Lead-acid batteries temporarily lose approximately 20 percent of their effective capacity when their temperature falls to 30°F (-1°C). This is compared to their rated capacity at a standard temperature of 77°F (25°C). At higher temperatures, their rate of permanent degradation increases. So it is desirable to protect batteries from temperature extremes. Where low temperatures cannot be avoided, buy a larger battery bank to compensate for their reduced capacity in the winter. Avoid direct radiant heat sources that will cause some cells to get warmer than others. The 77°F temperature standard is not sacred, it is simply the standard for the measurement of capacity. An ideal range is between 50 and 85°F (10–29°C).
Arrange batteries so they all stay at the same temperature. If they are against an exterior wall, insulate the wall and leave room for air to circulate. Leave air gaps of about 1/2 inch (13 mm) between batteries, so those in the middle don’t get warmer than the others. The enclosure should keep the batteries clean and dry, but a minimum of ventilation is required by the National Electrical Code, Article 480.9(A).
A battery enclosure must allow easy access for maintenance, especially for flooded batteries. Do not install any switches, breakers, or other spark-producing devices in the enclosure. They may ignite an explosion.
When installing new charge controllers or inverters in your system, make sure to program the appropriate charge setpoints for your specific battery type. Battery-based PV systems will usually have a solar charge controller and an AC battery charger, for use with an engine generator or the grid. The AC charger will typically be built into your inverter. Voltage settings appropriate for your type of battery must be programmed into these devices. If incorrect charge setpoints are chosen, sealed batteries can be overcharged and lose their internal moisture. Flooded batteries will be deprived of a full finish charge and will deteriorate if charge setpoints are too low.
When batteries are cold, they require an increase in the maximum charge voltage to reach full charge. When they are warm, they require a reduction in the voltage limit to prevent overcharge. Choose a charge controller and inverter/charger for your system that includes temperature compensation. To use it, you must have a temperature sensor located at the batteries. You may need a temperature sensor for each charging device (including the inverter), but networked systems communicate the temperature from a single sensor to all charging components. Some small charge controllers have temperature sensing built in. In that case, be sure the controller is located where its temperature is similar to that of the batteries. Otherwise, it will be “fooled” into setting improper charge limits.
Battery management is sometimes called a “black art.” That’s true only if the user (or supplier) is in the dark. Have you ever driven a car without a fuel gauge? It can be frustrating! Yet, many battery systems don’t have an equivalent device to show the state of charge (SOC), the level of stored energy.
Metering is not just bells and whistles. It provides crucial information for battery management, which in turn significantly increases battery longevity. Use a digital monitor, like the TriMetric (Bogart Engineering), IPNProRemote (Blue Sky Energy), or XBM battery monitor (Xantrex). These devices keep track of accumulated amp-hours and display the charge status of the battery bank. They also display other data that can be useful for maintenance and troubleshooting.
Install your monitoring device where it can be seen easily—in a central place in your home. Be sure the device is programmed properly, based on the parameters of your system. This needs to be done just once, during meter installation.