Continuing our example, at a battery charge voltage of 13.5 volts the SQ-75 will generate about 4.8 amps and produce 65 watts (13.5 volts x 4.8 amps) to charge the battery. Always use the module's current rating when sizing an array and not the rated peak power.
Some PV modules that look like good buys when comparing rated power turn out to be not so great when comparing battery charging power. When you are shopping for modules to charge batteries, compare the amp rating and not the rated watts! Modern Pulse Width Modulation (PWM) controllers convert excessive voltage into current - see the controller section for more details on this.
Choosing a Module
PV modules can be divided into three categories depending on the number of cells or the peak power voltage (Vpp). Low voltage modules, sometimes referred to as self regulating produce about 14.5 volts. Self regulating modules do not have a built in regulator. Medium voltage modules produce about 16 volts and high voltage modules produce about 17.5 volts or more. Each type of module is suited to different applications.
A charge controller will protect your battery from over charging, which leads to water loss and decreased battery life. A charge controller is an inexpensive device for preventing accidental damage to your expensive batteries.
Modules may not require a controller when the rated peak charge current (amps) of the PV module is less than 1.5% of the battery capacity in amp hours. For example, a module rated at 1.5 amps will not overcharge a 100 amp hour battery at normal operating temperatures.
Low voltage modules may be used without a regulator under certain circumstances. In hot climates the maximum voltage will drop due to the higher temperatures. This is what is meant by self regulating.
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