When installing new charge controllers or inverters in your system, make sure to program the appropriate charge set points 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 set points 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
set points 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.
lack of Monitoring Devices
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), IPNPro Remote (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.
The surest way to ruin batteries within a year or two is to keep them at a low state of charge (SoC) for weeks at a time. Active battery material will crystallize, covering the plates, which will become permanently inert. We call this "sulfation." Ideally, batteries should receive a 100 percent full charge about once a week for good longevity, and more frequently is better. If this takes a full day of backup charging with a generator, do it! Use your monitoring system to know when full soc is reached. If you don't have an amp-hour meter, watch for the voltage to reach maximum and the charge current to drop to a low level. This means the batteries are unable to accept much more energy, and are accepting only a "finish" charge.
In winter, some people run their backup generator for an hour a day just enough to prevent the system from shutting down. Bad idea! It may be better to run it for ten hours, once a week, or whatever it takes to fully charge the batteries, instead of partially charging them more frequently.
Finish charging a battery bank with an engine generator is an inefficient use of fuel, and results in extremely long generator run times. As a result, generators are typically shut down once the absorption charging stage is finished. But at this point in the charging process, the battery bank will only be at about 85 percent SOC. Since regular, full battery charging is important for battery longevity, make sure that your RE sources are topping off the battery bank after the generator has done the bulk of the charging. Relying on your PV system to provide the finish charge may be difficult during winter months. Another option is to set the inverter-charger to equalizing mode (see below) during generator charging about once a month to ensure that the battery bank is getting fully recharged.
The extreme of under charging is called "over discharging." Voltage should never, and I mean never, be drawn below about 11 V (for a 12 V system), or 22 V (24 V system), etc. System controls and inverters usually include a "low voltage disconnect" (LVD) function. If you have DC loads connected directly to the batteries without LVD, you are asking for trouble. It's better to lose power than to squeeze out another watt-hour and damage your batteries. Metering is vital here, because if you wait for the inverter to shut down or the lights to go dim, it's already too late, batteries will likely have lost a portion of their capacity and life expectancy.
Finally, flooded batteries need to be equalized at least four times a year. Exactly how often depends on several factors, including the size of the battery bank in relation, to your charging sources and the average depth of discharge during cycling. During normal battery discharging/charging, the individual cells of each battery will stray from a common and consistent cell voltage. Equalization can be thought of as a controlled overcharge of the battery bank that serves to both equalize cell voltage, and provide an aggressive and necessary mixing of the battery electrolyte. Equalization charging can be done with your PV system if your array is large enough, or with an engine generator or the grid. Most PV charge controllers and inverter-chargers have battery equalization functions.
Frequently asked questions and answers about batteries. www.batteryfaq.org
Love Your Batteries!
The lesson: Accept professional advice and service.
Lead-acid batteries are an old but durable technology. They are about 80 percent efficient at releasing stored energy, few high-tech storage systems come close to that efficiency and they rarely fail suddenly. With good management, you'll know when to replace them before they let you down. And even then, they are fully recyclable. Give your batteries what they need, and your batteries will do the same for you.
Batteries can produce thousands of amps if a direct short occurs. Be very careful when working with metal tools around battery terminals. If you do not feel competent to install or maintain your battery bank, do not hesitate to hire an experienced professional.
Gassing (bubbling) of hydrogen and oxygen is a normal occurrence, especially during final, or heavy, charging. This gas is potentially explosive, so keep sparks or flames away from batteries.
Electrolyte in flooded lead-acid batteries is an acid solution. It will burn eyes and skin, and eat holes in clothing. When working around batteries, wear goggles, gloves, and old clothes. Keep baking soda at hand to neutralize acid spills, but never allow any of this alkaline solution to get into the battery. This will diminish the strength of the acid and reduce the battery's capacity.
If you remove more energy from your battery bank than you put in, your batteries will suffer. It's not the batteries' fault, yet this is the most frequent cause of complaints about batteries" not holding a charge."
Here is one common scenario: A well-meaning appliance seller or mechanical contractor sells you a device that uses "very little electricity." Ha! They don't know about the initial expense of solar electricity. For example, about 3$ U.S. will buy you about 40 KWH per month of grid electricity. But adding more PV and battery storage to meet this load could mean an investment of several thousand dollars! Or, without upgrading your system, this would require frequent generator backup (especially in winter). The same also happens when a resident decides it's trivial to leave a coffee maker or large TV on all day. Even low power loads will add up if they're running 24/7. When people don't accept this reality, they overdraw their energy account, and often blame the batteries.
Exceeding Your Energy Budget
Lead Acid - Battery Maintenance
Renewable Energy Systems
#7 Lack of Proper Charge Control
#9 Improper Charging