Frequent short trips, with constant stopping and starting, make your battery work very hard, especially in winter when heater, headlights, heated windows and wipers may be working most of the time.

Eventually, because more current is being drained from the battery than the alternator can put back, the battery will not have enough power left to turn the starter motor. A battery in that state of discharge is said to be flat.

A flat battery can be avoided if you have a battery charger - a relatively cheap, but worthwhile accessory.

It uses mains current to replace the battery's lost charge through positive and negative leads that clip to the corresponding battery posts.

A typical battery charger, fitted with a charge gauge(ammeter).Clip the red positive(+) charger lead to thepositive (+) terminal.The high andlowcharging-rateswitch.The ammetergauge shows therate of charge.Clip the black negative (-)charger lead to thenegative (-) terminal.Make sure that thecrocodile clips are fixedsecurely and not liableto be knocked off byaccident.If the charging rate is muchmore than 4 amps, take offthe cell caps or coveringtrough to assist the gas toescape.

How to charge a battery

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.

Connecting a battery charger

Some batteries have a one-piece cell-cap cover fitting in a central trough.

Always check the electrolyte level before connecting the battery to the charger. Top up if necessary (See Checking the batteries) and clean the battery posts.

If there is a power point handy, the battery can be left in the car, so long as the charge rate is only 3 or 4 amps.

However, if the car has an alternator, disconnect the battery terminals beforehand: otherwise some alternators - generally the older type - can be damaged.