Burnt MOSFET on battery bms - need help

[datek]

I have a typical Hailong-style 36v16ah battery on my ebike. It's about 1.5 year old. I was riding one day, bike ran fine, but when I detached the battery from the bike, I saw this melted spot:


I opened the battery, expecting to see half-burnt pcb, but there doesn't seem to be any damage inside.
It's also nice to see how water-resistant the battery is. I've been riding in the rain probably for over a dozen times already.


Here you can see the molten spot from inside, and the rightmost mosfet (next to black -wire) that caused it


Here's the row of mosfets. Looks like heat has desoldered the rightmost one from it's base.


Here's a top view. Heat has also seemingly melted solder from black minus wire. I wonder how hot the battery cells underneath got.


So I'm making this post to ask your advice as to what to do next. I don't know much about mosfets. What is causing my mosfet to misbehave like this? Is it dead, do I have to replace it with new part, or can I just remove it altogether and make do with less.
Can the battery work with 3 of them (I assume the 5th mosfet is for charging)? Specs of the battery state max output current of 40A, and my bike is drawing 23A. So maybe not all of them are necessary for the current I'm drawing?

Anyway.. appreciating any answers!

 

[john61ct]

New better BMS is easier and not expensive

 

[datek]

I'm not really keen on changing the whole BMS if this is just a simple component failure and not something more serious.

 

[datek]

I googled a bit and found the same BMS for sale on Alibaba. It has one of the mosfets removed and current rating of 30A instead of 40A on mine. This makes me about 98% certain that I can just remove that mosfet on my BMS too. If no one chimes in I'll proceed to do that soon and see what happens..

BMS from Alibaba:

 

[mancuz28]

I am not experienced enough to give you advice on this...but very interested in the outcome, keep us posted. That does indeed look like the same BMS minus the FET...there are some other minor differences, but you may be right. Good luck!

 

[datek]

Ok, I removed the faulty mosfet. I also glued the temp sensors onto the remaining mosfets. Did a little riding and the battery works like before. The remaining mosfets stayed below room temperature while riding in cool autumn weather.