Slow charge vs fast charge for 12 volt car batteries?

Most battery chargers offer two different rates of charge. The slow rate is 2amps, and the fast rate is 6amps or 10amps, depending on the charger. It is always better for the battery, and safer for the owner, to charge at the 2 amp rate. It produces less heat and stress on the battery, and will help it last longer. The only downside is that it will take 8-10 hours (or overnight) to bring the battery to a full charge. The more rapid rate of charge will recharge a battery in just two hours. If you need a more rapid rate of charge than overnight (maybe you need your car to start ASAP), then it is OK to use the rapid rate on your battery charger but there are important safety precautions. The higher amps necessary for rapid charge heats up the battery, which shortens battery life. It also causes more gas to form in the liquid (acid) in the battery. This gas is hydrogen, and is highly explosive. Hydrogen gas can build up enough pressure to swell or explode the battery, which is extremely dangerous. Acid can spew all over the place and cause bad burns. To avoid this, the six cell-caps on top of the battery have vents in them to let the gas out, but rapid rates of charge may produce more gas than the vents can cope with. Thus it is much safer to unscrew or lever off the caps(depending on the type of battery), and set them loosely over the holes. This lets the pressure out, but keeps debris from falling into the interior of the battery. Secondly, and equally important, you must avoid open flames or sparks, which might ignite the hydrogen gas. In an enclosed space, turning on an electric light or switch can be enough to cause an explosion . Charging the battery outdoors is safer than indoors, but if the garage door is left open to provide good air flow, it is fairly safe. In conclusion, the slower 2amp rate of charge is easier on the battery and safer for yourself, but takes overnight. The rapid rate (6 or 10amp) is fairly safe if suitable safety precautions are taken.