VEC1090 A
REV101103
11
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. A cell with a very low specific gravity compared to the other cells probably means 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 HRS
4.5 HRS
6 HRS
20 AMPS
1.0 HRS
1.5 HRS
3.0 HRS
3.5 HRS
*** Not recommended for over 18 hours of charging 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 amp hour (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:
approximately 2.5 hours. Always round up the charge time by 10% to ensure full charge. In most cases, battery
charging 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.
