Auto Battery Chargers

Auto battery chargers range in price from fifteen dollars or so to hundreds of dollars but, like most things in life, you pretty much get what you pay for.

A cheap auto battery charger costing around $15.50 will only charge your car's 12 volt battery at a lower rate of charge. This is measured in Amperes (often shortened to just Amps).

The actual measured voltage of your car battery' will depend on its state of charge and the 'health' of the battery. A fully-charged battery in good health will often measure 13.8 volts DC with the engine running. A battery with a low charge or in poor health will measure less than 12 volts.

A slow charging rate for a 12v battery would be around 2 Amps or less, which is sometimes referred to as a trickle charge. A fast charge for the same auto battery would be 20 Amps or even higher. The higher the charging rate of your vehicle battery charger, the larger electrical transformer inside the battery charging unit needs to be -- and the higher its price tag.

The power transformer takes your AC mains voltage (usually 110V in the US or 220V in Europe) and steps it down to around 14 volts or so. The 14v AC voltage is then rectified into a DC current, and may be filtered further -- depending on the engineer's design criteria.

The best types of auto battery chargers will also have some built-in 'smarts' to sense when the battery has been fully charged. At this point they will stop trying to pump more current into the battery and change to maintenance mode, where they send the battery just enough electric current to keep it in a fully charged state. But they won't over-charge the battery.

A cheap and simple battery charger won't have the circuitry to detect when your battery has reached its fullest capacity. So you have to keep an eye on the charging process yourself, and switch off the charger once the battery has been charged long enough. The manufacturer's instructions usually state how long you should charge your auto battery; but if in doubt it is better to charge too little. That way you won't 'cook' the battery and damage the plates inside through overheating.