Photovoltaic charge controllers are devices used to protect batteries from
damage due to overcharging. They limit the voltage and current delivered to a
battery. Some have additional features such as a low voltage disconnect to
prevent your batteries from being over discharged.
Choosing a Charge Controller
When choosing a charge controller for your system keep in mind that you may
want to expand your charging capability. A larger controller may be a wise
Wires from a DC charging source, such as a PV array, can be large so ensure
that the wire connections on the controller can handle the wires you plan to
use. Small PV systems may require #12 - #8 AWG wire while larger systems can
require #6 AWG or even #4 AWG wire.
Poor connections are often made when the controller has its own wires that must
be connected using a wire connector (Marr type), or where the wires are held in
place by a screw which requires a crimped terminal connector. Soldering may be
the only way to ensure a reliable electrical connection. The best method of
connecting wires to the controller is a box type pressure connector where the
wires are compressed by a screw.
Most electrical codes (including the Canadian and the American Codes) require
that all electrical connections are done inside an enclosure to prevent the
possibility of accidental shorts. Ensure that the controller you purchase does
not have any exposed connections and that it can be easily mounted into an
Voltage Set Point
A battery¼s full state of charge voltage is dependent on the charge rate and
the temperature of the battery. While achieving an exact voltage setting is not
critical for small systems (under 15A) we recommend controllers that have
adjustable voltage set points. Some controllers feature automatic temperature
compensation to adjust the voltage set points based on battery temperature.
Many controllers have LED indicator lights. Indicator lights are good for
showing the stage the charge controller is in and the approximate battery state
of charge. LEDs are an inexpensive means of monitoring a small PV system.
However, the use of a good charge source ammeter and a battery voltmeter is
more valuable and larger systems should have accurate meters.
If your power system has no means of equalizing other than the renewable energy
source it is important that you can bypass the charge control function so that
an equalization charge can occur. If the controller does not have a bypass then
it will be necessary to disconnect the controller when you equalize your
If the battery bank is exposed to temperatures of greater than 30fC or less
than 0fC then the controller should be temperature compensated. A temperature
probe is attached to the side of the battery and the controller will
automatically adjust the voltage setpoints based on the temperature. Some
controllers attempt temperature compensation without a remote temperature
probe. This is not recommended since the controller may be at a different
temperature from the battery.