Ok, i moved it to a new thread to help concentrate your info for helpers.
To test the battery, you'll probably need to open it up.
Before doing that, be sure that you don't care about any warranty service on it, because it will probably void the warranty if there is one.
First, with it disconnected from everything, set the meter to 200vdc and test the voltage at the charge port and at the discharge port, and note them in a post here.
Next, if you can open the casing, and access the main cell block itself, set the meter to 200vdc and measure the voltage at the two thick wires coming from it to the BMS and/or the outside connectors, right where they join the cell block itself. Note that in a post here.
If you can also access the connector with 11 wires in a row on the BMS inside, set the voltmeter to 20vdc, then the black lead on the first wire from that connector (if there's a black wire, start there, otherwise just start at one end). Put the red lead on the next wire over, and note the voltage down. Move the red wire to the next one, and the black wire to where the red wire was, and note that voltage down. Repeat until you have 10 voltages, and post them here.
Once we have that info, we can say more about what is going on, and suggest other tests ro repairs. .