Home Power Systems
Solar Charge Controller:
What is a solar charge controller?
Why do I need one?
A
charge controller, or charge regulator is similar to the voltage
regulator in your car. It regulates the voltage and current coming from
the solar panels going to the battery. Most "12 volt" panels put out
about 16 to 20 volts, so if there is no regulation the batteries will be
damaged from overcharging. Most batteries need around 14 to 14.5 volts
to get fully charged.
Do I always need a charge controller?
Not
always, but usually. Generally, there is no need for a charge
controller with the small maintenance, or trickle charge panels, such as
the 1 to 5 watt panels. A rough rule is that if the panel puts out
about 2 watts or less for each 50 battery amp-hours, then you don't need
one.
For
example, a standard flooded golf car battery is around 210 amp-hours.
So to keep up a series pair of them (12 volts) just for maintenance or
storage, you would want a panel that is around 4.2 watts. The popular 5
watt panels are close enough, and will not need a controller. If you are
maintaining AGM deep cycle batteries, such as the Concorde Sun Xtender
then you can use a smaller 2 to 2 watt panel.
Detailed information on MPPT charge controllers.
The
charge controller regulates this 16 to 20 volts output of the panel
down to what the battery needs at the time. This voltage will vary from
about 10.5 to 14.6, depending on the state of charge of the battery, the
type of battery, in what mode the controller is in, and temperature.
(see complete info on battery voltages in our battery section).
Using High Voltage (grid tie) Panels With Batteries
Nearly
all PV panels rated over 135 watts are NOT standard 12 volt panels, and
cannot (or at least should not) be used with standard charge
controllers. Voltages on grid tie panels varies quite a bit, usually
from 21 to 40 volts or so. Some are standard 24 volt panels, but most
are not.
What happens when you use a standard controller
Standard
(that is, all but the MPPT types), will often work with high voltage
panels if the maximum input voltage is not exceeded. However, you will lose a lot of power
- from 20 to 60% of what your panel is rated at. Charge controls take
the output of the panels and feed current to the battery until the
battery is fully charged, usually around 13.6 to 14.2 volts. A panel can
only put out so many amps, so while the voltage is reduced from say, 33
volts to 13.6 volts, the amps from the panel cannot go higher than the
rated amps - so with a 175 watt panel rated at 23 volts/7.6 amps, you
will only get 7.6 amps @ 12 volts or so into the battery. Ohms Law tells
us that watts is volts x amps, so your 175 watt panel will only put
about 90 watts into the battery.
Using an MPPT controller with high voltage panels
The
only way to get full power out of high voltage grid tie solar panels is
to use an MPPT controller. See the link above for detailed into on MPPT
charge controls. Since most MPPT controls can take up to 150 volts DC
on the solar panel input side, you can often series two to four of the
high voltage panels to reduce wire losses, or to use smaller wire. For
example, with the 175 watt panel mentioned above, 2 of them in series
would give you 66 volts at 7.6 amps into the MPPT controller, but the
controller would convert that down to about 29 amps at 12 volts.
Charger Controller Types
Charge
controls come in all shapes, sizes, features, and price ranges. They
range from the small 4.5 amp control, up to the 60 to 80 amp MPPT
programmable controllers with computer interface. Often, if currents
over 60 amps are required, two or more 40 to 80 amp units are wired in
parallel. The most common controls used for all battery based systems
are in the 4 to 60 amp range,
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