Forney 52726 - Calculating Charge Time; Hydrometer or Electronic Method; Calculating Charge Time Formula; Chart Method for Charge Time

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CALCULATING CHARGE TIME
The Hydrometer or Electronic Method
To find the time needed to fully charge your battery, determine the battery’s charge level with a hydrometer or electronic
Percent-of-Charge Tester. The following table will help you convert hydrometer readings to percent of charge values.
SPECIFIC GRAVITY PERCENT OF CHARGE PERCENT OF CHARGE NEEDED
1.265 100% 0%
1.225 75% 25%
1.155 25% 75%
1.120 0% 100%
When you know the percent of charge and the Amp Hour (AH) rating of your battery, you can calculate the
approximate time needed to bring your battery to a full charge.
To convert Reserve Capacity to Amp Hours, divide Reserve Capacity by 2, and add 16:
Example:
Amp Hour Rating = Reserve Capacity/2 + 16
NOTE: The Reserve Capacity can be obtained from the battery specification sheet or the owner’s manual.
To calculate the time needed for a charge:
1. Find the percent of charge needed. (A battery at 50 percent charge that will be charged to 100 percent needs
another 50 percent (.50)).
2. Multiply the Amp Hour Rating by the charge needed (.50) and divide by the charge rate.
3. Multiply the results by 1.25 and you will have the total time needed, in hours, to bring the battery to full charge.
4. Add an additional hour for a deep-cycle battery.
Example:
(Amp Hour Rating x % of charge needed)/Charger Setting x 1.25 = hours of charge
(100 (Ah Rating) x .50 (charge needed))/20 (Charger Setting) x 1.25 = 3.125 hours
(100 x .50)/20 x 1.25 = 3.125
You would need to charge your 100-Ampere Hour Battery for a little more than 3 hours at the 20-Amp charge
rate using the above example.
The Chart Method
Use the following table to more accurately determine the time it will take to bring a battery to full charge. First,
identify where your battery fits into the chart.
NR means that the charger setting is NOT RECOMMENDED.
Find your battery’s rating on the chart below, and note the charge time given for each charger setting. The times
given are for batteries with a 50% charge prior to recharging. Add more time for severely discharged batteries.