With all but the smallest of solar panels, you’ll need a charge controller. A charge controller goes between the solar panels and the batteries. It’s job is to limit, or control, the power your panels put into your batteries. This keeps you from destroying expensive batteries by overcharging. There are two types charge controllers.
Power Width Modulation (PWM) controllers are relatively inexpensive. They work by literally switching the panels on and off very rapidly. That works fine as long as your panel voltage is fairly close to your battery voltage. If you’re a weekend boater who just wants a small panel to keep your battery topped up while you’re away, a PWM controller might do the job. Using a PWM controller will limit the size panels you can use because they don’t have the capability to change the voltage, only turn it on and off.
Multi Power Point Tracking (MPPT) charge controllers are a big step up in performance and price. These controllers actually adjust voltage/current in an attempt to wring the most power out of your panels. They also have the capability to step the panel voltage down to whatever your batteries need. An MPPT controller takes the 60 volts coming in from our panels and steps it down to our battery charging voltage (around 13.5). When the voltage comes down the amps go up, so very little power is lost in the controller.
Beware of cheap “MPPT” found on Ebay and the like. Some are actually PWM controllers with “MPPT” printed on them. Since the charge controller is the heart of your system, and could prove dangerous to you and your batteries, it’s best to stick with a quality unit from a reputable manufacturer.
A standard system with one MPPT controller is striving to optimize all panels, meaning that during partial shading some panels are running higher and some lower than optimum. No panels are really performing their best because the controller is working with an average. This is where MPPT optimizers on each panel can be useful. In an environment where shading is unavoidable, this allows each panel to work independently for optimum results. Obviously no two systems and shade scenarios are identical, but tests with optimizers show that during partial shading power output can be increased by 15-25%. In an environment where partial shading can’t be avoided, and space is limited this can be important. That sounds a lot like a sailboat doesn’t it?