Checking my thinking - combining batteries

[Madmarcus]

I'm interested in some 30.6V (8s) 67Ah batteries for use in different electric vehicles. I like the 67Ah but for at least one application I might prefer a series connection to give 61.2V. I have two thoughts on a BMS and I'd like to hear opinions of each.

1. Wire each battery with an 8s BMS.
This would allow me to separate the batteries when I didn't need 60V (and potentially for charging) but how do I use the two different BMSs while using/discharging the batteries?

2. Wire one 16s BMS.
Potentially annoying from a wiring and soldering aspect because the wires might need to be lengthened but that's not a big deal. Is there any drawback that I'm not seeing? Would I have to put on individual 8s BMSs first, charge them separately to the same point, and then take off the 8s and replace with the combined 16s or can I do the initial charging safely through the 16s BMS even though I don't know the initial state of the cells in the two packs?

Finally would the answers change if I had two battery packs (different batteries not the ones linked above) that I wanted to connect in parallel? It feels like series connection would be best with one BMS but parallel might be better with separate BMSs.

As a general note I do not want to get into breaking the battery packs into individual cells and rebuilding them.

 

[lnanek]

There's many long threads on this, but I think the conclusion was that, should you put them in series, you also need to add a diode across each pack to prevent them ever getting hit with unexpected voltages.

E.g. If you examine the BMS circuit board, the MOSFETs used might have a breakdown voltage of 40V, so can't really be relied upon in a 60V system. And MOSFETs can fail closed, which is a bad failure behavior.

 

[Madmarcus]

Thanks. I was hoping that there was a sort of consensus wisdom by now instead of reading through many threads with similar but not quite the same question (LiPo questions, ones that devolve into just break the pack and use the cells directly, and so on).

 

[neptronix]

Are you going to be using regenerative braking or not?

 

[lnanek]

Even without regenerative braking, and without having another serial pair in parallel, the caution may still apply. I've noticed that when I put my geared hub motor run by a Baserunner controller behind a DC-DC converter, it will occasionally throw over current faults when going fast downhill. Doesn't happen if I connect it directly to a battery. Maybe there's some friction in the clutch/freewheel or something.

I think many controllers depend on having a battery so that they can feed surge spikes and the like back into the battery. So even with no regenerative braking and only two packs in serial configuration, I think there are cases where the pair are going to get hit with voltage and need protection. Or you could maybe add extra hardware to prevent that like:

ODrive Regen Clamp

The ODrive Regen Clamp provides a high-performance braking energy management solution for ODrives without an external brake resistor driver. It automatically detects power regeneration from downstream motor drives, and diverts excess energy to an external brake resistor. Compatible and shown...

shop.odriverobotics.com

 

[Madmarcus]

Are you going to be using regenerative braking or not?

I'd like to but I'm not set on it. That's the type of thing I was worried about because I would have thought that wiring the two packs together into one BMS (either 16s or 8s depending on series or parallel) would have allowed the BMS or controller to deal with that.

Thanks for the possible solution Inanek - I'll look into that as a possibility if I decide I want regen braking.

 

[docw009]

There are some BMS designs that allow the user to have less cells in series than the max. For example, I have some 14S-10S units. Perhaps you can find a 16S-8S design, Then the components would be rated for the higher voltages automatically,

Otherwise, you must check the MOSFET part numbers on a BMS you have, and look up the max drain voltage on their datasheets.

 

[E-HP]

I'm interested in some 30.6V (8s) 67Ah batteries for use in different electric vehicles. I like the 67Ah but for at least one application I might prefer a series connection to give 61.2V. I have two thoughts on a BMS and I'd like to hear opinions of each.

1. Wire each battery with an 8s BMS.
This would allow me to separate the batteries when I didn't need 60V (and potentially for charging) but how do I use the two different BMSs while using/discharging the batteries?

2. Wire one 16s BMS.
Potentially annoying from a wiring and soldering aspect because the wires might need to be lengthened but that's not a big deal. Is there any drawback that I'm not seeing? Would I have to put on individual 8s BMSs first, charge them separately to the same point, and then take off the 8s and replace with the combined 16s or can I do the initial charging safely through the 16s BMS even though I don't know the initial state of the cells in the two packs?

Finally would the answers change if I had two battery packs (different batteries not the ones linked above) that I wanted to connect in parallel? It feels like series connection would be best with one BMS but parallel might be better with separate BMSs.

As a general note I do not want to get into breaking the battery packs into individual cells and rebuilding them.

the advice often provided would be to use a common port BMS for each pack, so each BMS will protect its own pack, regardless of whether the two packs are connected in series or parallel. I think the most important thing is to have a disciplined charging routine that you stick to, whether you decide to charge the pack separately or in bulk.