Battery Charger Selection
To specify a charger, you need to know:
- The ac input voltage
- The dc output voltage
- The dc output amps
The battery type dictates the charger output voltage rating & number of cells being charged. For this reason, it is preferable to specify dc output voltage as a specified number of cells of a particular battery type, such as 12 cells of lead acid instead of 24 vdc.
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:
A = Output rating of the charger in amperes.
k = Efficiency factor to return 100 percent of ampere hours removed. Use 1.1 for lead-acid batteries and 1.4 for nickel-cadmium batteries.
C = Calculated number of ampere hours discharged from the battery (calculated based on duty cycle).
H = Recharge time to approximately 95 percent of capacity in hours. A recharge time of 8 to 12 hours is usually recommended.
Lc = Continuous load (amperes).
The above sizing method is recommended, but tends to provide an optimistic recharge time. The actual recharge time is usually longer than indicated above because the charging current tends to decrease as the battery voltage increases during recharge.
It is important that the charger rating be set by the battery charger & the continuous load. Oversizing the charger is not desirable since excessive charging current will overheat the battery. Undersizing is equally undesirable. If the charging current is too low, it will not drive off the crystalline lead sulphate adhering to the plate and capacity will be reduced.