Hi.
I'm Amy Beaudet from the altE Store.
Thank you for watching the 4th of our video series on designing an off-grid solar system.
This video will explain how to size the charge controller for your off-grid solar system.
We'll discuss the 2 major types of charge controllers, and figure out how to select
the right size for your system.
Our previous videos helped us determine how much power we needed to make, what size battery
bank was needed to store it, and what solar panels were needed to generate that power.
Now we'll figure out what charge controller is needed to manage that power.
We're getting closer to completing the system, we've figured out the house loads, done the
batteries and panels, now let's do the charge controller.
We've got another video available that goes into great detail of the difference between
a PWM and MPPT charge controller, so I'm not going to repeat much of that.
The short version is, Charge controllers are available with 2 different technologies, PWM
and MPPT.
How they perform in a system are very different from each other.
An MPPT charge controller is more expensive than a PWM, and it is often worth it to pay
the extra money.
PWM charge controllers operate by making a connection directly from the solar array to
the battery bank.
During bulk charging, when there is a continuous connection from the array to the battery bank,
the array output voltage is "pulled down" to the battery voltage.
As the battery charges, the voltage of the battery rises, so the voltage output of the
solar panel rises as well, using more of the solar power as it charges.
As a result, you need to make sure you match the nominal voltage of the solar array with
the voltage of the battery bank.
A 12V panel can charge a 12V battery.
A 24V solar array is needed for a 24V battery bank, and 48V array is needed for 48V bank.
MPPT charge controllers measures the Vmp voltage of the panel, and down-converts the PV voltage
to the battery voltage.
Because power in = power out, when the voltage is dropped to match the battery bank, the
current is raised, so you are using more of the available power from the panel.
So you can use a higher voltage solar array than battery, like the nominal 20V grid-tied
panels that are more readily available.
With a 20V panel, you can charge a 12V battery bank, or 2 in series can charge a 24V, and
3 in series can charge a 48V.
This opens up a whole wide range of panels than now can be used for your off-grid system.
To determine what the specs are for a solar panel, you can look at its datasheet or the
label on the back of the panel.
When sizing a charge controller, the 2 numbers you are most interested in are Open Circuit
Voltage, or Voc, and Short Circuit Current, or Isc.
These are the volts and amps the panel outputs when it is not connected to anything pulling
it down.
You keep hearing me talk about Nominal Voltage.
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.
So let's start with sizing a charge controller with matching panels and batteries.
This can be done with a PWM charge controller.
Our first example is going to be one string of panels.
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 4 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.
So here we go with 4 parallel strings.
4 strings x short circuit current x 1.25 NEC = 43.4A, equals 43.5A, so I'll round up to
a nice Morningstar TriStar 45 charge controller.
Now let's get into the MPPT charge controller.
If you are using the same nominal voltage panels as battery bank, the math is the exact
same as with the PWM charge controller.
An MPPT charge controller can manage the same or higher nominal voltage panels as batteries.
There are some "Boost" charge controllers that can take a lower voltage panel, and Boost
it up to the correct voltage to charge a higher voltage battery bank.
These are generally designed to take a single panel to charge a 36V or 48V golf cart.
However, most MPPT charge controllers need the nominal voltage to be equal to or higher
than the battery bank.
Because watts equals volts x amps, and the watts are constant, if the voltage drops,
the current goes up.
for example, if you have a 200W panel with a voltage of 40V and a current of 5A, if the
voltage drops in half to 20V, the current doubles to 10A.
200W remains the same.
When the voltage of the array is higher than the battery, the charge controller will reduce
the output voltage to charge the batteries, and increase the current output.
A lot of MPPT charge controllers can handle a higher input voltage that nominal 48V.
This allows you to use several solar panels in series to increase the voltage in the line
going from the panels outside to the charge controller and batteries inside.
When you see the max voltage rating of a charge controller, that is the highest Voc voltage
of the string of panels, PLUS, a temperature compensation for the coldest temperature the
panels will be exposed to during sunlight hours.
So if it ever gets to -20 degrees Fahrenheit during a cold winter morning, like it does
here in New England, you need to multiply the Voc of the panel by 1.21 to determine
the highest voltage the panels may potentially put into the charge controller.
So to figure out how many panels you can do in series for a charge controller that is
rated for 150V, you multiply the Voc of the panel by the number of panels in series, by
the temperature correction factor.
If we try putting 4 nominal 20V panels with a Voc of 39.4V in series, when we multiply
x 4, we are already over the 150V limit, even without the temperature correction.
So if we drop down to 3, times 39.4V x 1.21 temperature correction, we are just under
at 143V.
Within the acceptable voltage range.
So let's look at what happens when we put a string of 3 240W panels in series and send
it into a 12V battery bank through our MPPT charge controller.
We have 240W panels times 3 of them gives us 720W.
Next, we take the 720W divided by a 12V battery bank equals 60A out of the charge controller.
OK, so I know I need a charge controller that's rated to handle at least 60A.
Outback makes a fabulous FlexMax 60, I can see if that will work for me.
The next step is to confirm with the specs of the charge controller you want to use that
it can handle the full wattage of the array at that battery voltage I see in its specs
that for a 12V system, it can handle up to 750W.
720W is less than 750W, so we are good to go!
So, let's use the off-grid example we've been using throughout this video series to figure
out what equipment we need.
Looking back through our list of steps, we have everything we need to calculate the charge
controller needed from our previous videos.
We had decided to do a 48V battery bank, and need an array at least 1168W.
First let's do a PWM charge controller.
We know we need at least 1168W of solar.
We also know we need to have the nominal voltage of the array equal the 48V battery bank.
So we will use 2 24V panels in series to equal 48V.
If we use the SolarWorld 315W panels, 1168 divided by 315W = 3.7, which means we need
4 of the panels.
So we'll have 2 parallel strings of 2 in series.
To figure out what size charge controller we need, we take the 2 strings times 9.19A
short circuit current time safety factor of 1.25 equals 22.9A. So we can use a 25A or
30A charge controller, depending on which model we like, for instance the BlueSky Energy
Sun Charger.
Now let's do an MPPT charge controller.
We know we need at least 1168W of solar.
We also know we need to have the nominal voltage of the array equal to or greater than the
48V battery bank.
So, if we wanted to use nominal 20V panels, traditionally used in grid-tied, we would
have to use 3 in series to get at least 48V nominal.
If we use the Kyocera 260W panels, 1168 divided by 260W = 4.5, but we know we have to do strings
of 3, so we have to use a number divisible by 3, so we round up to 6 panels.
So we'll have 2 parallel strings of 3 in series.
So let's look at what happens when we take our 2 parallel strings of 3 Kyocera 260W panels
in series and send it into a 48V battery bank through our MPPT charge controller.
The wattage of each panel is 260W, and we have 6 of them, for a total of 1560W.
Next we take that 1560W divided by a 48V battery bank equals 32.5A out.
OK, I can probably use MorningStar's Trisar 45.
Let's double check with their specs So this is interesting, even though our math says
we need at least 32.5A output, the Morningstar specs are saying that their 30A charge controller
can handle up to 1600W with a 48V battery bank.
That is because MPPT charge controllers are designed to current limit their output.
Even if I am putting enough power in to generate 32A out, the charge controller will limit
it to 30A.
So I have a choice, I can slightly undersize the charge controller and get the TriStar
30, knowing that on occasion it may clip the output, or I can move up to the TriStar 45A
charge controller, which can output up to 45A.
Either one is fine, I just need to decide if I want to pay a little more for a bigger
charge controller that will not limit the output, or if the occasional loss of 2A is
adequate.
With the 45A controller, I also have room for additional panels in the future if I want
to expand.
Being able to handle up to 2400W, I could add a third string of 3 panels in the future,
bringing me up to 9 panels and 2340W.
You may be adding additional loads in the future, so it may benefit you to have some
wiggle room for growth.
That's it for the this.
You can go to our web site to check out our selection of PWM and MPPT charge controllers.
Watch the final video in this series for how to select the inverter, using the numbers
we came up with from our loads list.
Also watch more of our various Video Series on our web site.
We've got a team of highly trained Technical Sales Reps available to help you plan your
system, give us a call.