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8
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13. 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.
VOLTAGE
READING
SPECIFIC
GRAVITY
PERCENT
OF CHARGE
PERCENT OF
CHARGE NEEDED
12.65
1.265
100%
0%
12.45
1.225
75%
25%
12.06
1.155
25%
75%
11.89 or less
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 Ca 16
2
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).
2.
Multiply the Amp Hour Rating by the charge needed (.50) and divide by the charge
rate setting.
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 x 1.25 = hours of charge
Charge Rate Setting
100 (Ah Rating) x 0.50 (charge needed) x 1.25 = 1.56 hours
40 (Charge Rate Setting)
100 x 0.50 x 1.25 = 1.56
40
You need to charge your 100-Ampere Hour Battery for a little less than 2 hours at the 40
Amp charge rate, using the above example.
