Charge Controllers: MPPT or PWM?
Power & Water
For Everyone
When choosing a charge controller, you will inevitably come across the terms PWM and MPPT. But what do they mean, and why does it matter?
What Does A Charge Controller Do?
While there are several excellent articles elsewhere that provide explicit detail on exactly what is going on inside each of these controller types, most people are only interested in the benefits and drawbacks of each type, and for them, it's only necessary to understand that MPPT charge controllers provide more power from a panel than PWM controllers by using the panel more efficiently within its programmed limitations (you can set those parameters on the CC itself).
The charge controller takes power from the solar panel and conforms it to a safe and reliable charging frequency for your chosen battery type. A decent controller should be capable of charging all the usual battery types and should have an input for an external temperature sensor, as well. That's because your battery's charging efficiency at any given temperature depends on the current it receives as well as how much charge it already has. At different temperatures, those values change. Knowing that it's a hot day helps your charge controller to modify the amount of power your specific battery type will receive at its current state of charge, optimizing charging time and getting your battery back up from a discharged state sooner, which can prolong its life significantly.
Charge controllers can also look after batteries, keeping them topped up, actively preventing overcharging, and tracking the power coming into and out of them during use. There are other features, as well, such as short circuit protection, high temperature cut-off, etc., but almost all charge controllers have these, regardless of type.
Price Matters, Right?
When shopping for a charge controller, you might be tempted by the cost of a PWM unit, as they can be picked up for pennies. But you get what you pay for - why are they so cheap?
In a nutshell, MPPT is a more modern technology that constantly measures the energy being drawn from the battery and adjusts the input from the solar panel accordingly. This helps your battery to charge more efficiently. This constant monitoring is referred to as Multi Point Power Tracking (MPPT).
PWM controllers are cheaper than MPPT for a good reason. It's an older charging method that draws power from the panel at just above the current voltage of the battery, regardless of power drain from your devices and regardless of the current charge state. This is inefficient at best, and may shorten your battery life by charging it improperly. While you may pick up a PWM charge controller cheaply enough, batteries are much more expensive to replace, so this is not a good trade.
For further explanations and comparisons of both types, I highly recommend Will Prowse's "DIY Solar Power" channel on Youtube, and specifically, this video: