A charge controller is a key component of battery‑based PV systems. A charge controller performs several functions, the most important of which is preventing batteries from overcharging.
How Does a Charge Controller Prevent Overcharging?
To prevent batteries from overcharging, a controller monitors batteryvoltage at all times. When the voltage reaches a certain pre‑determinedlevel, known as the voltage regulation (VR) set point, the controller either slows down or terminates the flow of electricity into the battery bank (the charging current), depending on the design. In some systems, the charge controller sends surplus electricity to a diversion load (Figure 1).
This is an auxiliary load, that is, a load that's not critical to the function of the home or business. It is often a heating element placed inside a water heater or wall‑mounted resistive heater that provides space heat.
In PV systems, excess power is often available during the summer months during periods of high insolation. In these instances, the diversion load may consist of an irrigation pump or a fan to help exhaust hot air from a building.
Diversion loads must be carefully sized according to the National Electrical Code, something an installer will be sure to do.
Why Is Overcharge Protection So Important?
Overcharge protection is important for flooded lead‑acid batteries and sealed batteries. Without a charge controller, the current from a PV array flows into a battery in direct proportion to irradiance, the amount of sunlight striking it. Although there's nothing wrong with that, problems arise when the battery reaches full charge.
Without a charge controller, excessive amounts of current could flow into the battery, causing battery voltage to climb to extremely high levels. High voltage over an extended period causes severe out gassing, water loss, and loss of electrolyte that can expose the lead plates to air, damaging them. It can also result in internal heating and can cause the lead plates to corrode. This, in turn, will decrease the cell capacity of the battery and cause it to die prematurely.
Fig.1: Diversionary charge controllers send surplus electricity to a dump load, either a resistive heater or fan or pumps, as explained in the text.
Charge controllers protect batteries from high voltage, but also often incorporate overdischarge protection, that is, circuitry that prevents the batteries from deep discharging. When the weather's cold, overdischarge protection also protects batteries from freezing. This feature is known as a low‑voltage disconnect.
Charge controllers prevent overdischarge by disconnecting loads ‑ active circuits in a home or business. Overdischarge protection is activated when a battery bank reaches a certain preset voltage or state of charge but only protects against deep discharge caused by DC circuits. This feature prevents the batteries from discharging any further. Overdischarge not only protects batteries, it can protect loads, some of which may not function properly, or may not function at all at lower than normal voltages.