Maintaining the capacity and cycle life of your deep cycle battery bank
By Jeff Myles
When properly designed and maintained, a deep cycle lead-acid battery bank offers an economical, reliable and long-lasting energy storage solution for off-grid and grid-connected back-up systems. This proven battery technology is widely chosen for use in motive power, marine and industrial equipment requiring deep cycle capability, and provides renewable energy customers with a selection of voltage and capacity options to suit any residential or small-scale commercial or micro-grid application.
A battery-based system is only effective if the required energy is there when you need it. Experienced PV installers and off-grid homeowners understand that the key to success and longevity of deep cycle batteries is proper system design, sizing, installation and performing necessary ongoing maintenance.
Maintaining a balanced, healthy battery bank
Following the proper guidelines and manufacturer's recommended installation, charging and maintenance instructions will protect your investment and provide many years of battery life. When improperly charged or maintained, a battery bank will lose its capacity, gradually or potentially with abrupt cell failures, which requires replacement. For off-grid customers, capacity loss is not only an inconvenience, but may also result in a power outage as they await service or replacement.
No battery-based system is maintenance-free. Routine inspection and maintenance help to identify potential issues early on, troubleshoot and make system adjustments to correct and prevent premature battery failure.
The batteries should be inspected upon arrival and placed on an initial refresher charge as it is normal for batteries to self-discharge in transit or storage. The electrolyte level in each flooded cell should be inspected to ensure the plates are fully covered before charging. It is normal for electrolyte levels to drop in new batteries shortly after manufacturing as the sides of the case relax or bulge slightly.
Watering is an essential maintenance requirement with all flooded deep cycle batteries. Preventive maintenance involves, at a minimum, checking the electrolyte levels in each cell every few months. If the electrolyte levels are very low before the battery is put into service, consult the battery dealer or manufacturer for assistance. Only distilled water should be used when filling cells to prevent contamination and cell damage. The cells should be watered back to the original acid level (1/4" - 1/2" below the bottom of the vent well) as required. Watering frequency will vary based on ambient and charging temperatures, depth of discharge, cycling frequency and humidity.
At full charge, the specific gravity readings of each flooded cell should be periodically tested using a hydrometer or refractometer and tracked for the life of the battery. Good record keeping is helpful in determining the health of the battery bank and can prove invaluable in identifying changes if system problems develop. Cleaning and re-torqueing the battery terminals are also good practice. Follow the recommended cleaning and torque guidelines provided by the manufacturer as these may vary by design.
Testing specific gravity is one of the easiest and most effective ways to determine the health, state-of-charge and if a battery has become sulfated.
A deep cycle lead-acid battery will naturally sulfate during discharge. The accumulation of sulfation on the plates is reversed during charging, granted the charge voltage and current is adequate. By ensuring the system is designed to meet the recommended charge rate, sulfation should not accumulate, and battery capacity is maintained. Sulfation build-up is the most common cause of capacity loss and premature battery failure.
A noticeable decrease in battery capacity, voltage drops under load and lowered specific gravity readings are often indicative of sulfation build-up. Often, the specific gravity readings for each cell will be consistently lower than normal or may vary from one battery to the next. Sulfation hardens on the plate and becomes more difficult to remove over time. Eventually, if not addressed, this causes significant capacity loss, cell or battery failure.
What causes sulfation build-up? The most common cause of sulfation is insufficient charge time or current. Increasing the size of the PV array to generate more power is often the best solution. Sizing the charge source to support the battery bank will ensure they're reaching full charge efficiently and as often as possible.
- Sulfation may accumulate when batteries are stored for extended periods without cycling or without a float charge to maintain capacity.
- Improper charge settings and undercharging. Rolls recommends a 3-phase charge cycle (Bulk, Absorption & Float) and a charge rate equal to 10% of the C20 (20 hr AH rating) of the battery bank for flooded models.
- Deficit cycling or operating at partial state-of-charge. A battery bank should be brought to full charge as often as possible to prevent sulfation buildup and capacity loss.
Jeff Myles is Marketing Manager at Rolls Battery Engineering (rollsbattery.com)