Regular battery maintenance for energy-storage systems used with renewable-energy (RE) applications is critical to for maximum system performance. Batteries in RE systems should be engineered for deep-cycle use, meaning the battery’s design is optimized for the deep discharge and recharge cycles characteristic of RE systems, especially in support of the variable power production from wind and solar equipment.
Deep-cycle batteries fall into two groups: flooded lead acid (FLA) and sealed valve-regulated lead acid (VRLA), such as Absorbed Glass Matt (AGM) and gel. To achieve maximum life, each battery type requires particular regular care.
When deep-cycle flooded batteries charge, they produce and vent hydrogen gas. This “off gassing” of hydrogen reduces the battery’s electrolyte level, which requires periodic “watering” of the battery (with distilled water) to ensure maximum life. It is important to add distilled water only when the batteries are fully charged in float mode, which is when the charge current and voltage are reduced to maintain a full battery. Although variations exist between manufacturers, electrolyte levels should never be allowed to drop so low as to expose the battery plates to air.
While monthly watering is typically recommended, the ideal frequency depends on how the batteries are used and the operating temperatures of the battery bank. Bearing this in mind, careful monitoring of new installations is key (say, once a week to start) to determine the correct frequency of watering required. As flooded batteries age, their gassing rate will increase requiring more frequent watering. However, regardless of the amount of watering a battery bank receives, it is important to do so on a regular schedule.
Equalization is also an important part of maintaining the longevity of FLA batteries used in RE systems. The process for equalizing FLA batteries involves periodically overcharging the batteries at a higher voltage for a set period of time to reduce the effects of electrolyte stratification (the accumulation of sulfuric acid at the bottom of a battery cell), sulfation (the formation of deposits in the active battery material), and other battery cell inconsistencies that develop over time.
Stratification and sulfation typically occurs when a battery is deprived of a full charge. If left unchecked, these conditions eventually diminish the overall efficiency and performance of the batteries.
Although equalization is an important maintenance procedure for FLA batteries, the same is not true for VRLAs. VRLA batteries should never be equalized because they are incapable of properly venting excess hydrogen produced in the process.
VRLA batteries are typically referred to as maintenance-free because of the minimal amount of maintenance needed to function reliably. Because VRLA batteries are sealed, adding distilled water is unnecessary. However, it is important to check VRLAs regularly for terminal corrosion. In addition, such batteries should be cleaned to remove dirt or dust that accumulates on top of their surface.
State of charge
Evaluating the state of charge (SOC) of a FLA or VRLA battery bank is a necessary maintenance procedure. To conduct an SOC check, simply take an open-circuit voltage reading of the battery bank using a voltmeter. For FLA batteries, it is possible to use a hydrometer to take specific gravity (SG) readings of individual cells. Both voltage and SG readings should be taken under a “no load” battery condition to ensure accuracy.
It is wise to refer to the battery manufacturer’s user’s guide to determine proper equalization and SOC methods, and measurements required for each specific battery configuration.
With proper care and maintenance, deep-cycle batteries are more likely to achieve the manufacturer’s optimum design life, extending the battery investment and keeping the total cost of ownership to a minimum.
Putting safety first
Effective battery maintenance for peak system performance and longevity means committing to a regular maintenance schedule. With FLA or VRLA deep-cycle batteries, it is important to adhere to similar safety and maintenance tips.
- Always wear protective clothing, gloves, and goggles when working with batteries. The electrolyte in a flooded battery is a solution of acid and water, so take extra precaution to avoid contact with skin and clothing.
- Check that all cable connections to the terminals are properly tightened — connections that are too tight or loose may result in post breakage, meltdown, or fire.
- To safeguard against short-circuits, use only insulated tools when maintaining batteries.
- Avoid placing tools or objects on top of the batteries.
- Charge batteries in a well-ventilated area.
- Clean battery tops and terminals with a solution of baking soda and water, and dry thoroughly.
- Apply a thin coat of petroleum jelly or terminal protector spray to terminals.
- Never add acid to a battery.