Drill battery cutting off voltage on output

[nukecat]

Hi. I have a cordless drill, which I bought not long ago, but the battery quit working on the other day. When I tried to use the drill, led on the front lit up for not even a second, then it went off but the drill never turned. After that I checked the drill voltage, and found out it was 7 volts and was dropping like a capacitor would do when slowly discharging it (keep in mind the battery is an 18v pack). That was strange given that it was charged not even a full week before and I haven't used it much after charging, but I tought it was just simply discharged, so I put it on a charger. The battery has a green and a red led on it along with a dc barrel jack and the charger is just an off the shelf wall plug pack, so you just plug the adapter into the battery, the red led means its charging the green means its charged. It charged for half an hour, put it back in the drill, guess what the same thing happened, the white led lit up and went out immediately but it never turned. After that I put it on the charger again to see if its really charged, the charge light lit up for a moment, went out and the green led lit up indicating it was charged up. I checked the voltage on the battery again at it was the same 7-8 volt-ish but it was dropping. At this point I took apart the battery and measurred all the cells and they were just fine at 4.2V, together adding up to 21V. To check them under load i jerry rigged up some alligator clips to the drill part and it worked, so it must be the BMS module that went bad, but then why is it charging the cells correctly? I put a 180ohm resistor on the cell to discharge it overnight, and in the morning I charged it up again, which went fine It charged for 3,5 hours, the charging stopped correctly at 21V, but when I attached the drill again the same problem persisted. Since then I've measured the thermistor on the battery pack which was at 8k, unsoldered the BMS and resoldered it, reflowed the board with the hot air gun, checked all the passives and mosfets (except for 3 SOT packages which I was unable to identify one read RK and a sideways 8, the other XU something, and the third I cant remember), checked the shunts, but nothing worked. The mosfets are IRLR8726PBF and the chip is CW1053). I really don't know if I'm missing something or what. Anyways, at worst I need to buy a new 5S BMS module and hack it into the pack.

[tunk]

I'm no expert on this, but I think there's at least two safety mechanisms, over-temperature
and over-current. Given the LED lights up, I would guess one of them kicks in. You could try
to short the current sensing resistors (most likely RS1 and RS2). As for the thermistor, you
may (temporarely) replace it with resistors of lower and higher resistance to see if that's
the culprit. If you buy a new BMS, I guess you should get something which can handle 20-30A.

[narkeleptk]

XU may be BZX99-C7V5.
Don't really see anything for the RK.