Lithium Battery Balancing
Battery Cell Balancing
Typically, the individual cell group in a battery have somewhat different capacities and may be at different levels of state of charge (SOC) due to the manufacturing variances, assembly variances, and different charge/discharge histories experienced. When cell groups are connected in series, these differences may limit the energy that can be taken from or return to the battery and result in overcharge or over-discharge without effective and appropriate balancing circuit.
The Renogy Smart Lithium Iron Phosphate battery employs bypass circuit to maintain the balance between each cell group in the battery. Each cell group is connected with a bypass resistor and a switch in parallel. During the charging process, when the voltage of the highest-voltage cell group reaches the set balancing starting voltage and the voltage difference between the highest-voltage cell group and the lowest-voltage cell group exceeds the set voltage difference, the switch connected to the highest-voltage cell group will be closed to shunt the charge current around the highest-voltage cell group through the bypass resistor until the voltage difference drops below the set value. To avoid excessive energy loss, the battery cell balancing is only performed during the charging process.
Paralleled Battery Balancing
Typically, the voltage difference between individual batteries is larger than that between individual cell groups. When batteries are connected in parallel, the balancing will start automatically between batteries as the current flows from the higher-voltage batteries to the lower-voltage batteries. However, due to the small internal resistance of the battery, the balancing current will be so large that trigger the over-current protection of the battery when the voltage difference is too large. As the number of paralleled batteries increases, the voltage difference will become more restrictive. As a result, few or no restrictions are imposed on the voltage difference between batteries when the number of paralleled batteries is small, while a voltage difference smaller than 0.1V between the highest-voltage battery and the lowest-voltage battery is recommended when the number of paralleled batteries is large.
