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HOW TO DETERMINE MAXIMUM VEHICLE BATTERY CAPACITY

A security system installed in a vehicle presents a constant (although minimal) current draw on the vehicle battery. If the vehicle is driven each day, the alternator ensures the battery is topped up.

If the car is only driven on weekends, the time between re-charging is increased, and the wear and tear on the battery is exaggerated. Lead acid batteries prefer to be charged. The longer the time a battery spends partially discharged, the shorter it's life span. It also suffers a decrease in the ability for the battery to accept charge from the alternator.

The calculator below can determine a new battery's maximum time without re-charging.

Capacity Calculator

1. Current draw of immobiliser

(in mA)

2. RC rating of battery

Capacity

days

this figure includes a 50% error margin

Standby tim formula

Reserve Capacity (RC) indicates how long a vehicle may be left without recharging the battery. In a laboratory, RC is the number of minutes a fully charged battery at 26.7 degrees C can be discharged at 25 amps until the voltage falls below 10.5 volts.

To convert Reserve Capacity to approximate Ampere-Hours, multiple RC by 0.4.

For example, a battery with a 120 RC rating will have approximately 48 AH of capacity at the 25-amp discharge rate. More RC is better in every case!