Why do we want more balancing current from a BMS?

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#1

I’ve been wondering about this, tossing it out for discussion…

I can understand wanting a high balance current rating if putting together a new pack, for the initial balancing, but very little current is needed for balancing after that.

Over time the BMS just needs to compensate for the differences in leakage current rates between the cells/p-groups since this is what causes the imbalance. These differences are a lot less than 1mA for new (round Li-ion) cells.

There’s nothing we can do about the difference in capacity between the cells/p-groups resulting in them dropping voltage at different rates. There’s nothing we can do about the difference in internal resistance between the cells/p-groups, causing different amounts of voltage sag (during discharge) or voltage rise (during charge).

The BMS just has to do its job to keep any cell/p-group from going too high or low in voltage and protect against overcurrent and short-circuits.

So why do we want more than a few mA of balancing current per cell?

If we want it to help balance a new pack quickly then we’re paying more for a larger BMS that generates a lot of heat just to use once or twice.

If we want a lot of current to quickly rebalance a pack that’s been in storage for a very long time are we willing to pay more and deal with a larger BMS just for that rare occurrence?

If we want it to help compensate for aging cells, which self-discharge and become unbalanced faster, then we are forcing ourselves to run a pack with badly performing cells that will have also lost a lot of capacity and will have more voltage sag. Why try to keep those cells balanced? In my opinion it’s better to replace them, or the pack, than to desperately try to keep crappy performing cells balanced.

A technical concern…the higher the balancing current the longer the cells need to rest, after balancing current flows, in order to come to their true resting voltage. If the BMS doesn’t take this into account then the balancing accuracy will suffer. Whether this makes a practical difference depends on the cells, the current level, etc.

So, why do we want more balancing current from a BMS?
How much balancing current do we really need?

Just thinking out loud…I’d love to hear your thoughts on this!

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#2

@DerelictRobot @JTAG @hardunclejohn @ENNOID @SimosMCmuffin

Tagging in BMS makers/designers.

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#3

I have some kinda shitty used cells that I like to abuse and they tend to drift and have cells go out.

That always needs balancing, and it really never completed charging for whatever reason the BMS would quit instead of balancing.

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#4

I think a BMS should be able to balance a 0.1V differential in a day, and we have packs from 10Ah to 50Ah these days

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#5

What would unbalance a pack that much though?
Or are you referring to new packs or ones out of storage?

What voltage difference (when charged) do you feel means a pack is balanced? Something very tight like 10mV or is something looser like 30mV or even more okay?

(Edit) At what cell voltage this 0.1V spread exists at is critical. If in a fully charged pack (as far as the BMS can go, that is) that’s easy to correct. If at any voltage that’s much lower than that the voltage spread doesn’t matter and shouldn’t be balanced out.

#6

part of the problem with this, is that the cells are so cheap that putting a high end balancing current BMS would be worth many times more than the cells it is attached to, therefore something external you can hook up when needed and remove is better for that.

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#7

That would definitely work but a lot less current is needed if balancing is available 24/7, down to some rather high cell voltage (4.0V). Which would be better, I don’t know. I guess the details matter for this one.

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#8

Running the battery down real low can easily do that, from empirical experience. Theoretically, it shouldn’t but IDK :man_shrugging:

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#9

I think logically you are correct but most here including me are man childs who want the biggest and more of everything. :joy:

People using 10awg wires, 6374s on street boards, 6mm bullet connectors, xt90s etc etc. Totally unnecessary for the most part but it feels good.

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#10

I feel like a pack is balanced if it’s within about 10mV but there is no basis for that. Just gut feel.

I also feel like the closeness of a “balanced pack” has a lot to do with the measuring equipment used to check it. The more accurate the measurement, the more off it looks :rofl:

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#11

I mean 10awg is only rated for 55A

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#12

If, for example, you brought a pack down to 2.5V per cell one or more times in a short period of time you would find that the cells had become unbalanced?

Would you remember roughly how bad it got for the times it happened to you?

#13

I’ve killed packs running them down to 3.0V / cell. I’d personally never go down to 2.5V on current dumb BMSs, despite the specifications indicating it’s fine.

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#14

More like 100+ continuous. Any street board will be fine with 14awg wires.

and this is worth a watch

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#15

Any idea why they died? If no cell went below 3.0V/cell and weren’t overheated, etc., they should have been fine for hundreds of cycles. I’ve done some pretty abusive cycle life testing down to 2.5V every cycle and many different cells were fine.

Something other than 3.0V/cell is killing them IMHO.

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#16

The details matter a lot here though…
Those tests are single wires in open air. Inside a board enclosure, next to another hot conductor, with no convective cooling, means those wires will get a lot hotter. Add on pressure and vibration from being in a board and I’d be wary of running at current levels that high if it’s happening a lot.

This is ignoring the loss in power you experience with smaller gauges.

Many can get away with things like 14AWG with 100A setups because we rarely hit current levels that high and only for short periods of time.

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#17

No idea; not enough visibility into the state of health at the time. Probably being unbalanced which means some p-groups were lower than others? :man_shrugging:

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#18

The ideal BMS would shift the energy from the most charged cell to least charged, so the only losses are efficiency losses during transmission. It seems like it’s just the cheap BMS’s (like the ones we use in this hobby) that bleed the excess power via resistors to equalize pack voltage.

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#19

Most production boards are running 14awg power and 16awg phase. Personally I think it’s a liability. Even at the low power ratings that these boards can do, i’ve seen the phase wires get so hot they’ve desoldered themselves from the motor controller

Then you’e got people like metroboard who run 8awg into an xt60 for a 50a max battery :crazy_face:

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#20

That may have more to do with shitty soldering than the wire its self. A really poor solder joint can greatly increase the resistance of the wire+connector system.

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