Nowadays, battery-based inverters offer more than just a DC-to-AC power
conversion—they can charge batteries and select from multiple power sources.
A battery charger in a battery-based inverter greatly enhances the inverter’s versatility and utility. First, it provides an alternate means for charging the battery bank from a power source other than the sun, which is useful in times of short days, poor weather, and high loads. It also increases complexity, weight, and cost, but the additional application flexibility usually outweighs these negatives.
Connecting an inverter/charger to an AC power source opens up a whole new realm of application flexibility. An inverter can similarly use energy from the grid for backup, or be connected to the grid and a backup generator via a transfer switch, which may be internal or external. This capability can meet a wide range of useful applications and benefits.
One method of doing this is by “low battery transfer”—if the battery voltage falls below a predetermined set point for a prescribed period of time, the inverter automatically transfers loads to an AC source and turns on the battery charger. When the battery voltage exceeds the “high” set point for a prescribed time period, the inverter transfers the loads from the grid back to the battery bank. Other methods border on “smart grid” status. For example, a “grid use” feature can be set to instruct the inverter when to connect to the grid. Typically, this setting coordinates with time-of-use (TOU) billing, connecting loads to the grid when rates are low to operate AC loads and perhaps help recharge the batteries. When TOU rates are high, the inverter disconnects from the grid, and the RE power source and the batteries power AC loads.