Hi. I’m Amy from the altE Store. I’m going to go over the basics of solar charge controllers
with you. First let’s take a look where a solar charge
controller fits into an off-grid solar system. The charge controller goes between the solar
panel and the deep cycle battery. A Charge controller is an important component
in battery based systems. They are not used in straight grid-tied systems, as they do
not have batteries to charge. Their primary role is to manage charging the
battery bank. It prevents it from overcharging, and many of them control the rate of the current
and voltage at which it charges. More on that in a moment.
Some charge controllers have load control, where you connect the DC load right to the
charge controller instead of to the battery, and it will turn it on and off based on voltage
of battery and/or time of day, for example turning the load off if the battery gets too
low or turning on a light from dusk to dawn. At night, the voltage of the battery bank
is higher than that of the solar array that is connected to it. Since electricity flows
from high voltage to low, without a charge controller, the tendency would be for the
electricity to flow out of the battery bank. A charge controller prevents that from happening,
allowing the flow to only go one way, into the batteries.
Many charge controllers manage charging the batteries by varying the voltage and current
to the battery bank based on how full the battery is. Much like pouring a glass of water,
when the glass is fairly empty, you can have the faucet on full blast, but when it starts
to get full, you want to turn down the flow to prevent overflowing. Likewise, a charge
controller sends a lot of power to the battery when it is low, but as it approaches full,
it slows it down. Once it is full, it will send a smaller amount of power, a trickle
charge, to keep it topped off. This is called multi-stage charging. Here’s
an example from Morningstar of 4 stage charging. With Bulk Charging, when the battery is low,
it will accept all the current provided by the solar array.
At Absorption, the battery has reached the regulation voltage, the controller begins
to hold the voltage constant. This is to avoid over-heating and over-gassing the battery.
The current will taper down to safe levels as the battery becomes more fully charged.
Equalization is done with flooded batteries, not sealed batteries like AGM and Gel. Many
batteries benefit from a periodic high voltage boost charge to stir the electrolyte, level
the cell voltages, and complete the chemical reactions. Your battery specs will tell you
how often and at what rates it wants to be equalized.
Float charge is when the battery is fully recharged, the charging voltage is reduced
to prevent further heating or gassing of the battery.
There is a wide variety of features that are optional on some, but not all controllers.
In most cases a display does not automatically come with the controller, but can be added
separately for a remote display. A few even have Ethernet connections, allowing you to
monitor your system across the web. Temperature compensation will improve the
battery bank charging, by adjusting its output based on the temperature.
Low Voltage Disconnect is a great feature that allows you to connect your DC load to
the charge controller. If the battery voltage gets low, it will turn off the load, preventing
the batteries from becoming too low and getting damaged.
Some controllers can be used as a diversion, or dump, load controller, turning power on
to a heater to burn off excess power. There are others that have light control functions,
turning lights on and off automatically based on dusk and dawn.
You’re going to hear me talking about Nominal Voltage, Voc, and Vmp. Voc is Open Circuit
Voltage, or what you will measure from a solar panel in perfect test conditions with nothing
but a volt meter connected to it. Vmp is voltage at Max power, or what the solar panel will
put out when it is connected to equipment like a charge controller or inverter. Nominal
voltage is a way to categorize battery based solar equipment. Because a higher voltage
is required to charge a battery, nominal voltages are used to help see what equipment goes with
what. So a nominal 12V panel, which actually has a Voc voltage of around, 22V, plus or
minus a volt or 2, and a Vmp of around 17V. And if you count the number of cells, or silicon
squares on the front, it will likely have 36 cells.
Likewise, a panel that was designed to charge a 24V battery bank will have a Voc of around
44V and a Vmp of around 36V. Counting the cells will come up with 72, twice as many
as a 12V panel. If you wire 2 24V panels in series, or 4 12V panels in series, you can
charge a 48V battery bank. This was all well and good for battery based
systems, but then along came grid-tied systems, and 12, 24, and 48V became meaningless. So
the industry sort of standardized on 60 cell, 20V nominal panels. Alone, they are too big
to charge a 12V battery, and too small to charge a 24V battery. An MPPT charge controller
solved that, by reducing the voltage down to the required range, and in doing so, increasing
the current output, so you are not losing power.
There are 3 main types of charge controllers. Shunt controllers, that just turn the flow
to the batteries on or off are rarely used anymore, so we won’t go into them.
The 2 main types you’ll find these days are PWM and MPPT. Let’s discuss them in
greater detail. PWM are generally the less expensive option
of the two. A PWM charge controller pulses the power sent to the battery bank, allowing
it to do the different charging stages we discussed.
When using a PWM charge controller, the nominal voltage of the solar panel must be the same
nominal voltage as the battery bank. So if you are using a 12V battery, you must
use a 12V solar panel. If you have a 24V battery bank, you must wire two 12V panels in series,
or one 24V panels to make 24V. If you have a 48V battery bank, you must wire four 12V
panels in series, or two 24V panels in series, to make 48V. Make sure the charge controller
you select is designed for the battery bank voltage. Some can support multiple voltage
ranges, others are designed only for 1 voltage. Note if a PWM charge controller says it can
support 12 or 24V, both the panel and battery bank must be one or the other. It is NOT saying
it can take a 24V panel to charge a 12V battery. It is saying it can work in EITHER a 12V or
a 24V system. Selecting the right charge controller for
a PWM system is pretty simple. For a single string, we check the label or datasheet and
confirm with the Voc of 22.1V that it is a nominal 12V panel, and the Isc is 8.68A . We
then multiply the Isc by the number of parallel strings, 1, and multiply it by NEC’s safety
factor of 1.25, to get 10.85A minimum charge controller amperage requirement.
Great, I’ll round up to a nice 15A Morningstar ProStar 15M with a meter.
Now let’s try it with 2 parallel strings of the same 140W panel. Notice I’m not talking
about how many panels are in each string, because I’m using a PWM charge controller,
I know that I’m using the right number to match the voltage of my battery bank, so I’ve
got 1 for a 12V, 2 in series for a 24V, and 4 in series for a 48V battery bank. In this
example, I have 2 parallel strings of 2 in series for a 24V system.
8.68A short circuit current x 2 strings x 1.25 NEC required protection, equals 21.7A,
so I’ll round up to the Midnite Solar 30A Brat charge controller.
Now we move on to MPPT charge controllers. A Maximum Power Point Tracking, or MPPT, is
the more sophisticated, more expensive type of charge controller.
It tracks the output of the solar array and adjusts itself so that the output is always
maximized. In doing so, it can increase the production of the array by up to 30%.
Another great advantage is that most MPPT charge controllers can take a higher voltage
array, for instance a 60 volt array, to charge a lower voltage battery bank, like a 48V.
This is required if you have a 60 cell, 20V grid tied solar panel, that are common, and
thus less expensive, and use it to charge a 12V battery. It’s also very useful if
you have to go a distance from your array to your battery bank. The higher the voltage
of the solar array, the lower the current going across the wire. Therefore, you can
use smaller gauge wire, which will cost less, and have a lower voltage drop, which gets
more of your power to the batteries. There are also a few MPPT charge controller
that can take a lower voltage panel and charge a higher voltage battery bank. These are great
to use a 12V panel to charge a 36V golf cart. But MOST MPPTs require a higher or equal to
voltage panel. Be sure to read the specs carefully. To see how an MPPT charge controller works,
let’s look at a system with one 60 cell PV panel and one 12V battery. The charge controller
takes the 30 volts from the solar panel and converts it down to around 14V to charge the
battery. Unlike a PWM charge controller, it doesn’t just throw away the extra voltage,
it increases the current on the output to maximize the power out. So if it is taking
30V in and sending 14V out, that is a decrease of 2.14. Since it is taking 9A in, it will
INCREASE that by the 2.14, and output 19.28A out. So power in equals power out.
So the simplest way to size an MPPT charge controller is to take the total watts of the
array and divide it by the voltage of the battery bank. So 270W panels times 4 panels
divided by 24V battery bank times 1.25 for NEC equals 56A charge controller. Cool, I’ll
use an Outback Power FlexMax 60. In addition to amps, Solar Charge controllers
are also rated by voltage. A typical 150V charge controller can support
up to three 20V panels in series. You may be saying, but three 20V panels in series
equals 60V, that’s waaay below 150V. But the voltage the specs are referring to are
the Voc voltage, the actual voltage the panel puts out. That is much higher than the nominal
20V or the Vmp. The Voc of a 20V panel is actually around 38V, so three in series is
114V. Also note that cold weather increases the voltage output of a solar panel. So if
we also figure in the cold temperature in the winter, we increase the volts. At -5 degrees
Fahrenheit, it adds 20% to the voltage, bringing us up to 136Voc. So you can see why at least
in cold climates, three 20V nominal panels would be the max for a 150V charge controller.
There are now higher voltage charge controllers available, with some accepting as much as
600V in. This is very useful if the array is a long distance away from the battery bank.
So again, check the specs to find the right charge controller for you.
That’s it for a quick summary of solar charge controllers.
Check out our website for a great selection of solar charge controllers and all of the
other components needed for a solar power system.
Also watch more of our Videos on our web site to learn more.
We’ve got a team of highly trained Technical Sales Reps available to help you plan your
system, give us a call.
And don't forget to check out the rest of our website at altEstore.com where we're making renewable doable!