How Do These BMS Work?

http://www.ebay.com/itm/322058052636

These devices are all over eBay and I've even seen them in some videos. They seem utterly useless to me. Am I missing something or do these actually work?

It says the max charge current is 40A. Okay. Then it says the balancing current is 60mA. At 4.2v, that's what, 0.25W? Is it supposed to work by slowly bleeding off a tiny bit of juice over time, rather than "ok, your battery is at 4.2v, time to dump the 20A of charge current coming in".

Surely they have to do something as there are hundreds of them on eBay, and a lot of them are selling...

That's pretty dang slow if it is really balancing at 60mA, especially if you are trying to balance big packs in series. The ones that UKDIYPW found move around 10A

http://s.click.aliexpress.com/e/A6Uvnun

Dunno - Maybe that's just the operating current. It sure does say balancing current though.

A 60mA balance bleed current is fine for low Ah packs but rather pointless for our use - I've seen a few on here say they do work but I'll bet that's either on a pack that's already very well balanced, or on a battery that isn't being used (so no charge/discharge cycles) - perhaps the placebo effect is comforting.

Those Ali express ones have good reviews, Simon has purchased them for his packs, Iwonder if he has formed an opinion on them yet.

Sean said:

Those Ali express ones have good reviews, Simon has purchased them for his packs, Iwonder if he has formed an opinion on them yet.

Click to expand...

Im keen to hear feedback too, quite a few aussies have bought from this seller.

The only thing stopping me is the seller doesnt specify paypal, I just cant bear giving out credit card details.

Yes they bleed slowly. They are made for low amp packs.
Many of them also doesnt start balancing untill you reach like 4.15-4.18 what i have heard so far.
You could potentially mod them but since they are pretty dumb its still perhaps not what we are looking for.

Im building a bit smarter such device now that will be actively monitoring the packs and then you can set your sel when you want to balance. And with external balance you can also define how much current to be drawn to do the balance. Though its just passive balancing actively controlled. Best would be to charge the one being to low instead of dumping load of the one to high

For those wondering if you could upgrade a cheap BMS with a better mosfet and larger resistor, yes you can, sort of.
Probably too much extra work to fit say 7 20A mosfets + suitable driver, and 5W or 10W resistor. Would also be rather messy given a 5W resistor is rather big compared to the original circuit.

station240 said:

For those wondering if you could upgrade a cheap BMS with a better mosfet and larger resistor, yes you can, sort of.
Probably too much extra work to fit say 7 20A mosfets + suitable driver, and 5W or 10W resistor. Would also be rather messy given a 5W resistor is rather big compared to the original circuit.

Click to expand...

I wonder if one could use a dump load coil intended for use with wind generators.

mike said:

station240 said:

For those wondering if you could upgrade a cheap BMS with a better mosfet and larger resistor, yes you can, sort of.
Probably too much extra work to fit say 7 20A mosfets + suitable driver, and 5W or 10W resistor. Would also be rather messy given a 5W resistor is rather big compared to the original circuit.

Click to expand...

I wonder if one could use a dump load coil intended for use with wind generators.

Click to expand...

You could, if the required current to dump was high enough and the resistance was suitable - but it would have to be very high to warrant their use - and they are usually just high wattage wire wound resistors anyway, or badly made (and expensive) lengths of nichrome wires coiled in a open cage.

They'd be ideal for load testing a bank.

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