Charge Controllers

Batteries have to be protected against overcharging and deep discharging by a charge controller. Charge controllers are also referred to as charge regulators and battery control units (BCU).

During charging, the battery voltage increases. At a certain voltage, the battery starts gassing. Gassing, the production of hydrogen and oxygen, reduces the life expectancy as well as the required maintenance periods and should be avoided. A controller avoids excessive gassing by maintaining the voltage at the end-of charge level of about 14.0 V, depending on the battery type and temperature. The end-of-charge level may be adapted to the battery temperature by means of a temperature sensor.

In contrast to charging, the voltage of a battery being discharged declines. In a deep-discharged state, chemical processes occur which will reduce the battery lifetime quickly. To protect the battery, the loads have to be cut off at a certain voltage. High-quality batteries are less sensitive to deep discharging.

For automotive batteries, manufacturers recommend discharging of only 30%, while 80% is common practice.

Battery type	Max. discharge	Cut-off voltage
Automotive battery	30 - 50%	11.8-11.5 V
Heavy-duty battery	50 - 70%	11.5-11.2 V
Solar battery	60 - 80%	11.4-11.0 V
High-quality battery	80%	11.0-10.8 V

A new generation of controllers adapts the charging process and the load management to the state of charge and the ageing process of the batteries. System efficiency and battery lifetime benefit from this, but such controllers are sophisticated and currently hardly available in developing countries.

Charge controllers may provide a lot of additional helpful functions and features, but this makes them more expensive and reliability may suffer as a result. Controllers are designed for maximum input and output currents. Safety measures may include protection against short circuit, overvoltage, overloads, reverse polarity, wrong connection and environmental hazards. Controllers should indicate the state of charge and a load cut-off.

Individual batteries will develop differently after some time of operation. Worn-out cells have to be identified and removed in order to protect those that are still in good condition.

Source: Klaus Haars, GATE-ID technical brief: Electricity from Sunlight, Solar Energy Supply for Homes and Buildings, Eschborn, 2002.

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