Bulletin #11

Calculation of battery charger size with automatic two-rate charger - Engine Starting Application

The following formula can be used to determine the required charger ampere rating to recharge a battery used in an engine-starting application.  This formula assumes that there is little or no continuous current drain on the charger and it is useful only in calulating ampere rating of two-rate chargers.  It is also assumed that the charger will replenish only the ampere-hours withdrawn by the engine-cranking event.  The voltage of the battery system is immaterial to this calculation.

1. Determine the current that the starter draws for the entire starting cycle.  Assume, for example, that:
   
   The starter draws 900 amps rolling current, worst case
   The maximum cranking time per start attempt is 15 seconds, which equals 0.0042 of an hour
   The maximum number of start attempts will be 5
   
   Ampere-hours (AH) drawn by the starter for this example is (900) (0.0042) (5), or 18.9 AH

2. Decide how quickly you wish the battery to be recharged.  Assume, for example, that you wish to recharge this battery in 5 hours

3. Plug variables into this formula to find the charger ampere rating (use 1.4 for lead acid and 1.8 for nickel-cadmium when asked for inefficiency constan):
   
   Total AH drawn by starter X recharge inefficiency constant / desired recharge hours

4. Answer (for above example):  18.9 x 1.4 / 5 = 5.29 amp charger

Davidson Sales Company represents Stored Energy Systems (SENS), a recognized supplier of battery chargers and DC rectifiers for industrial and communications markets.  If you have any questions about battery charger sizing or application of the topic discussed here, contact us.

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