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APPENDIX A — CHARGING TIMES
To calculate the approximate charging time required to fully charge a battery, it is necessary to determine the
specific gravity (or, percent of battery charge) using a hydrometer. Use this technique if battery vent caps can
be removed. Check each cell. If there is one cell with a very low specific gravity compared to the other cells,
there is probably a shorted cell in the battery. Replace the battery.
The following chart converts hydrometer readings into percent of charge values.
SPECIFIC GRAVITY PERCENT OF CHARGE PERCENT OF CHARGE NEEDED
(HYDROMETER READING) IN BATTERY BY BATTERY
1.265 100% 0%
1.225 75% 25%
1.190 50% 50%
1.155 25% 75%
1.120 0% 100%
Refer to the chart below for approximate charging times.
PERCENT OF CHARGE 75% 50% 25% 0%
2 AMPS 6.5 HRS 12 HRS NR*** NR***
10 AMPS 1.8 HRS 3.0 HRS 4.5 HRS 6 HRS
20 AMPS 1 HRS 1.5 HRS 3 HRS 3.5 HRS
40 AMPS* NR** NR** NR** NR**
* For Charging Batteries 80 to 100 AH
** Not Recommended for more than 1 hour
*** Not Recommended because of the 18 hour charge time
The times shown in the chart above are approximate and refer to an average automotive battery. For smaller
batteries, the charge time should be adjusted using the formula shown below and adding 1 hour to the time
calculated.
To estimate charging time for a discharged battery, divide the AH rating of the battery by the charge rate selected.
This is the number of hours required to recharge the battery. For example, a 50 AH (12 volt) battery is
discharged (10 volts). How long should it be charged at the 20 Amp rate. Divide the 50 AH by 20.
Answer: is approximately 2.5 hours. Always round up the charge time by 10% to ensure full charge. In most
cases, battery recharge times will vary depending on the age and condition of the battery. Smaller batteries
should be charged at a lower rate (2 Amps) and add an extra hour to charge time.
VEC1093 A
REV101103
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