Hi.
I'm Amy Beaudet from the altE Store.
Thank you for watching our video series.
If you missed our previous videos, I recommend you go back and watch the earlier ones in
the series to get a good foundation.
This video will help you understand the difference between PWM (or Pulse Width Modulation) and
MPPT (or Multi Point Power Tracking) charge controllers, and when to use each.
Before we go further, I want to go over a quick reminder power calculations.
Solar panels are rated by their actual output when connected to a load.
For instance, a Canadian Solar 300W 24V nominal panel has an actual output of 36.1 volts Vmp
and 8.3 amps Imp.
When you multiply them together, you get the rated watts, 299.63, rounded up to 300 Watts.
You can see the specs on the back of the module, or on its datasheet.
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.
If prevents it from overcharging, and many of them control the current and voltage at
which it charges.
More on that in a moment.
At night, the voltage of the battery bank is higher than that of the array that is connected
to it.
Without a charge controller, the tendency would be for the voltage 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.
Charge controllers are available with 2 different technologies, PWM and MPPT.
You often can't tell the difference between a PWM and MPPT charge controller just by looking
at them.
Here you see a Morningstar Tristar-45.
One model is MPPT, and one is PWM.
How can I tell the difference?
By looking at the label.
Both ensure the batteries are being charged at the right voltage based on their state
of charge.
However, how they perform in a system are very different from each other.
An MPPT charge controller is more expensive than a PWM, let's go over why 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 raises, so the voltage output of the
solar panel rises as well, using more of the solar power as it charges.
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 let's see how this affects our system with a 100W 12V panel with a 12V battery bank.
We'll do the math assuming 100% efficiency, which isn't what you'll see in the real world,
but will illustrate the difference between PWM and MPPT quite clearly.
With PWM, when the battery voltage is low, say 11V, when the charge controller connects
the panel's output to the battery, the solar panel's output is pulled from 18V down to
11V.
With a maximum power current of 5.56A, the charge controller's output into the battery
is 11V x 5.56A = 61 Watts.
When the battery is fuller, at 14V, more of the panel's available voltage is used, and
with the same 5.56A, the charge controller is outputting 78W into the battery.
Now let's do the math with an MPPT charge controller.
When the battery is low, it drops the voltage from 18V down to 11V, and that drop is a ratio
of 1.6 (18V / 11V).
So when it drops the volts by 1.6, to keep power constant, it increases the current by
1.6 as well, increasing the current from 5.56 to 8.9A. 11V x 8.9A = 97W, compared to the
PWM 61W.
If we do the same math for a fuller battery at 14V, the in vs out ratio is 1.28, so that
increases the current from 5.56A to 7.1A, times 14V equals 99.4W. Compare the outputs
with the same battery and same panel, just going from PWM to MPPT.
So, you've seen how an MPPT charge controller can maximize the output when the solar panel
nominal voltage matches the battery bank.
Here's another huge advantage of using MPPT.
If you have a solar panel array that has a higher nominal voltage than the battery bank,
a PWM charge controller is just going to throw the extra voltage away.
Let's give an example of a 12V battery bank with 2 different 100W solar panels, one a
12V and one a 24V.
Since watts equals volts times amps, a 100W 12V nominal panel with a Vmp of 18V has an
Imp of 5.56A. 18V x 5.56A = 100W.
Likewise, a 24V nominal 100W panel has twice the voltage, but half the current, 36V x 2.78A
= 100W.
As we saw in the previous slides, a PWM charge controller brings the panel voltage down to
the battery voltage.
So on the 12V panel, the volts get pulled down from 18V down to 13V, 13V times 5.56A
= 72W.
So on the 24V panel it does the same thing, it pulls the voltage from 36V down to 13V,
but remember that the current was half of that on the 12V panel, so with the current
at 2.78A times 13V = 36W.
You have lost most of the power from that 24V panel by throwing away the volts.
Now let's do that with an MPPT charge controller.
The 18V is dropped down to 13V at a ratio of 1.38, increasing the current by 1.38 to
7.7A. 13V x 7.7A = 100W.
The 24V panel's 36V is dropped down to 13V with a ratio of 2.7, so the current is raised
by 2.7 to 7.7A. Hmmm, that looks familiar, that's the same current output from the 12V
panel.
So we are able to use all 100W of the power of the 24V panel in the 12V battery bank.
This is very useful, as higher voltage panels are usually also higher wattage, so you can
get a panel that is 200-300W in a 20V or 24V nominal panel, whereas 12V panels tend to
be well under 200W.
The higher wattage panels are also generally less expensive per watt than the smaller panels,
so depending on your system, it can often be a less expensive solution to go with the
bigger panels and the MPPT charge controller.
So, when should you use a PWM vs an MPPT charge controller?
PWM works great on smaller systems, and where the nominal voltage of the panels match the
voltage of the battery bank.
Remember that wiring panels in series increases the voltage, so if you have two 12V panels
and a 24V system, you can wire them in series to make 24V.
In most cases, with a small system, the cost of increasing the panel size to get more power
is less than the extra cost of going from PWM to MPPT.
Going from a 100W to a 130W panel to get 30% more power will cost less than going from
a PWM to an MPPT charge controller.
An MPPT is worth it if you have a larger system that can benefit from the extra percentage
of power from an MPPT.
For instance, if you have a 2000W array, and you can increase the output by 25%, that's
like adding another 500W of panels.
At a dollar a watt, that's $500 for just the panels, not to mention the extra wires and
racking needed.
In that case, it's probably cheaper to buy an MPPT charge controller to get that increase.
An MPPT charge controller is required when the voltage of the array doesn't match the
voltage of the battery bank.
Check out our website for a great selection of charge controllers available.
Also watch more of our 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.
And please check out the rest of our website, altestore.com, where we are making renewable,
do-able.