Batteries Need Charge Control
Every battery-based renewable energy system requires a device called a charge controller. Charge controllers use various strategies, but they all have the same primary purpose: to keep the battery from overcharging and becoming toasted.
Most solar charge controllers simply open or restrict the circuit between the battery and the array when the voltage rises to a set point. Then, as the battery absorbs the excess electrons and voltage begins to drop, the controller will turn back on. Newer solid-state controllers use power transistors and PWM (Pulse Width Modulation) technology to turn the circuit rapidly on and off, effectively floating the battery at a set voltage. The latest generation of PV controls employs an electronic trick called Maximum Power Point Tracking, or MPPT. This allows the controller to run the solar array at whatever voltage delivers the highest wattage. This is often at a higher voltage level than the batteries would tolerate and is well above their nominal rating. The excess voltage is converted to amperage that the batteries can digest happily before it leaves the controller. On average, an MPPT controller will deliver about 15% more energy per year than a standard controller, and they do their best work in winter, when most off-grid homes need all the charging help they can get.
Additional features of some charge controllers are timing switches and low voltage disconnect circuitry. The former is perfect for systems where a light is needed from dusk until dawn. The latter isolates the battery from further discharging below a minimum voltage set-point. Low voltage disconnects are common whenever a non-critical load can be switched off to preserve battery health. In systems greater than 20 amps, this function is usually managed by the inverter.