It sounds like it must be either the
BMS playing up or perhaps a single faulty cell causing the
BMS to shut the power off, but it's unusual for a cell to give up completely without prior notice
(i.e a greatly reduced range).You will need to remove the handle end of the battery to access the
BMS board. hopefully you should be able to measure the voltages of each group of cells at the balance lead connector to see if they are all similar voltages.
The balance lead connector can be seen in the lower left hand side of this photo:
As yours is a LiFePO
4 pack it will have 16 single rectangular 10Ah cells instead of the 13 groups of 3 x 4Ah cylindrical cells shown above.
In the above diagram you need to put the meter probes between Battery -ve and pin 1 of the connector to measure cell no 1, and between Battery +ve and pin 12 of the connector to measure cell no 13.
It might be easier to leave the black meter lead on the battery -ve connection and simply note down the readings obtained from each of the 12 pins and the Battery +ve lead.
Then you simply subtract the lowest of two adjacent readings from the highest to find the cell voltage.
To find the problem you will have to test cell voltages while the battery is turned on and plugged in to place a load on the
BMS and battery cells.
The
BMS board usually has a common +ve connection for the battery, the power output and the charging input, but the -ve connections are usually separate and are joined electronically through the
BMS board.
If you can
post some photos of the
BMS board I will try and explain what each of the connections is for if they are not clearly marked already.
Take a look at
Gary's post showing the dismantling of the LiFePO4 battery pack to get a better impression of what's inside.
Unfortunately Gary's photos show very little of the
BMS board itself so I still don't know how many balance wires are connected, but I'm sure we will find out soon enough.
Alan 
