Wire to the 12 V power
To protect the battery, the BMS must be powered whenever there is any current in or out of the pack.
If there is any current in or out of the pack, yet the BMS is off, your pack IS NOT PROTECTED!
If there can be any drain on the battery when the ignition is off, then the BMS has to be powered at all times, and the BMS has to be able to shut off that load as well.
A load that is permanently connected to the battery pack will damage your pack, even if it has a low voltage cutoff. That is because the lowest capacity cell can be overdischarged before the pack voltage is low enough to trip the low voltage cut-off.
In a properly designed EV with a Li-Ion pack, there is no load on the pack whenever the ignition is off (and the vehicle is not plugged into the AC power).
In that case, the BMS is powered by the ignition 12 V.
These instructions assume that:
User supplied required parts:
Here are some suggestions for proper tools.