Help with LiFePO4 battery problems

I own a Huasha HS50QT7 electric scooter with a 1.5kW motor. It was powered by 4 Greensaver 12 v 38ah Silicon Lead acid batteries connected in series, with a household circuit breaker in circuit. The range was poor as two of the four batteries are shot, so I inquired with many manufacturers and bought a 48V 30ah LiFePO4 battery and charger from yxmzone. This was the largest capacity, at the time, that would fit in the battery bay (with a club hammer and cold chisel).

On connecting the charger, it blew up, I did return for repair but bit was not collected and came back again. The battery was charged to 48 v when I fitted it, and as it had a BMS, I removed the Circuit breaker. I got a new charger from VPower and on charging it dropped to -4.5V. The polarity was wrongly connected as it had been with the charger. I fixed that and charged to 52 v and it ran OK. After an MOT, it stopped on the way home and will not charge above 48v since and just runs the scooter.

The question is whether it is the BMS or have some of the 3.2V cells been damaged. I have a multimeter and am ready to remove the duck tape wrapping the battery. So I do not completely wreck it, any advice on how to proceed would be appreciated. Otherwise a one year old LiFePO4 battery will be consigned to waste.

Leckyman, I have done a new exciting project not posted here involving a Ford Ranger with a AC system and opperating at top voltage of 385 volts with 104 100 amphr High Power LifePo4 batteries.
What you need to do is to be able to take readings of the batteries while driving the scooter and also while charging to tell what the BMS is doing. My suggestion is to hook a wire to a gauge on each battery (Scooter Supply on ebay has cheap ones) it will tell you what the cell is doing under a load, and if it goes below the safe discharge voltage while under a load, also if it flies out of control while charging. it only takes one bad cell to make it not run right. My guess is at least one of the cells is damaged. Re post the results.
On my truck I can acess the SOC (state of Charge) on each set of four batteries for temperature and I have a interface on a can bus that takes readings on each battery for voltage and controlls the charger to turn it off when a cell reaches maximum charge and a alarm for undervoltage, Theese settings are set so as to go off before the cell reaches minimum voltage (while driving down the road) and Maximum (while charging)
I hope this helps.

Bob, new dawn, as in the dawning of a new era