In battery-based systems, a power source besides PV, wind, or microhydro is usually needed to provide battery charging during times of cloudy weather, no wind, or low stream flow. Engine generators are commonly used in off-grid systems; the utility (and sometimes generators) is often used in grid-tied systems.
Both are AC power sources, but the batteries are DC, so AC to DC conversion is needed. Battery-based inverters commonly include integrated battery chargers that convert AC to DC and control the charge flowing into battery bank.
Inverter/chargers eliminate the need for an external battery charger, reducing system cost and complexity. Just like with other multistage charge controllers, users can program bulk and float voltages and absorption time to accommodate their particular battery bank. Additionally, users can adjust the maximum charge rate flowing into their battery bank, which is crucial since the maximum rate is different for various battery banks.