Tech Tuesday: Solar Panel Solar Charge Controllers Explained

Posted June 09, 2020

In this video, we explain two different types of charge controllers to pair with your solar array and connect to RELiON Batteries.

What is a solar charge controller? It protects the battery during charging and also prevents the battery from sending current into the solar panels overnight. For more information you can check out: Tech Tuesday Videos


I’m Simon and welcome to RELiON’s Tech Tuesday. Lithium batteries and solar panels make a great power source for an off-grid home or RV, but they need one more component to work: a solar charge controller.

The charge controller is the gatekeeper between a solar array and a battery: it protects the battery during charging and also prevents the battery from sending current into the solar panels overnight, when the battery’s voltage is higher than the solar array’s.

There are two kinds of charge controllers, and both can connect your solar array to a RELiON battery.

The first is a pulse width modulation controller. A PWM is like an on/off switch that connects and disconnects the solar array from the battery.

Based on the battery’s voltage, the charge controller allows bursts of current to pass from the solar array to the battery. The lower the voltage in the battery, the longer the bursts. The higher the voltage, the shorter the bursts. This is what we mean by pulse width modulation - the amplitude of the pulse remains the same, but the duration varies.

The other common type of solar charger is a maximum power point tracking controller. The MPPT monitors the voltage on both sides: the solar array as well as the battery. When the voltage of the array is higher than the voltage of the battery, the MPPT steps down the input voltage. This topology allows the controller to efficiently operate at a higher solar array voltage. . Higher array voltage means lower current while delivering the same amount of power. This translates to smaller and less expensive wiring and switch gear between the solar array and battery bank.

An MPPT controller is always calculating and delivering the greatest amount of power it can safely send between the solar array and the battery: that’s the maximum power point. This maximimum power point will vary depending on sun angle, changes in ambient cell temperature and cloud cover. As a result, an MPPT charge controller operates more efficiently and lets you harvest more energy over time. This can translate into faster and more efficient battery charging over time. An MPPT controller sees the most benefit when operating during colder times of the year but still provides the advantage of higher voltage array wiring regardless of temperature.

Pulse width modulation controllers are less expensive and are appropriate when you have a relatively small, simple set-up like an RV or boat. If you’re somewhere with a lot of sunlight and you don’t draw too much from your battery, a pulse width modulator could be the right choice.

If you have a larger or more complex system with greater physical distance between the battery bank and array, or you are using your system in a location with variable weather, a maximum power point tracking controller is a better option.

Before buying a charge controller, take the time to sketch out how much power you can generate from your solar panels, what sort of activities you will be powering and the conditions where you’ll need this power. That will help guide you to the right purchase. And if you’re still not sure, contact us and we’ll be happy to work with you.

Thanks for joining us. Let us know what tech topics are on your mind, and we’ll see you next time.



  • Gary Hart

    We have your 75Ah LiF3PO4 battery hooked to a 20' RV trailer charged with two 60W Zamp solar panels with wired in controller. We've discovered the hard way on a very cold night that the PWM cycle hit max voltage settings for the BMS, thus nulling out programming as if it was turned off...a safety feature? As a result, the charging never really achieves 100% while the newly wired gauge (with shunt) shows the PWM cycle from 11 to 15V swing. We were not able to utilise the small furnace/fan to heat the trailer (24F that night) due to uncharged battery. I believe a better charge controller is in order and prefer any guidance you may have to replace the one wired into the Zamp controller.