Battery chargers have come a long way in recent years with the coming of age of “smart” chargers. These chargers can automatically detect the type of battery being charged (Flooded, AGM or GEL) and charge them appropriately.
They start by being connected to the battery and then send a pulse into the battery to determine the internal resistance of the battery. From this it will know the type of battery being charged and will charge it at the voltage that best suits that type of battery.
These smart chargers are usually staged and will usually have a “float” charge as its final stage allowing the user to not worry about the battery being overcharged or when they must remove the battery from the charger. When using an older charger, it’s important that the charger be compatible with the type of battery you are charging. If you charge an AGM battery with a charger designed to charge wet cell batteries alone, it will overcharge the AGM and damage it, shortening its life span. As a rough guide battery chargers should be rated at 10% of the AH capacity of the battery that it’s charging. For example, if you are charging a 100AH battery, then a battery charger rated to 10amps would be the ideal size to charge it. Up to 5% either side of this 10% figure is fine but placing a small battery charger on a large battery or battery bank may take it too long to charge or it may never reach full charge and placing a charger that’s too large for a battery can cause it to charge too quickly and cause damage to the battery.