A lead-acid battery makes electricity by mixing sulfuric acid with water, creating a chemical reaction that generates free electrons, or electricity. Over time, the sulfuric acid attaches itself to the lead plates, weakening the battery until it dies. Battery chargers for lead-acid batteries use outside electricity to force the sulfuric acid back into solution in the water, recharging the battery. There are also different kinds of battery chargers.

 

Ideally, a lead-acid battery is charged in three stages. In the first stage, the battery is charged at a constant current as the voltage rises in the battery. This completes about 70 percent of the charge and takes from five to eight hours. In the second stage, the current should be decreased, and for another seven to 10 hours the battery is charged to its peak voltage.

 

In the third stage, a lowered current provides what is called a float charge for around four hours, during which the battery "rests" and the voltage falls back a bit from peak to prevent corrosion. Because batteries can be charged enough for use in the first stage, many different types of chargers can charge more quickly.

 

The "simple" type of battery charger with a constant current can complete stage one, but it is incapable of completing stages two and three, and can damage the battery by overcharging. The battery charger must match the amperage on the battery to prevent rapid overcharging and damage. Simple chargers deliver 12V or 24V at one rate, and so require close monitoring. They can charge far faster than the ideal, but they will not safely charge past 70 percent in the first stage.

 

Other types of chargers are designed to track the recharging stages and reduce current appropriately for each stage. These types of chargers include trickle chargers, timer-based chargers and "intelligent" chargers. Trickle charger produce a consistently low current to prevent damage from overcharging, but they are slow. Timer-based chargers time the charge stages, but without any direct feedback from the battery. Intelligent chargers adjust to the stages based on monitored feedback from the battery.

 

Recharger designs are also matched to two different basic types of lead-acid batteries: starting batteries (such as the one that starts your car) and deep-cycle batteries (such as the ones that run trolling motors for boats). Starting batteries deliver a short, powerful charge to ignition systems. Deep-cycle batteries have lower, slower voltage, but they deliver a steady current for a long time.

 

The latter are generally marine batteries, and there are three types of chargers used for marine batteries. One is ferro-resonant chargers, which are cheap but slow. Linear chargers are slow and vulnerable to current surges, for example from a generator. Switched mode chargers correct the problems of the other two and are lightweight; but switched-mode chargers are the most expensive.