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[princec]

So regarding a conversation elsewhere, say we've got a 28s1p battery...

Exactly *how* does the BMS measure any one cell in it?

Cas

[TheRan]

Lots of wires. It literally has a connection to each cell.

[princec]

I see. I haven't ever looked at the inside of a battery so I don't know of course Makes total sense, I just thought for some reason they didn't do that and there was some cleverer/bodgier way it was being done.

Cas

[TheRan]

I've watched a few videos of people building DIY electric vehicles and making their own battery packs for them, it's a tedious a laborious job and I can see why they can cost what they do. Also why BMS's can be quite expensive, it's essentially 20+ tiny multimeters in one and then they also need to be able to route the power where needed. I think it's because they use the sense wires to move that power about that balancing can take so long, there's a limit to the current they can put through them. Not sure if there's really any other way to do it because if you measured the voltage of multiple cells and then did the math to divide it that's just going to give you an average.

[Richard230]

When I rebuilt the bloated batteries of my 2010 Electric Motorsport GPR-S I used what I recall was a mini-BMS between each cell. I don't think it actually used a central brain to balance the cells after the original one failed. They seemed to work - at least longer than the High Power batteries did. Photo attached.

[Specter]

They measure each cell, and any differences, depending on the BMS are either diverted between the cells to try to balance them off, or burned off, and the entire pack charged, burn off next higher, and keep until they all are in the range of the bms setpoints.  The diversion is not that hard, they can use a wheatstone type of balancing to measure and deflect power, but that can add some weight AND cost, and of course take room, which many battery containers simply do NOT have much of.

This is kind of another reason why it's important to keep an eye on your batteries state of balance.  If you let it get too out of whack, yah it's not good for the battery BUT, because of the correction method, can take several DAYS to finally correct it.  Can you leave your car sit idle for 4 days?

Aaron