How long does it take to recharge a fully discharged Discover VRLA DRY CELL AGM or GEL 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 40% of the total charging time to put the last 10-20% of the charge back into the battery.

The 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.

How can you tell if a battery is fully charged?

The only true way to tell if a VRLA DRY CELL AGM or GEL battery is fully charged is by using a good voltmeter to determine the open circuit voltage (OCV) without any load applied to the battery. Accessible flooded-type batteries can also use a hydrometer.

Table 5 - State of Charge vs. OCV Charge

Charge % Open Circuit Voltage Flooded Battery Open Circuit Voltage GEL Battery Open Circuit Voltage AGM Battery
100% >12.60 >12.85 >12.80
75% >12.40 >12.65 >12.60
50% >12.20 >12.35 >12.30
25% >12.00 >12.00 >12.00
0% <11.80 <11.80 <11.80

Divide the above values in half for 6-volt batteries or by six to determine cell voltage. The TRUE OCV can ONLY be measured after the battery has been removed from the charge or discharge load for 24 hours.

How can undercharge harm my Discover battery?

In many respects, undercharging is as harmful as overcharging. Keeping a battery undercharged or continually undercharging allows the positive grids to corrode, lead sulphate to build up, and plates to shed, which can dramatically shorten life. Also, an undercharged battery must work harder than a fully charged battery, which also contributes to shortened life. A constantly undercharged battery has a significantly reduced capacity because of the effects of acid stratification. It will readily be inadvertently over-discharged and eventually damaged.

How can you tell if a battery has been damaged by under or overcharging?

The only way is with a load test. Use the same procedure for VRLA DRY CELL batteries that you would use with a flooded cell battery:

  1. Recharge if the open circuit voltage is below 75%.
  2. If adjustable, set the load at three times the 20-hour rate.
  3. Apply the load for 15 seconds. The voltage should stabilize above 9.6 volts while on load.
  4. If the battery has a CCA rating, you can apply a load equal to ½ the rating for 15 seconds. The voltage should stabilize above 9.6 volts while on load. To apply a more determined test, you may apply a load equal to 100% of the rated CCA for 30 seconds or a load of 5 to 7 times the AH rating for 30 seconds. The voltage should stabilize above 7.2 volts while on load.
  5. If below 9.6 volts (7.2 volts for the 100% CCA test), recharge and repeat the test. If below 9.6 volts (7.2 volts for the 100% CCA test) a second time, replace the battery.