Choice of solar charge controller, MPPT versus PWM
Lately I have written about my own experience installing a solar system on our ship. Each of us has different reasons to consider installing a solar system, from simply wanting to keep our starter battery charged between visits, to our reason, to live aboard boaters who want to reduce energy use on land. From our point of view, the greatest benefit is obtained when we disconnect from the network. Free electrical support for our leisure batteries.
The purpose of this article is to help the reader choose the correct solar controller for their own use. For your own peace of mind, work on the basis that each solar installation requires a controller. Batteries are expensive items and do not deserve to be damaged by overcharging. A low-end charge controller is inexpensive and will ensure that your battery is not overcharged, and just as important, it will ensure that there is no flyback voltage on the solar panels when the panel is not producing power.
The two most common charge controllers are PWM (pulse width modulator) and MPPT (full power point tracking). Each one has its own characteristics. Let’s take a look at both.
Cheap PWM charge controllers are actually quite smart, although not particularly efficient. They are simply a high speed electronic switch. They operate by turning on and off extremely fast and the switching speed controls the level of voltage delivered to the battery. If you simply want to take care of a starter battery while away from the boat or RV, then a small 15 or 20 watt panel connected via a 10 amp PWM charge controller will do exactly what you want. That same panel, without the controller, could easily damage your battery.
However, MPPT solar controllers are very smart and extremely efficient. They are also more expensive than PWM. However, I would like to put this in context. Due to the size of our ship, we are limited to just four leisure batteries. These are Trojan T105 deep cycle batteries and it would currently cost around £ 500 to replace them. Our Victron MPPT 75/15 solar charge controller costs less than a single battery. I have total faith in him and know that I can trust him to squeeze every last electron from the panels without damaging the batteries.
The reader should always remember that 12 volt solar panels will produce much more than 12 volts even in moderately bright climates. We have three solar panels on the roof of the boat, and since MPPT really likes higher voltages, we have wired them in series. The thing is, these ‘so-called’ 12 volt panels produce up to 22 volts each! Wiring the panels in series means that we are feeding up to 66 (yes, sixty-six) volts into our charge controller. You may have noticed the ’75 / 15 ‘in the Victron controller name. It means this controller is happy to get up to 75 volts from the panels and will happily draw up to 15 amps from the controller to the batteries! Briefly, the controller converts the panel voltage from DC to AC. It is then converted back to DC at the voltage required by the battery. Any excess voltage turns into extra amps for the batteries!
This type of controller is a full-fledged smart charger in its own right.