UFC 3 -520-01
June 10, 2002
Size the charger to be large enough to supply the normal continuous loads
while also recharging the battery within a reasonable time period. The charger sizing
formula is as follows:
= Output rating of the charger in amperes.
= Efficiency factor to return 100 percent of ampere-hours removed.
Use 1.1 for lead-acid batteries and 1.4 for nickel-cadmium
= Calculated number of ampere-hours discharged from the battery
(calculated based on duty cycle).
= Recharge time to approximately 95 percent of capacity in hours. A
recharge time of 8 to 12 hours is usually recommended.
= Continuous load (amperes).
The above sizing method is recommended, but tends to provide an optimistic
recharge time. The actual recharge time is usually lo nger than indicated above
because the charging current tends to decrease as the battery voltage increases during
EXAMPLE: Determine the charger rating if a) the continuous load is 100
amperes, b) 300 ampere -hours are discharged from a lead-acid battery, and c)
the battery is to be recharged within 10 hours.
+ 100 = 133 amperes
EXAMPLE: Suppose that the above system has 50 amperes of noncontinuous
loads that can be energized at any time. In this case, the total charger load is the
sum of the continuous and noncontinuous load before consideration of battery
recharge requirements. At any time, the charger load can be as high as:
A = Lc + Ln = 100 + 50 = 150 amperes
If the charger in the previous example was selected to have a capacity of 133
amperes, the battery would have to supply the additional load whenever the
noncontinuous load is energized. So, the charger should instead be sized to
provide the expected system loads, or 150 amperes in this example. Note that
this assumes the noncontinuous loads will not be energized for long periods
when the battery is being recharged.