Problems with LiFePo4 cells   V 1.3  2012

The most important rule with LiFePo4 cells is to keep all cells in a battery pack at the same charge level. You can measure each cell and there should be no difference in voltage of more than 0.02 volts. Usualy the balancers or the BMS balance small differences. Cells have a different resistance, which can cause larger discrepancies over time. Balancers only work during charging. So the balancer shunts the charging current when voltage is over 3.3-3.5 V. But at high charge currencts and if battery is never fully discharged, there could occur a higher inbalance. The charger used, should automaticaly reduce the current  over 54 V (to C 0.1) and stop charging at 58 V. Only active BMS systems charge each cell individualy, but are very expensive and have the disadvantage of limited charging current and a more complicated setup.

 

If a battery cell has a higher or lower voiltage than the other cells it has to be disconnected and dis– or recharged with an electronic LiFePo4 single cell charger. Only charge one cell at the time! Idealy the battery pack is fully charged and then discharged to 3 volts per cell.

The inbalanced cell then is disconnected and charged / discharged to the same voltage. Repeat this process, to verify the result.

 

The electronic chargers usualy show the charging voltage, not the voltage of the cell. Therefore you have to measure the cell voltage with a separate volt meter. After the charging process, you have to disconnect the cell for a few hours and recharge if the voltage drops.

Then the voltage should not drop faster than the voltage of the other cells. Otherwise the cell is damaged and has to be replaced.

 

If a LiFePo4 cell is over discharged or over charged, it can expand. To avoid this, the cells have to be stored in a stable frame. Often used steel band is not sufficient! If a cell is expanded, it has to be secured to avoid the shell to be damaged. A lithium battery with a damaged shell cannot be used anymore!

 

Please note—no warranty applies, if cells are over discharged or overcharged. If a BMS shuts down the battery pack or indicates an error or a balancer indicates an error — first measure ALL cells. Do not use the battery unless the problem is solved and ALL cells have the same voltage and all cells are over 3.2 volts. Do not use cheap BMS systems.

 

The following information / procedure should only be carried out by specialists in a secure environment. All4solar is not reliable for any damage this can cause.

 

Over discharged cells below 2 volts or expanded cells can be re-used but need to be pressed into a frame and then charged to 3.2-3.3 volts. Then discharged to 2.5 volts and charged again. ATTENTION: Do not dismantle cells or drill holes etc.

Only if the cells keep the voltage over a few days and the expansion is completely reduced, the cell can be reused.  NEVER use cells which get hot when charged / discharged at a 1 C rate or which are physicaly damaged.

IMPORTANT: LiFePo4 cells from different manufacturers have different voltage limits. We recommend to use a BMS with a adjustable voltages.