Why a Battery Management System (BMS) is needed:
1. A LFP cell will be damaged if the voltage over the cell falls to less than 2,5V.
2. A LFP cell will be damaged if the voltage over the cell increases to more than 4,2V.
Lead-acid batteries will eventually also be damaged when discharged too deeply or overcharged, but not immediately. A lead-acid battery will recover from total discharge even after it has been left in discharged state during days or weeks (depending on battery type and brand).
3. The cells of a LFP battery do not auto-balance at the end of the charge cycle. The cells in a battery are not 100% identical. Therefore, when cycled, some cells will be fully charged or discharged earlier than others. The differences will increase if the cells are not balanced/equalized from time to time.
In a lead-acid battery a small current will continue to flow even after one or more cells are fully charged (the main effect of this current is decomposition of water into hydrogen and oxygen). This current helps to fully charge other cells that are lagging behind, thus equalizing the charge state of all cells.The current which flows through a fully-charged LFP cell however, is nearly zero, and lagging cells will therefore not be fully charged.
Over time the differences between cells may becomeso extreme that, even though the overall battery voltage is within limits, some cells will be destroyed due to over-or under voltage.
A LFP battery therefore must be protected by a BMS that actively balances the individual cells and prevents under-and over-voltage