Cheap "BMS" battery killer?

Thread Starter

DanDare

Joined Apr 29, 2021
15
Hi,
I'm trying to evaluate this (very popular) cheap "BMS" connected to Li-ion cells. (attached images)

Connecting a 12V lamp to it's output (around 1.2A initially), I have probe readings:
Probe_3 -> 11.4V
Probe_2 -> 7.6V
Probe_1 -> 3.8V

When batteries reach around 3.2V individually, the "BMS" shuts off the power terminals. But then probes readings are:
Probe_3 -> -4V
Probe_2 -> -0.16V
Probe_1 -> -7.76V

It persists this condition until I disconnect the load. After disconnecting load voltages returns to initial condition.
What the heck this "BMS" is doing? I guess it looks very bad.
It looks like it's activelly killing my batteries. Or I'm missing something?

This is a 3S "BMS" but I see 4 MOSFETS. I wonder if them are some type of H-BRIDGE, doing misterious/unexplained/dumb things.
Any clue? Or just getting over with this directly to the trash can?

Thanks!
 

Attachments

Thread Starter

DanDare

Joined Apr 29, 2021
15
Now I see. Scope 'ground' was attached to "BMS" '-' output. In common for all probes.
But measuring each battery individually, disconnected, reads normal values as expected.

This thread is irrelevant now. Could not find a delete thread Button. So, that's.
Thanks
 

Thread Starter

DanDare

Joined Apr 29, 2021
15
Thanks Delta Prime!
Yeah I know batteryuniversity and it's pretty good indeed.
Didn't understand the relation you pointed about horrible solder connections and exploding batteries. If concern is temperature I've set iron to a very hight temperature but then soldered terminals really quick, almost instantaneously.
 

Thread Starter

DanDare

Joined Apr 29, 2021
15
Ah OK.
Not nitpicking. Actually enjoyed your detailed explanation, thanks for that. Yeah spot welder is the *only acceptable* way to connect batteries in industry. Interesting to think (and care) about solder conductivity, may be even helping batteries unbalance to develop. Must think more about this....

Just googling:
"The electrical conductivity of soft and hard solders is considerably less than that of copper, varying with composition between approximately 9 percent and 13 percent for soft solders and 20 percent and 40 percent for silver solders."
 

bassbindevil

Joined Jan 23, 2014
422
That "BMS" looks like it doesn't include balancing. Boards with usually have a second row of six-legged ICs, each accompanied by a (relatively) big resistor. You can get add-on modules for balancing, either resistive or active (with inductors to transfer energy between tiers).
 
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