Only charge GEL batteries using a confirmed and reliable, temperature sensing, voltage regulated charger. Never use a constant current charger!
- At 25°C/77°F keep charging current in the range of 2.38 volts to 2.42 volts per cell for GEL cell batteries.
- At 25°C/77°F keep charging voltage in the range of 2.4 volts to 2.465 volts per cell for AGM batteries
They are usually rated by their voltage and there start rates or the rate in amperes that the charger will supply at the beginning of the charge cycle. When selecting a charger, the charge rate should be between 10% and 30% of the battery’s 20-hour AH capacity.
For example: a battery with a 20-hour capacity rating of 100 AH should use a charger rated between approximately 10 and 30 amps (for multiple battery charging use the AH rating of the entire bank to determine the charger rating required.
The “Ideal Charge Curve”
Bulk phase: This is where the heavy lifting takes place. Charge at a rate up to 20 percent to 40 percent of the batteries capacity in amp hours to a voltage of about 14.6 volts (gel: 14.1 volts). For example, a 200 amp-hour battery would be charged at 40–80 amperes. This will bring the battery to about 75 percent of full charge, and is efficient (more amp-hours replaced per hour of charge time) since the battery accepts more current when it is discharged. AGMs require slightly different voltages, and unless there is an AGM setting, should be charged using lead-acid settings.
Acceptance phase: Maintains battery at 14.6 volts (Gel: 14.1 volts) while the amperage is steadily reduced. This will restores the next 25 percent of capacity at a declining rate. Your battery can be considered fully charged if it will accept current equal to 2 percent of C at 14.6 volts (a 200 amp-hour battery will only accept four amps).
Float phase: When the battery’s acceptance declines to two to four percent of C, the voltage is reduced to 13.4 volts (Gel: 13.8 volts) to maintain the battery without losing electrolyte from the cells. This is a maintenance phase, not a charging phase.
Equalization: This stage is used to prevent flooded lead acid batteries from aging prematurely, and is an optional, frequently omitted phase. After the battery reaches the end of the acceptance phase, the battery continues to be charged at four percent of C until the voltage stops rising, usually around 15.5 to 16.2 volts. This forces the battery to its highest possible state of charge, boiling the electrolyte in a controlled manner and dissolving the lead sulfate crystals that have collected on the battery’s plates.
In industrial applications where maximum energy storage is important, this phase is done every charge cycle. In the marine environment, it is more likely to be done every 20–50 cycles to extend the life and capacity of wet batteries. Gel and AGM batteries should not be equalized. Since electrical equipment and light bulbs can be damaged by high voltage, the battery should be disconnected from all loads during equalization.
This type of battery charging, consisting of multiple stages, is not possible with automotive-type alternator regulators, unregulated solar panels, ferroresonant chargers, or taffrail generators. We strongly encourage the use of efficient charge devices, both for shorepower charging and alternator regulation, that use modern multiple-step regulation.
How to rapidly kill a battery
Undercharging: Consistently failing to fully recharge batteries leaves them with lead sulfate that hardens on their plates—they become sulfated—and gradually lose their ability to perform. Increased resistance when charging causes falsely elevated voltage readings, essentially fooling the battery charger, leading to further undercharging, in a downward spiral. Beyond a certain point, a sulfated battery cannot be returned to a healthy state, and you need a replacement. Keep your batteries charged, and equalize your wet cell batteries every six to eight weeks in temperate climates, and more frequently in the tropics.
Overcharging: Especially fatal to Gel and AGM batteries, consistent overcharging (NOT equalization) boils the electrolyte out of the cells, and can even lead to thermal runaway, with the battery becoming hotter and hotter. One of our writers experienced thermal runaway on his liveaboard Catalina 30, caused by a ferroresonant “dumb” charger, with nearly catastrophic results.
Excessive deep discharge: Don’t completely discharge a deep cycle battery if it can be avoided. The deeper the discharge the less life you will get from the battery. The ideal method is to charge and discharge the batteries through the middle range (50 percent to 85 percent) of their capacity and, if they are flooded batteries, to equalize them periodically. Leaving the battery in a fully-discharged state, for example during winter storage, causes it to become sulfated.