Words of wisdom: “Any Solar is better than no solar, but having too much solar is difficult. The best solar systems are those which can keep the batteries the happiest. Happy batteries are those which are recharged quickly after depletion.”¹⁾ - SternWake

Counterintuitively, the job of a solar charge controller (CC) is to keep the batteries from being overcharged. Solar panels run at higher voltages than batteries, often at voltages high enough to damage the batteries. The CC provides the correct amount of power the battery needs at the correct time; this is called smart or three stage charging.

The two main types of CCs are Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT). As is the case with inverters, one costs much less than the other but may not work as well in some cases. It is the subject of Holy Wars, particularly concerning small PV systems. The RVer will have to be comfortable with their own decision.

The PWM charger runs the solar panels at the voltage needed to charge the battery at a given time. This is unlikely to be at the panel's rated output but that may not be a practical concern. An ideal scenario for PWM might be: low voltage panels (nominal 12v) in parallel, hot ambient conditions, and shallow Depth of Discharge, a surplus of sunlight, lots of room for extra panels, limited funds.

The MPPT charger runs the panel at panel's maximum power output under present conditions then downconverts the voltage to match charging needs. Ohm's Law requires the amperage increase when the voltage decreases, so the MPPT charger can provide more amps to the battery than PWM if the charging stage warrants. Since the bulk phase is the only phase that uses heavy amperage the increase may only be noticeable during that phase. Additionally, around 2% of the max power is consumed during downconverting. Overall, an MPPT CC will generally harvest 10-30% more power than PWM. An ideal scenario for MPPT might be: high voltage panels in series, cold ambient conditions, heavy battery use, limited or poor quality sunlight, limited room for panels, no limit on funds.