There are a number of factors affecting an alternator’s ability to adequately charge a battery. The greatest factors are:
 

1) How much current (amps) from the alternator is diverted to the battery to charge
2) How long the current is available (drive time)

3) Battery temperature

4) Battery age

Generally, running the engine at idle or short stop-and-go trips, during bad weather at night, will not recharge the battery effectively.