Some homeowners elect to install batteries in their grid-connected systems to provide backup power in case grid power goes down, for example, if a tree branch falls on a utility line and shuts down the electrical power system supplying you and 65 million other people. A typical grid-connected system with batteries is shown in Figure 7-5.

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Image caption: Fig. 7-5

In grid-connected systems equipped with battery banks for backup power, the solar array produces electricity that feeds live circuits in the house during the day. Excess electricity is stored in the batteries. When the batteries are full, excess electricity is diverted onto the electrical grid. At night, power required for household use is typically delivered by the grid. The batteries are called into action only when grid power fails. (Batteries can also be charged by grid power when voltage runs low.)

A grid-connected system with battery backup includes all of the components of a grid-connected system, including disconnects for safety. Grid-connected solar electric systems with battery backup, however, also require an additional component, one we haven’t discussed yet. That is a charge controller, as shown in the schematic in Figure 7-5.

Charge controllers are electrical devices that monitor battery voltage and then utilize the information to protect the batteries. The charge controller is typically housed in a metal box that is mounted on the wall near the inverter next to the DC safety disconnect.

The charge controller protects batteries against overcharging and is also known as a high voltage disconnector. When the device detects high battery voltage, indicating batteries are full and in danger of overcharging, the charge controller interrupts the flow of electricity from the PV (Photovoltaic) array into the batteries, protecting them from overcharging. Overcharging can damage the metal plates in batteries, reducing battery life.
Grid-connected systems with battery backup also contain meters to monitor electrical production and consumption and battery voltage. Battery voltage provides a measure of the state of charge of a battery bank — that is, how much electricity batteries are holding at any one time.

Checking Your Batteries

Most sophisticated solar electric homeowners monitor battery performance with an ampere-hour meter. Besides supplying information on battery voltage, this device keeps track of how many amp-hours of electricity are stored in the battery bank at any one time. They also typically monitor how many amps are going into and how many are being drawn from the batteries at any given point in time.

Learn more about Powering Your Home with Solar Electricity .