How long does it take to recharge a fully discharged GELL or AGM (VRLA DRY CELL) battery?

A specific time is difficult to determine because recharging depends on so many variables:

  • Depth of discharge
  • Temperature
  • Size and efficiency of the charger
  • Age and condition of the battery

The initial charging current with a Discover battery should be 15% to 30% of the battery's C20-hour capacity rating.

It will take about 60% of the total charge time to bring a VRLA DRY CELL AGM or GEL battery from 0% SOC to 95% SOC. 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 the rate of flow (Amperes) multiplied by time (hours) and is usually expressed in Ampere-hours (Ah). Once the charger has been turned on for 1 to 2 minutes, the charge rate in amps 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 battery's 20-hour rate at 80°F. The 20-hour rate is the battery's capacity divided by 20 hours.

Typically, the total charge (capacity of the recharge) that must be returned to a VRLA DRY CELL AGM or GEL battery to achieve a 100% state of charge is from 104% to 112% of the charge removed. For comparison purposes, the returned charge for flooded electrolyte batteries must be between 115% and 130% of the charge removed.

NOTE: Variables such as the rate of charging current, ambient temperature during the 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.