Using a car battery charger
An average car battery has a capacity of around 48 amp hours which means that, fully charged, it delivers 1 amp for 48 hours, 2 amps for 24 hours, 8 amps for 6 hours and so on.
A basic charger usually charges at around 2 amps – and so needs 24 hours to deliver the 48 amps needed to fully charge a flat, 48 amp hour battery.
But there is a wide range of chargers with different charge rates on the market – from 2 to 10 amps. The higher the charge output, the faster a flat battery is recharged. Fast charging, however, is undesirable as it can buckle the battery plates.
The loads imposed on your battery may be gauged from the amount of current used by the various electrical components: headlights take about 8 to 10 amps, a heated rear window about the same.
Theoretically, a fully charged battery, without taking in current from the generator, should work the starter for about ten minutes, or the headlights for eight hours, and a heated rear window for 12 hours. As the battery nears full discharge, the lights gradually grow dimmer and finally go out altogether.
There are also causes other than short trips and cold weather which can affect the state of your battery. Failure is more common on cars equipped with a dynamo rather than an alternator, because the alternator produces more electricity and charges better at low engine speeds (See How the charging system works).
The answer in all these cases is frequent testing with a hydrometer (See
Checking the batteries) to see how much capacity is left in the battery, and using a battery charger to top up its charge when necessary.