4S BMS failure - Why?

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UDO
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4S BMS failure - Why?
I was using this BMS: https://www.aliexpress.com/store/product/4S-15A-lipo-lithium-Polymer-BMS-PCM-PCB-battery-protection-board-for-4-Packs-18650-Li/2344164_32707615617.html for about 1 month and now stopped working as one chip on the bms pcb burned out. I marked it red circle:

I use BMS in 4s2p config of 8x liitokala 26650. MY stock e-bike has 10S config with its own BMS and now I added in serial this 4s2p booster pack for a total of 14s. I wonder what was the reason BMS failed. The max current is below 14A@ 59V (its a 250W e-bike with 14A controler)

I PM user Barkuti for some help and his thinking is:

Quote:
From what I can see I can’t say for sure, that’s a 2A schottky diode with 40V maximum peak reverse voltage, it may have something to do with BMS logic protection against load transients but really don’t know. Maybe installing an SS28+ could fix the issue

Any ideas?

Thanks!

HKJ
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If you ever run the batteries dry that diode will have to handle all the current from the other pack, no wonder it died.

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Lexel
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yeah its a polarity protection diode under normal circumstances it does not have to run any current

UDO
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Makes sense, What if I just solder together 2 contacts left in the place of burned diode? I will loose polarity protection, but overdischarge and over current protection should not be affected by this?

HKJ
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UDO wrote:
Makes sense, What if I just solder together 2 contacts left in the place of burned diode? I will loose polarity protection, but overdischarge and over current protection should not be affected by this?

Wont work, running two BMS in series is a bit tricky, you need heavy bypass diodes on both of them or you risk burning them. You where lucky this time (if it was only the diode).

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Enderman
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You should not be running two separate battery systems in series.
If you want higher voltage you will need a BMS that can handle that many series cells, and all cells should be identical.

cera@1967
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 As I can see from your photo , the diode is just a reverse polarity protection diode  , so no current is going thru it , except the case when , by mistake , you reverse the polarity of your power supply.., then the diode is in direct bias and poof.., will short the PS...!

You can use the BMS without this diode (if everything else is O.k , and I do not see why not ). Barkuti seems to be right , the overvoltage (59V) could damage it.

Remove it ,give it a try without diode , and if is working , you can solder an other Schottky diode with a higher reverse voltage (100V) , just to have the reverse polarity protection back...

And something else , be sure AT ALL TIMES , that your batteries are installed with the right polarity !!!

Good luck !

Adrian

kennybobby
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How do you know only the diode is failed?

Just losing that diode wouldn’t disable the board, something else is burned up or out of range and the four large FETs along the bottom are not turning on. The gate drive of the FETs is usually 12V obtained from a tap on the 4S pack.

Even though it looks like it’s not needed, i would still run a small gauge sense wire from the top of the 4S to the solder pads, e.g. B+ solder pad is shown without a connection, and use large gauge wires for B- and P+.

Those boards are cheap on ebay, probably not worth trying to fix it. Most boards have over-voltage protection for charging and under-voltage protection for discharge, plus an overcurrent protection. Your board does not have any current protection, so you may want to look for one that does. Some even have a balancing circuit to even out the cells. Good luck with your project.

Now i used to think that i was cool,
drivin' around on fossil fuel,
until i saw what i was doin',
was drivin' down the road to ruin. --JT