BMS fried - help!!!

[fafrd]

I just fired up my 280Ah LiFePO4 battery with 300A Heltec BMS for the first time for a full discharge test at 80A for 3.5 hours continuous.

Everything went fine for the first hour then the BMS put out a plume of stinky smoke and I immediately shut everything down.

I assume the BMS is fried and this surprises me because it was rated for 300A Max Working Current and 100A Max Charge Current (single-port).

I’m in dialog with Heltec to understand whether I received a defective unit but I also want to understand experience of anyone else who has successfully tested their BMS at 80+ hours for 3.4+ hours.

The BMS as passively cooled only and the room was warm at 77F / 25C (because I was running space heaters for the test).

So I’m also interested in whether anyone who has a BMS which passes a test like this is relying on passive cooling only or is actively cooling with a fan, as well as whether there are any ‘danger zones’ in terms of ambient temps and/or temps on the metal BMS sleeve to provide an early warning that the BMS is in the way to overheating.

The voltage at the BMS output was 0.45V below the voltage at the input, so at 80A we’re talking about 36W...

Appreciate advice from anyone who is successfully running 80A or more through their BMS for ~4 hours of continuous discharge..,

 

[BiduleOhm]

It was already dissipating 36 W @ 80 A? that's a very low efficiency...

Can you do a teardown to see exactly what failed?

 

[fafrd]

It was already dissipating 36 W @ 80 A? that's a very low efficiency...

Can you do a teardown to see exactly what failed?

I’m pretty sure it was one or more of the power transistors that burned up (that’s where the plume of smoke came from).

I’m in dialog with the manufacturer now and will do a tear-down as soon as they give me a green light.

What is a typical voltage drop to see between BMS P- and B- leads when ~80A of current is flowing (@ 26V in my case - 8S LiFePO4)?

At 80A, the BMS is putting through 2080W, so losing 36W in the process amounts to 98.3% efficiency - is that poor?

 

[BiduleOhm]

I’m pretty sure it was one or more of the power transistors that burned up (that’s where the plume of smoke came from).

I’m in dialog with the manufacturer now and will do a tear-down as soon as they give me a green light.

Ok

What is a typical voltage drop to see between BMS P- and B- leads when ~80A of current is flowing (@ 26V in my case - 8S LiFePO4)?

I don't know what the typical is for chinese BMS but as a data point I expect 3.8 W @ 100A, 15 W @ 200 A and 34 W @ 300 A on mine (and that's with Tj = 120 °C) so 36 W @ 80 A is more than 10 times worse.

If it's supposed to handle 300 A you would look at at least (extrapolating without accounting for Rdson increase due to temp increase) 506 W at that current... and there's no way it would handle even half that (even with a fan...). So either it's a super bad design, or you had a defective one.

At 80A, the BMS is putting through 2080W, so losing 36W in the process amounts to 98.3% efficiency - is that poor?

It's not a lot if you compare it to an inverter for example, but in the world of SSRs it's quite a lot (and in my book it should be far lower).

 

[fafrd]

Ok




I don't know what the typical is for chinese BMS but I expect 3.8 W @ 100A, 15 W @ 200 A and 34 W @ 300 A on mine (and that's with Tj = 120 °C) so 36 W @ 80 A is more than 10 times worse.

If it's supposed to handle 300 A you would look at at least (extrapolating without accounting for Rdson increase due to temp increase) 506 W at that current... and there's no way it would handle even half that (even with a fan...). So either it's a super bad design, or you had a defective one.




It's not a lot if you compare it to an inverter for example, but in the world of SSRs it's quite a lot (and in my book it should be far lower).

Thanks. If your BMS draws 3.8W @ 100A, I’d appreciate to know which manufacturer/model it is.

And 3.8W @ 100A translates to only a 38mV drop, so if that is typical, I’ll inform the manufacturer of my 0.45V drop since that may indicate a defect.

And for next time, I’ll get guidelines from the manufacturer and check voltage drop between P- and B- before letting the BMS run for an hour and burn up.

 

[BiduleOhm]

Thanks. If your BMS draws 3.8W @ 100A, I’d appreciate to know which manufacturer/model it is.

Hmm... me: https://diysolarforum.com/threads/summary-of-diy-bms-design-and-reflection.17169/ ?

I calculated 0.38 mOhms typical and 0.48 mOhms max (1.9 mOhms/MOSFET and 2.4 mOhms/MOSFET respectively) which translate to the power numbers I gave above