If you ever decide to buy solar panels for your boat you will also need a charge controller. In fact, choosing the right charge controller is as important as buying the right solar panels. Cheap charge controllers just close the connection between the batteries and the solar panels until a desired voltage is achieved. As a result, directly connecting the batteries to the solar panels will lower the voltage down to the battery voltage, greatly reducing the power output of the solar panels.
Another (slightly more expensive) type of controller is a PWM (Pulse-Width Modulation) controller. By applying short bursts of higher voltages, this type of charger has the effect of "cleaning" off unwanted build-up on the lead plates in the batteries, extending their life. However, this unit does not optimize the performance of the solar panels.
The most expensive but by far the best type of charge controller is a MPPT controller (Maximum Power Point Tracking) which can not only increase the charging current by 30% or more, but also incorporates the PWM charging systems to optimize battery life and protects it from overcharging. As its name suggests, the MPPT applies an appropriate resistance to obtain the maximum power output on the current/voltage curve for the given environmental condition. It is important to choose the right size controller for the solar panels you have.
A rule of thumb in sizing a MPPT controller is taking the total system short circuit current produced by your solar panel system and multiplying it by a safety margin of 1.5 to select the current rating of your MPPT controller. There are two ways to find the total short circuit current of your system if you have an array of more than one panel. If your panels are wired in parallel, your total system short circuit current is the sum of the short circuit current of each panel. If your panels are wired in series, the system short circuit current will be equal to the lowest short circuit current rated panel. ?