>>MPPT vs. PWM Charge Controllers – What’s the Difference?

MPPT vs. PWM Charge Controllers – What’s the Difference?

MidNite Classic 150

Before solar panels became interesting for people that are on the grid, there were only 36-cell ones for charging 12 Volt batteries (and if you had a 24V battery you’d need two panels in series, to make 72 cells). With 36 cells the panel runs at about 18 Volt, and when it gets warm in summer, the Voltage will drop but still be enough to charge a 12 Volt battery (which really takes about 15 Volt).

Charge controllers were developed for these 36-cell panels, to prevent overcharging and damage to the battery. These charge controller are essentially just switches that rapidly (many times per second) connect the panels to the batteries, and then disconnect them. By changing how long the panels are connected vs. disconnected the effective charge current changes, and that’s how the controller keeps the battery Voltage in check. These controllers are called Pulse Width Modulation controllers, or PWM controllers for short, because that’s how they do the charging.

Then solar became interesting for people that are on the grid, and the standard there is for 60-cell panels. Almost all the panels you see on rooftops are 60-cell. These panels have an open Voltage of about 38 Volt, and run at about 30 Volt (though on a hot day in the sun they’ll run as low as 24 Volt). You can hook them up to a 12 Volt battery with a PWM controller, but since those controllers directly connect the panel to the battery (they’re just a switch), it forces the panel to run at 14 or 15 Volt, half of what it can do, and power output will be half as well. So by using a PWM controller with a 60-cell panel you will get about 100 – 130 Watt out of a 260 Watt panel.

To make 60-cell panels work with batteries a different type of charge controller was developed, called Maximum Power Point Tracking or MPPT charge controllers. Those run the panel at whatever Voltage it takes to make the most power, and then convert that Voltage down to the battery Voltage. So, with a 60-cell 260 Watt panel and an MPPT controller the panel could be running at 30 Volt – 8 Amp, while charging the battery at 14 Volt and 17 Amp!

To charge a 24 Volt battery takes at least 2 x 36 = 72 solar cells. That means you need two of the 60-cell panels in series (120 cells), and an MPPT charge controller. Likewise, a 48 Volt battery bank needs 3 of the 60-cell panels in series to reach a Voltage that’s high enough to consistently charge the batteries.

Now, MPPT type charge controllers have much more electronics inside and are much more complex than PWM type charge controllers. That means a 40 Amp MPPT controller is quite a bit more expensive than a 40 Amp PWM controller. But 60-cell panels are MUCH cheaper per Watt vs. 36-cell panels, and that more than makes up for the price difference in charge controllers.

You’ll find a variety of PWM and MPPT type charge controllers on our Web site.

Hopefully this helped clear up the mysteries around charge controllers!

Rob Beckers

By | 2017-08-03T10:22:35+00:00 November 16th, 2016|Rob's Rumblings|2 Comments

About the Author:

Rob Beckers is the founder and president of Solacity Inc. He is passionate about renewable energy, and wants to leave the world just a little greener than he found it.

2 Comments

  1. Daniel Matew April 24, 2018 at 10:46 am - Reply

    I was hoping you would continue the presentation with controllers that would transfer the maximum power for both situations when the input voltage is also lower then the output voltage. So there is no need to add another panel for a 24V battery – just select another output voltage on the controller. And I bet the schematics are known since such controllers are used for recovering break energy on EVs and hybrid cars. The breaking generator may deliver a large span of voltages (and powers) to be supplied to a relatively constant voltage battery.

  2. Rob Beckers April 25, 2018 at 9:32 am - Reply

    Daniel, it’s indeed possible with electronics to make what’s called a “boost converter” where the output Voltage is higher than the input Voltage. Unfortunately I don’t know any brand/type of MPPT charge controller that uses this. They are all fairly simple “buck converters”, that can only lower the Voltage from input to output.

    So, for all intends and purposes, all the MPPT controllers on the market need an input Voltage from the panels that is higher than the battery Voltage.

    -Rob-

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