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GB Industrial Battery Capacity Ratings and Conversions:

AH (Ampere-Hour) Ratings

6 Hour vs. 20 Hour Ratings:

Rating a battery over a longer time period always means a lower current draw over a longer period of time. The lower the continuous amperage during discharge, the more efficient the battery becomes. The opposite is true when the continuous current draw is increased.

          Forklift applications usually use the 6hr rating.

          Off grid power and specialized power applications usually use the 20hr rating.

          You cannot directly compare a battery rated at 6hrs to a battery rated at 20hrs. Always use the same rate when making comparisons.



          Convert a 6hr rating to 20hr by multiplying: AH @ 6hrs x 1.5764705 = AH @ 20hrs.

          Use the GB Capacity Calculator to convert 6hr through 24hr rates: *Battery Capacity Calculator.



The AH rating divided by the number of hours of the rating = continuous amps (current draw) for that number of hours, to a maximum of 80% depth of discharge.


80% Depth of Discharge (DOD): A Fully Charged battery, discharged to 1.75 volts per cell.

          Example: A 600AH battery (rated @ 6hrs) will provide a full 600AH over 6hrs and still have 20% reserve capacity that is never used.

          You are not limited to 80% of the 600AH (480AH) as one might assume.

          Discharging a battery below 80% will drive the voltage below 1.70 volts per cell, which will overheat and damage the battery and any equipment it is powering.

          Sealed batteries usually have a 50% depth of discharge, and will be damaged by an 80% discharge.

          All GB batteries have an 80% depth of discharge, but can be used in 50% (or less) applications.


Rate Comparison

6hr rate: A fully charged battery discharged 80% over a six-hour period.


6 hour AH rating divided by six hours = continuous amps available over six hours


20hr rate: A fully charged battery discharged 80% over a twenty hour period.


20 hour AH rating divided by twenty hours = continuous amps available over twenty hours

AH Comparison

EXAMPLE: 6hr vs. 20hr - Battery Model 6-100-13


600AH @ 6hrs: 600AH divided by 6hrs = 100 amps continuous current for 6 hours.


The same battery rated at 20 hours

945AH @ 20 hours: 945AH divided by 20hrs = 47.25 amps continuous for 20 hours.

Definition: Ampere-hour or AH:

          A measurement of electrical capacity the amount of energy the battery will store. Current multiplied by time in hours equals ampere-hours. A current of 50 amps for one hour would be 50 AH at the 1hr rate; a current of 30 amps for 5 hours would be 150 AH at the 5hr rate.


          AH ratings will vary with temperature, and with the rate of discharge. For example, a battery rated at 100 AH at the 6-hour rate would be rated at about 135 AH at the 48-hour rate.


Ampere-hours (AH) designate the storage capacity of deep cycle batteries. SLI batteries are not rated in AH, but in "CCA", or cold-cranking amps (marine batteries are often rated in "marine cranking amps"). "6 hour rate" and "20 hour rate" indicate that the battery is discharged steadily over 6 or 20 hours, and the Amp-hour capacity is measured by amperage over time, until the cut-off voltage (usually 1.75 volts per cell) is reached.

Copyright GB Industrial Battery USA. All rights reserved.

Last Updated: Monday, August 18, 2008 - 10:11 P.M. Eastern Time.


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