A specific time is difficult to determine because recharging depends on so many variables:
* Depth of discharge
* Size and efficiently of the charger
* Age and condition of the battery
It will take about 60% of the total charge time to bring a VRLA battery from 0% charged to 80-90% charged. It will take the remaining 40% of the total charging time to put the last 10-20% of the charge back into the battery. Charge is a quantity of electricity equal to rate of flow (Amperes) multiplied by time (hours), and is usually expressed in Ampere-hours (Ah). The charge rate in amps — once the charger has been turned on for 1 to 2 minutes — will indicate the approximate charge time in hours.
A battery with a 0% state of charge is defined as having been discharged to a point when the terminal voltage is equal to or less than 1.75 volts per cell (10.50 Volts for a 12 volt battery)— measured under a steady load at the batteries 20-hour rate at 80°F. (The 20-hour rate is the batteries capacity divided by 20 hours). Typically, the charge (capacity of the re-charge) that must be returned to a VRLA battery to achieve a 100% state of charge is from 110% to 120% of the charge removed. For comparison purposes the returned charge for flooded electrolyte batteries needs to be between 110% and 130% of the charge removed.
NOTE that variables such as the rate of charging current, ambient temperature during charge cycle and the control of the voltage during the charging cycle will impact the ability of the battery to be properly replenished and the ongoing performance of the battery.
IMPORTANT: Always use an automatic temperature-sensing, voltage-regulated charger! Set charger at 14.2 to 14.5 volts at 25°C/ 77°F for gel, or 14.4 to 14.7 volts at 25°C/ 77°F for AGM. Do not exceed 14.5 volts for gel or 14.7 volts for AGM.
In many respects, undercharging is as harmful as overcharging. Keeping a battery in an undercharged condition allows the positive grids to corrode and the plates to shed, dramatically shortening life. Also, an undercharged battery must work harder than a fully charged battery, which contributes to shortened life as well. An undercharged battery has a greatly reduced capacity. It may easily be inadvertently over-discharged and eventually damaged.