Basic Operation Principles for Lead Acid Batteries
By Steve Bell, Technical Support Specialist
Batteries are one of the most important and most poorly understood components in an off-grid system. Because batteries are now one of the most expensive system components, it is wise (prudent) to maximize the effective service life of the battery bank. There are multiple factors that directly affect the service life of any lead-acid battery, such as proper charging, average Depth of Discharge (DOD), and battery bank configuration and wiring.
GEL, AGM (Absorbed Glass Mat), and Flooded batteries are all lead-acid batteries. The primary difference is how the electrolyte is handled. Each type of battery has its own optimal charging characteristics. Always consult the battery manufacturer for their charging recommendations.
Proper charging of the battery bank is critical to good battery performance and service life. Lead-acid batteries need to be charged to the recommended Absorption voltage and held at that voltage until the battery is 100% fully recharged. When the battery first reaches the Absorption voltage, the battery is only about 85% to 90% recharged. The Absorption charge needs to be continued until the battery is fully recharged. This is called the Absorption Time. Depending on output of the charging source, the size and age of the battery bank, and amount of load being powered, the Absorption Time can be quite long. Once the battery is fully recharged the charging voltage is reduced to a somewhat lower voltage, called the Float voltage. If the full Absorption cycle is not completed, the percentage of the battery that remains uncharged builds up lead sulfate crystals on the battery plates. This is called Sulfation and it both reduces the effective amp-hour capacity of the battery and decreases the battery’s charge efficiency. The longer the lead sulfate remains on the battery plates, the more difficult it becomes to remove it. Sulfation has a cumulative effect and over time battery capacity is permanently lost. The single most significant cause for premature battery failure is a buildup of Sulfation caused by chronic under-charging of the battery bank. Ideally the battery bank should receive a 100% full recharge at least every 7-10 days, more often is better.
Proper charging also requires adjusting the Absorption and Float voltages based on the actual battery temperature. This is called Temperature Compensation. The ‘standard’ charge voltages are based on a battery temperature of 25C (77F). As a battery gets colder the charge voltages need to be increased, and as the battery gets hotter, the charge voltages need to be decreased. Using a Remote Temperature Sensor (RTS) connected to the battery bank will provide the most accurate Temperature Compensation.
How deeply a battery is discharged, called Depth of Discharge (DOD), has a direct effect on how long the battery will last. A battery has a finite number of charge/discharge cycles before the battery is worn out. This is known as the Cycle-Life of the battery. As the battery’s average DOD becomes greater (deeper), the total number of available charge/discharge cycles decreases. Many deep-cycle batteries can be discharged up to about 80% DOD but limiting the average DOD to about 50% often provides the best balance of battery performance vs battery life.
The physical configuration of the battery bank and its wiring can have a significant effect on battery performance and life. All the individual batteries comprising a battery bank need to be of the same size/capacity, the same age (time in service) within 1 year max, and ideally the same brand and model. The battery wiring needs to be configured so that the total cable length going to each battery within the battery bank is the same. All these factors are to ensure the total resistance is the same going to each battery in the battery bank. If there is a significant difference in the resistance between individual batteries in the battery bank, the lower resistance batteries will charge more quickly and the higher resistance batteries will charge more slowly. Because the charge controller is charging to the average voltage of the entire battery bank, the higher resistance battery circuits will be chronically undercharged, and the lower resistance battery circuits will be chronically overcharged. This will cause premature aging of the entire battery bank.