There are a number of factors affecting an alternator's ability to adequately charge a battery. The greatest factors are:How much current (amps) from the alternator is diverted to the battery to chargeHow long the current is available (drive time)Battery temperatureBattery ageGenerally, running the engine at idle or short stop-and-go trips, during bad weather at night, will not recharge the battery effectively.In the following situations, the alternator will not adequately recharge a battery:The battery is drained because an interior light was left on in the car for 18-24 hours.The battery is drained because the vehicle has not been driven for a month or more.The car is only driven at 60 km/h (35 mph) to a nearby store and back 2 or 3 days a week.