11
AMER-EN-IOM-ODY-0523
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batteries is 13.6 volts for a 12-volt battery at 77°F (25°C).
There are also chargers available that charge using constant current. These chargers do not have
multiple charge phases as shown in Figure 2. They simply supply current to the battery, which causes
the battery voltage to rise. Once the battery voltage reaches the setpoint for a fully charged battery,
the charging will terminate. How long it takes to charge a battery with this method depends on how
deeply the battery is discharged and how much current the charger provides. See Table 6 below as
an example of recharging a battery rated at 100Ah from various depths of discharge using a 10-amp
constant current charger.
Open Circuit
Voltage
Depth of Discharge
Charge Time
(hours)
12.6
25%
4
12.2
50%
8
11.9
75%
12
11.5
0%
16
Table 6
Temperature Compensation of Charge Voltages
Proper charging of all AGM
2
batteries requires temperature compensation of the charge voltage. This
is especially true in float applications where the batteries are on-charge constantly. The temperature
compensation coefficient is approximately ±18 mV per °C variation from 25°C per 12-volt battery.
Temperature and charge voltage are inversely related. Therefore, charge voltage must be reduced as
temperatures increase and charge voltage must be increased as temperatures decrease. Regardless
of temperature, the minimum charge voltage is 13.2 volts as lower voltages will damage the battery
grids and shorten life.
Recovering a Severely Discharged Battery
Many commercially available chargers must detect a certain minimum voltage for the charging process
to start. This is a common safety feature to prevent using a 12-volt charger on a 6-volt battery.
Chargers of this type are unable to charge a battery that has been severely over-discharged. For
example, a 12-volt charger might not start the charging process if the battery connected to it has an
OCV of 5 volts. Depending on the size of the battery, there are two ways to try to recover the battery.
• For batteries used in starting applications, the alternator can be used to recharge the battery after
jump-starting the battery to start the vehicle.
• For batteries not used in starting applications, a second battery that is known to be at a high SOC
can be connected to the discharged battery in parallel. Refer to
Section IV, B (Battery Sketch 2)
for details related to connecting batteries in parallel. Once the low battery’s voltage reaches 11.5
volts, the batteries can be disconnected from each other. The standard A/C
charger normally used by the system can then be connected to the battery that needs to be
charged. Please refer to “Procedure to Recover Deeply Discharged ODYSSEY Batteries” found
on www.odysseybattery.com for more details.
It is important to understand why the battery became over-discharged, so the situation does not
happen again. The most common reasons for batteries to be over-discharged are system issues
related to parasitic loads (see page 13) or malfunctioning/non-existent low voltage disconnect
equipment.