Two packs of batteries

Hi all.
I have two packs of batteries 36 V each, since I wanted to use them both in my bike I first connected both in paralel at the discharge side but then I thought it was not a good idea since one pack might be charging the other.
I then disconnected the discharge and put a 2p 2t switch to discharge one and then the other, but kept connected the charge side.
In this configutation both packs discharge at the same time regardles of the switch position.

First question. Would this configuration be OK ? since one pack is charging the other but through the other BMS ?

Second question. In case it is not OK, would it be wise to eliminate one BMS and just solder the second battery pack to the first so as to make 10 groups of 8 batteries instead of 10 groups of 4 ?

Thank you very much.

Hi amaurer. Welcome to ES!

Connecting two packs in parallel is fine if they are both so similar that they don't have different amp draw characteristics. As an example, if one is older and has more internal resistance it will stress under acceleration or other heavy amp draw like going up hill while the second newer pack won't. That will cause heat and consequently faster degradation of the weaker pack. There are a bunch of other problems that you can run into as well. And, there are plenty of solutions also.

Check out this link and then search for more links on multiple battery packs in parallel. https://endless-sphere.com/forums/a...zn6g8&q=battery+packs+in+parallel&sitesearch=

If you want to charge them at the same time (connected on the charge side.) that should be ok if you isolate the packs with applicable diodes to keep the current only going to the packs and not back and forth between packs. When one is charged the BMS should stop the charge to the battery, and the other should continue charging until both battery packs are only balancing with the BMS's. Again, the two battery packs should be as closely matched as possible to avoid problems.

As for the one battery charging the other while riding, you can do that if the older less efficient battery is the first battery in line so the second battery is not stressed when supplying the first.

Getting rid of one bms is probably not the best answer unless both packs are exactly the same and you properly wire the one bms properly to all the cells.

This is what I would do. I would mount the two batteries on the bike. Keep the charge ports separate. Get two chargers so each pack can charge separately. Put switches on the discharge side so only one pack at a time will run the bike. And then, and this is important, turn off the drained battery before you switch on the full battery. This is to keep an inrush of power into the drained pack from the full pack. You might also consider applicable diodes to stop the transfer of current from one pack to the other. They might cost you a bit of capacity do to energy being converted into heat.

Hope it works out for you.

First thing, I'm assuming they are the same type cells. not one lifepo4 and one NMC or something like that.

The issue of one pack charging the other should be not a problem. But though I'm an idiot, or at least a child in my electronic knowledge, I thing you should disconnect them at the charge side.

Most bms can handle 5 amp input, such as regen input, though the discharge wires. But you might confirm that as fact with your particular packs. What happens when you run them parallel, is a big jolt of power will be split between the packs, but if one pack is very strong cells, and the other is weak, the strong cells will take more of the load. After the amp spike ends, then there will be a small difference in voltage, and the power will flow from one to the other, till both are at the same voltage again. But unless you have a really great battery connected to a really shit battery, this difference is not much, and not that much amps flow back in the input leads of one of the batteries. So if its only 1 or 2 amps, briefly, its just like having regen, and its fine with most bms these days.

On the charge side, I'm less sure its ok to connect them. The routine advice given here for a long time has been charge full, connect parallel, run the bike, then disconnect and charge them separate. Re connect only when both are full again.

There is a proper way to connect the charge side parallel, but its not just a simple connect at the inputs.

If you want to, you can remove one bms. Then connect them in parallel at the positive and negative. Then also, connect all the bms wires in parallel as well, and then connect to one bms.