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 of 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 120 VAC 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 middle 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 andcorrosive fumes. Also, the water consumption increases; distilled water should be used to maintain the water 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 modest dump load can be 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 (there is a fair bit of guesswork involved here). The voltage set-point should be about 13.5 to 14.5 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 state of charge, the voltage limit should be on the low end of the range.

Some examples of good charge controllers are the TRACE C-30, C-40 and the ENERMAXER. Both switch power to a dump load when their set point is reached. The C-30 has "on" and "oft" settings and uses a relay to switch the load either fully on or fully off. The ENERMAXER has one set point and uses solid state switches to dump the power gradually at the one voltage. Dump loads are usually resistive, such as heaters, but can be anything that is compatible with the system.

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. As you gain experience, the battery voltage can be used to assess the charge level more accurately.