AN3971

Application overview

Doc ID 022119 Rev 2

5/28

The primary function of a charge controller in a standalone PV system is to maintain the

battery at the highest possible state of charge, and to protect it from overcharge by the array

and from over-discharge by the loads.

Although some PV systems can be effectively designed without the use of charge control,

any system that has unpredictable loads, user intervention, optimized or undersized battery

storage (to minimize initial cost), typically requires a battery charge controller.

The algorithm or control strategy of a battery charge controller determines the effectiveness

of battery charging and PV array utilization, and ultimately the ability of the system to meet

the load demands.

Important functions of battery charge controllers and system controls are:

●

To prevent battery overcharge: to limit the energy supplied to the battery by the PV

array when the battery becomes fully charged

●

To prevent battery over-discharge: to disconnect the battery from electrical loads when

the battery reaches a low state of charge

●

To provide load control functions: to automatically connect and disconnect an electrical

load at a specified time, for example, operating a lighting load from sunset to sunrise.

The most common battery type used is the valve regulated lead acid (VRLA) battery,

because of its low cost, maintenance-free operation and high efficiency characteristics.

Although the battery installation cost is relatively low compared to that of PV systems, the

lifetime cost of the battery is greatly increased because of the limited service time.

The lifetime parameter is reduced if there is low PV energy availability for prolonged periods

or improper charging control, both resulting in low battery state of charge (SOC) levels for

long time periods.

An increase in the lifetime of the battery results in improved reliability of the system and a

significant reduction in operating costs. The life of a lead acid battery can be extended by

avoiding critical operating conditions such as overvoltage and overcurrent during the

charge.