Charge controllers are used to regulate the voltage coming from the solar panels to avoid overcharging the battery bank. The two most popular types of charge controllers are the Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).
PWM basically works by sending charging pulses to the battery. Depending on the voltage of the battery, the controller will send pulses of varying length. For example: if the battery bank is fully charged, then the controller will send few very narrow pulses. If the battery is discharged, the pulse frequency is higher with wider pulses.
For PWM controllers, the nominal battery bank voltage should match the nominal PV array voltage. The higher the nominal PV voltage compared to the batteries, the less efficient the solar system will be.
An MPPT charge controllers is a DC to DC converter. This basically means that they convert from a higher PV voltage to the lower battery voltage level in an optimal way. The controller adjusts the power from the solar panels to charge the batteries at the highest current (Amps) possible. Being a true DC to DC converter it can accept higher voltage panels to charge lower voltage batteries.
The table below summarizes the two types of controllers
| PWM | MPPT | |
| Efficiency | Depends on temperature, battery state of charge and other factors. | Depends on temperature, battery state of charge and other factors but on average expect 10% to 20% more power |
| PV Voltage | Is limited by the battery bank voltage | Can accept higher voltage panels that do not match the battery bank voltage |
| Cost | Cheaper than MPPT | Typically 2x to 3x more expensive compared to PWM |
Most common PWM and MPPT controllers use a three-Stage Battery Charge mode:
Bulk Stage: At this stage batteries are charged at “bulk” voltage, where bulk refers to the maximum allowed voltage for charging.
Absorption Stage: The battery voltage is held at bulk voltage (usually for 1 hour). Current gradually declines as the battery capacity is reached.
Float Stage: batteries voltage is held at float, which is a lower voltage setting.
Taken from Xantrex C-Series Multifunction DC Controller Owner’s Manual