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11.2 The state-of-charge of sealed-top (maintenance-free and recombination-type) batteries must be
checked with a high resolution voltage tester.
11.3 If uncertain about type of battery you will be charging, or the correct procedure for checking the
battery’s state-of-charge, contact the seller or manufacturer of the battery.
11.4 The temperature of the battery, and the equipment the battery is used with, has a dramatic effect
on battery efciency and system power requirements. For example, at 0
o
F (–18
o
C), a battery
is operating at 0% of its rated efciency, while the engine it is attempting to start requires over
twice as much power as would be necessary at 80
o
F (27
o
F).
12. BATTERY CHARGING FORMULA
12.1 Determine battery’s state-of-charge by following the instructions and chart in Section 11, connect
charger as per instructions in Section 9 and/or 10.
12.2 From the Charging Time Chart (located on the Table of Contents page and also on the Quick
Reference Guide), pick the ROW which has the Reserve Capacity rating and measured battery
voltage that is closest to the battery being charged. Follow across the row until you are under the
desired charging rate. This value indicates the approximate time required to recharge the battery.
NOTE: As a manual charger, the NAPA 85-8100 will continue to charge for as long
as the Timer is selected. This is potentially dangerous, as battery thermal runaway
and gassing are not detected. For this reason, only set the Timer for slightly over
the needed time to perform a charge. More time can always be added later, as the
Timer keeps track of accumulated time and charge.
12.3 An alternative method for calculating the time required to recharge a battery in good condition is
to follow this formula:
(reserve capacity in minutes) + 15.5 = amp hour capacity
2
(amp hour capacity) x (percent of charge needed) = time in hours
charger amp rate setting
(time in hours) x (recharge factor of 1.25) = recharge time
Example: Suppose a battery has an RC of 120 minutes and is discharged down to 25%.
We need to put in 75% more charge and we want to use our charger’s 40 AMP charge setting:
**Initial specic gravity readings vary, depending on battery type and
manufacturer. Contact the seller or manufacturer of the battery for this
specication.