Proper configuration for a 72V battery composed of several 36V kicscooter packs?

[redmouse]

I'm trying to make a big battery pack for an electric motorcycle.

I sourced a bunch of second hand kickscooter batteries, each has 36V nominal, has it's own BMS, and my total pack voltage will be 72V.

I also have bunch of 36V/2A chargers and one 72V 4A charger.

What do you think is the best configuration?

I kinda wanna do C, because I think it would allow me for fastest charging, and allow me to spot which packs are weak. What worries me is what if one of the packs degrades or fails - what will happen to the other pack on the same branch? Will it get overcharged by the packs in parallel?

But I don't know what I don't know, so I'd welcome some pro advice. TY so much!

 

[JackFlorey]

A and C are somewhat the same. In both cases, all batteries are in parallel so you are charging them all at the same time. If you unparalleled option C with switches or relays you could get individual charging. Paralleling them all would be fastest charging.

B is somewhat risky because there is no way to balance the packs, and if there was a significant imbalance between top and bottom you'd see the batteries cut out via their BMS (at best) and fail catastropically (at worst.)

 

[docw009]

If the packs have separate charger inputs, you're not going to like this answer, but if you charge one pack when it's connected in parallel with the others, you've bypassed the charge protection circuits on the other five batteries. This may not matter most of the time, but for that one instance where a battery has a balance issue and normally couldn't be recharged, you're forcing charge thru it. The safe way to charge parallel packs is for all of them to use a one port BMS. Otherwise, you have to separate them.

The other comment has to do with batteries in series. You won't like thid either, It covers the second part of JackFloery's answer, The BMS in each battery has to be able to take the combined voltage of both batteries. In this case, that's 84V. This only happens when a BMS shuts off, but it could shut off thru over current or whatever. Then you get the voltage from both packs across the MOSFET's in the off battery, That could be as high as 84V.

A careful designer would examine the BMS boards, pull the part numbers and look it up the MOSFETS's. To illustrate this, here's a small 36V BMS. It uses four SFS7508 devices. which I looked up and found are 75V transistors. That's too low.

 

[redmouse]

JackFlorey, docw009>​

Thanks guys, great info, I wouldn't have thought of that. Makes total sense.
Yes, the charge and discharge ports are separate on the pack, so that's unfortunate. I thought I'd make things easy with those packs, but alas I played myself instead.

I thought about multiswitches, multipin connectors and relays.. but then I'd have to also make it work with precharge to connect to the controller, and make it reasonably foolproof for when another person uses the bike... and all that sounds like a much bigger headache than just straight up gutting the packs and hardwiring them together as a single 20S pack with one BMS. So that's what I'm going to do.

 

[redmouse]

AkbarRamzan made this simulation to illustrate what's happening at the mosfets, I think it's very cool:
Falstad.com simulation

 

[2Phast2Delirious]

Option "A" has always appealed to me, and the fact that it is lithium-compatible is handy.

An individual battery maintenance system was what option "C" meant for when we used 12v deep-cycle batteries for electric vehicles. Instead of spending $400 on a 120v 10A charger, I spent $120 on ten 12 volt (1,3,6 Amp) smart chargers.

 

[problemaddict]

Quick question, are you sure the BMSs on your packs actually do balancing?
I built a pack as you described using hoverboard 36v packs. The BMSs on those packs were only for protection, they didn't balance. So I had to periodically check every pack and balance it if needed...

 

[redmouse]

Yes, they're from commercial fleet rental scooters, so I believe they have to be pretty bulletproof. But that doesn't matter anyway because I'm not goign to use those BMSes.

 

[problemaddict]

Ok good, that would've been my next suggestion, LoL.
Not sure what to suggest on your original question. I just charged in bulk @ 72v and checked the packs often....

 

[DaLanMan]

Yeah... Beyond my skill level to answer, aside from some muttering about "only good idea if those units are designed to be wired like that"

That said, why not peel em apart and brew up your own?

I have a multi-battery unit charger I am assembling, it has a pre-line controller that kicks the batteries out of connection with each other, then direct-feeds the charger power to the 2 units (charges one, then the second) took me about 30mins to come up with a circuit and well, honestly just back from hospital with a Crohns event kicking off a 105f fever, so behind on my stuff by a bit, hopefully will be back on things soon.