I am still in the learning mode on Lithium batteries, BMS`s Chargers etc. Is it the BMS that switches the charger off. My Charger cuts off when my 13 S battery reaches 53.9 Volts. Is this normal??
The BMS balances the cell groups to make sure they all end up at the correct voltage. Almost all BMS's limit the amount of current that can go into the battery pack, and most of them will limit how much current can go out of the battery.
The chargers job is to charge to the 54.6V that the pack needs to end up at. The BMS will allow the cell groups to charge up to a full charge (in this case 4.2V). When a cell group reaches max voltage, the BMS stops the charge and begins balancing the individual cell groups.
So for your 13S battery pack, the BMS will take a reading for each of the cell groups. Most cell groups will be at 4.20V, but one cell group might only be at 4.15V. The BMS will then drain the other cell groups down to 4.15V. The BMS then allows the charger to send another full charge to the cells.
If you are still ending up with 53.9V after a full charge, it means that one or more of your cell groups is not taking and holding the full charge.