Solar charge
controllers are rated and sized by the solar panel array current and
system voltage. Most common are 12, 24, and 48-volt controllers. Amperage
ratings normally run from 1 amp to 60 amps, voltages from 6-60 volts.
For example, if one
module in your 12-volt system produces 7.45 amps and two modules are utilized,
your system will produce 14.9 amps of current at 12 volts. Because of light
reflection and the edge of cloud effect, sporadically increased current levels
are not uncommon. For this reason we increase the controller amperage by a
minimum of 25% bringing our minimum controller amperage to 18.6. Looking
through the products we find a 20-amp controller, as close a match as possible.
There is no problem going with a 30-amp or larger controller, other than the
additional cost. If you think the system may increase in size, additional
amperage capacity at this time should be considered.
MPPT
Charge Controllers:
Outback Power's MX60, an MPPT Charge
Controller
Traditionally, you
would assume that the nominal voltage of your battery and your
solar panel array would be the same and that you would also choose that voltage
for your charge controller. However, in recent years, a more efficient charging
technology called Maximum Power Point Tracking (MPPT) has become available on
some models of charge controllers. One of the interesting features of this
technology is that it usually allows you to have a solar panel array with a
much higher voltage than your battery bank's voltage. The MPPT charge
controller will automatically and efficiently convert the higher voltage down
to the lower voltage.
MPPT
Charge Controllers Save You Money on Wiring Costs:
A big advantage to
having a higher voltage solar panel array is
that you can use smaller gauge wiring to the charge controller. And since a
solar panel array can sometimes be over a 100 feet away from the charge
controller, keeping the cost of the wiring down to a minimum is usually an
important financial goal for the whole project. When you double the voltage
(e.g. from 12 to 24 volts), you will decrease the current going through the
wires by half which means you use a quarter as much copper (or cable with half
of the diameter). See our wire sizing seminar for more information.
So, for instance, you
could have a 1000 watt solar panel array that operates at 48 volts DC and your
battery bank is 24 volts DC. MPPT charge controller are rated by the output
amperage that they can handle, not the input current from the solar panel
array. To determine the output current that the charge controller will have to
handle we use the very basic formula for power (watts), which is:
Power = Volts x Amps
Here we know the
power is 1000 watts, the battery bank is 24 volts, so:
1000 watts = 24 volts
x Amps
which gives us:
Amps = 1000 watts/ 24
volts
Amps = 41.7A
We still want to
boost this value by 25% to take into account special conditions that could
occur causing the solar panel array to produce more power than it is normally
rated for (e.g. due to sunlight's reflection off of snow, water,
extraordinarily bright conditions, etc). So, 41.7A increased by 25% is 52.1A.
In this case we'd probably choose a 60 Amp MPPT Charge Controller, like Outback
Power's MX60.
Another
Benefit of MPPT Charge Controllers:
Because MPPT charge
controllers can handle a different (but higher) input voltage from the solar
panel array than the battery bank's voltage, you can also use these charge
controllers with solar panels that have odd voltages that don't match any
typical system voltage (i.e. 12, 24 or 48V). For instance, you could have a solar
panel that has a nominal voltage of 57 volts and charge and battery bank that's
24 volts efficiently with an MPPT charge controller.
Be aware that MPPT
charge controllers have an upper voltage limit that they can handle from the
solar panel array. It's important that you make sure than there is no condition
that the solar panel array voltage will go above this limit or you could
potentially burn out the controller. You want to make sure that the open
circuit voltage of the solar panel array does not go above this limit. You also
want to give yourself a little bit of a margin for an error to take in account
the possibility that a solar panel array's voltage will actually increase the
colder it gets. If you give yourself a 10% margin of error you should be fine.
Here’s an example:
We'll use four 12
volt Evergreen 102 Watt solar panels all run in series for a nominal voltage of
48 volts and our battery bank is at 12 volts. We'd like to use BZ Product's
MPPT500 charge controller. If we look at the panel's specification page we see
that each panel has an open circuit voltage of 21.3V. That means the array has
four times that (because there are 4 panels in series). So the array open
circuit voltage is 21.3V x 4 = 85.2V. We'll boost this up by 10% for safety and
we get 93.7V. Now we'll look at the MPPT500's specifications and we see that it
can take a maximum of 100 volts. So we're ok! |
