Battery unbalance and equalization
After several charge-discharge cycles, differences appear among the SOC of the individual cells of a battery, due to manufacturing tolerances, uneven temperature distribution, differences in ageing etc. The battery then is said to be unbalanced or un-equalized. The less charged cells are said to be weak in contrast to the others being the strong or healthy ones. If the issue is not addressed properly, the SOC differences will continue to grow with the number of the cycles and, the effective capacity will be reduced and symptoms will arise such as a constantly running generator (low capacity) or the battery will “not hold the charge”.
In an unbalanced battery, the maximum charge that can be safely drawn cannot be more than the charge stored in the weakest cell and this is the autonomy provided by the battery to the system. The remaining energy in the rest (healthy) cells practically is not available to the system. Attempt to draw more charge from the battery, will result to reverse polarity and quick deterioration of that particular cell. Lack of proper means to detect this condition and to terminate immediately further battery discharge is a very common reason for early cell failure.
Attempt to fully charge the weak cells will necessarily result in overcharge of the healthy ones. Lack of proper means to detect this overcharge and to terminate immediately further charging will result to immediate and severe destruction of lithium batteries.
Long term unbalance of Lead-Acid batteries results in sulphation of the partially charged (weak) cells.
The problem of battery unbalance cannot be easily handled in stand-alone PV systems.
In diesel-battery and hybrid PV systems, with lead-acid batteries which are tolerant to overcharge, the common practice is to keep the diesel generator running until the weak cells are charged. This requires usually a 11-15% overcharge during which the healthy cells are forced to overcharge and gas.
The cost of this equalization over-charging phase is the fuel wasted and the engine wear during this phase plus the damage done to the healthy cells due to corrosion, overheating and electrolyte loss due to gassing.
Our Active Battery Management systems control battery equalization in the most efficient way, according to our Dynamic Battery Management Concept.
The necessary amounts of charge are exchanged directly, between the weak and the strong cells, through a powerful charge redistribution system consisting of high efficiency, two-way chargers individually dedicated to each one cell of the battery.
This system is superior to all competitive passive or active equalizing systems. It provides the highest flexibility in charge exchange and the fastest response in SOC control.