BATTERIES, INVERTERS & CONTROLLERS 

System Voltage 

A small system with a short transmission distance is usually designed to operate at 12 volts. Larger systems can also be 12 volts, but if higher power is desired or the transmission distance is long, then a system of 24 volts or higher may be preferable. This is especially true if all loads are inverter-powered. In a 12-volt system, operating at a low power level, it may be advantageous to operate all loads directly from batteries. Many 12-volt appliances and small inverters are available. 

In 24-volt systems, it may also be preferable to operate the loads directly (although not as many appliances are available). In higher power systems, it is usually better to use an inverter to convert battery voltage to regular domestic AC power. This has been made feasible with the advent of reliable high power inverters. Thousands of home power systems are in operation with only AC loads. 

Sizing Battery Capacity

A typical hydro system should have about two days of battery storage capacity. This will generally keep lead-acid cells operating in the upper end of their charge range where they are the most efficient and long-lived. Alkaline batteries like the nickel-iron and the nickel-cadmium types can have a lower capacity since they can be more fully discharged without harm. 

Batteries should be located outside of living space, or adequate ventilation should be provided, as a rising charge level tends to produce both hydrogen gas and corrosive fumes. Also, distilled water should be added as needed to maintain the electrolyte level. 

Charge Control 

A hydro system requires that a load be present so that the power has somewhere to go. Otherwise, system voltage can rise to very high levels. This situation provides an opportunity to do something with the excess power (i.e., a diversion load used for water heating). 

As the batteries become fully charged, their voltage rises. 
At some point, the charging process should stop and the power be diverted to the dump load. The voltage set point should be about 13.5 to 14.5vdc for a 12-volt system depending on the charge rate. The higher the charge rate, the higher the voltage can go. If batteries are often in a high state of charge, the voltage limit should be on the low end of the range. 

A voltmeter or a watt-hour meter can be used to monitor battery charge level. Battery voltage is roughly a function of the charge level, and varies according to the load level and charge rate. There are many commercially available monitors that conveniently display these features to the user, including the state of charge.