Technical Manual

Document No. 6-0101 Rev. F Page 19 of 40
5.4 Charging
Charging Lifeline® AGM batteries is a matter of replacing the ampere-hours removed during
discharge plus a little extra to make up for charging inefficiency. The ampere-hour input
necessary for a full recharge depends on the depth of discharge, rate of recharge, and
temperature. Typically, between 102% and 110% of the discharged ampere-hours must be
returned for full recharge. If the recharge is insufficient, the battery’s state of charge will
gradually “walk down” as it is cycled, resulting in sulfation and premature failure.
The recommended method of charging Lifeline® AGM batteries is to use a 3-stage charging
profile. In the first stage, a constant current is applied until the voltage reaches a pre-set limit.
The first stage is often called the Bulk charging stage.
In the second stage, the voltage is held constant at the same pre-set limit until the charging
current tapers to a very low value, at which point the battery is fully charged. The second stage
is often called the Absorption charging stage. A voltage setting of 14.3 volts ± 0.1 volts (7.15 ±
0.05 volt for a 6-volt battery) should be used when the battery temperature is 77°F (25°C). The
battery is considered to be fully charged when the current drops below 0.5% of the battery’s
rated capacity (0.5A for a 100Ah battery). The absorption stage will typically last 2 – 4 hours
before the current reaches this level.
In the third stage, the charging voltage is reduced to a lower value that minimizes the amount of
overcharge, while maintaining the battery at 100% state of charge. This third stage is often
called the Float charging stage. A float voltage of 13.3 ± 0.1 volts (6.65 ± 0.05 volts for a 6-volt
battery) should be used when the battery temperature is 77°F (25°C). The charging voltages at
other temperatures can be determined from the following table:
Table 5-1. Charging Voltage at Different Temperatures for a 12 Volt Battery*
Temp °F
Absorption
Voltage
Float
Voltage
Temp °F
Absorption
Voltage
Float
Voltage
-40
16.88
15.86
70
14.41
13.39
-30
16.58
15.56
77
14.30
13.30
-20
16.30
15.28
80
14.27
13.25
-10
16.03
15.01
90
14.15
13.13
0
15.78
14.76
100
14.04
13.02
10
15.54
14.52
110
13.95
13.00
20
15.31
14.29
120
13.87
13.00
30
15.10
14.08
130
13.81
13.00
40
14.90
13.88
140
13.76
13.00
50
14.72
13.70
150
13.73
13.00
60
14.56
13.54
160
13.71
13.00
* For a 6 volt battery, divide the voltage by 2. For a 2 volt battery, divide the voltage by 6.
See Appendix C for a chart of charging voltage versus temperature. Most chargers that have
automatic temperature compensation use a simplified equation with a linear coefficient. The
recommended linear coefficient for Lifeline® batteries is 0.0022V/cell per degree F
(0.013V/degree F for a 12V battery).