27/06/2017
MPPT vs. PWM Charge Controllers – What’s the Difference?
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.
