as a fuse instead of letting the fault current continue to flow, possibly leading to more trouble.

No “correct” way to handle this though. Our own preferences and requirements will push us one way or the other.


Some inline 2a fuses added to the lines could at least give it a known break point.
But if that ever popped the p group on the other end would stop getting balanced which wouldn’t be great.
Although that is assuming it hasn’t failed in some way.

If the charger is only connected to one bms, the second bms becomes obsolete, no?

I would assume that a bms will only balance cells and provide charge protections if you are charging through the bms.

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Wouldn’t that mean the battery pack A would get charged to 100% and battery pack B would stay at XY% when the problem was detected? Why the pack B can’t cut off charging when connected in parallel compared to only being connected to charger?

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Because it would be charged through the discharge leads rather than through the charge leads and its own BMS.

When you connect two packs in parallel, you join their discharge leads. If the packs are not at the same voltage, then current will flow through the discharge leads from the higher pack to the lower pack until the voltage evens out. Then if you charge one of the packs, the same mechanism kicks in and the other pack will start gaining charge from the first pack.

It’s like if you have two buckets with water and connect them with a pipe near the bottom. As you connect the pipe, the water will start flowing from one bucket into the other until the water is at the same level in both. Then if you start pouring water into one of the buckets water will flow into the other bucket as well. Due to physics/gravity, the water will naturally want to keep the same level in both buckets.

Sticking with this analogy an equivalent of a BMS would be a smart inlet that closes itself if something goes wrong inside the bucket so that it cannot take more water in. However if you connect the two buckets with a pipe, then even if they both have their own smart inlet, it’s of no use since the water can always get in through that interconnecting pipe from the other bucket.


Water analogies almost always make it so easy to explain in layman’s terms :sweat_smile: well said.


Related question then: is it fine to charge both packs from the same charger if you disconnect them from one another at the discharge end? Then a single BMS should be able to cut charge for a while if it needs to balance cells.

And to answer OP’s question: since both packs are using the same cells, the load should get distributed proportionally, so it should be safe to connect the packs, even if one is much smaller than the other.

This is really really really dangerous, probably more dangerous than charging them in parallel. This introduces a massive risk every time they are connected together, a risk far more serious and drastic than the thing you’re ameliorating.

Humans make mistakes and if you do this long enough, you’re going to burn your house down.

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No, the second BMS turns into a passive balance board. You could also just actually use a passive balance board instead.

Or you could connect your board lighting to the second BMS and so if your lights went out you’d know there was a p group issue on battery 2.