Battery chargers are all automatic these days and have been for years. So, don’t get taken in by the term "automatic battery charger" when buying a charger. Instead, focus on what kind of battery needs charging. Then determine how “smart” the automatic battery charger needs to be for ease-of-use purposes — “smart” being defined as the ability of a charger to detect the kind of battery it’s charging and to vary the amount of charging current needed at a given moment to ensure a steady and safe charge.
In their early years, battery chargers were manual. A person simply hooked up one to a battery and plugged it in or turned it on with a switch. More specialized, and costlier, models featured the ability to determine charging rates. The problem was that the person using the charger controlled the charging. If unskilled or inattentive batteries could end up undercharged or overcharged, which could shorten their lives or even ruin them.
Charger manufacturers developed automatic battery chargers to do a better, safer job of charging. They also recognized the need for automatic battery chargers to be smarter, meaning easier to use and with a minimum of human interaction. Traditional batteries such as car batteries were being made of materials that improved their life cycles and required more careful charging. There also was the digital revolution. Digital electronic devices of all kinds began showing up in the marketplace, with many of them using rechargeable batteries that required sophisticated charging technology.
This evolution in charging technology means that you don't have to know or do much to use an automatic battery charger. What you do need to know, however, is what kind of battery or batteries you want to charge. Then you need to decide how fast you want the charging process to be.
As an example, you might want an emergency charger for when your car battery loses a charge because someone left the lights on overnight or because it was discharged while trying to start the car during extremely cold weather. If charging time is not a concern, then a good choice might be an inexpensive automatic battery charger that provides a charging rate of about 10 amps. Ten amps will take about five or six hours to charge a standard car battery. If a faster charge is required, though, more expensive automatic battery chargers will put out more amps and can charge a battery in a much shorter time. Both types will be smart, meaning that they will regulate the charging process to make sure the battery is not overcharged or undercharged and that the battery is not damaged.
Another example is determining the best automatic battery charger to keep a motorcycle or recreational vehicle (RV) battery fully charged all winter. The best choice is the one that detects when the battery begins to discharge from just sitting there. It will then start a slow charge and automatically shut off when the battery is again fully charged. A similar automatic battery charger might be required to keep a sump pump operational at all times in case a power failure occurs. These automatic battery chargers are smart because they can precisely detect charge fluctuations in the batteries and keep the charging rate in balance at all times without human intervention.
You also might have more than one kind of battery to charge, maybe for a car and a boat or for different portable electronic devices, but you might want only one charger for all the batteries. First, determine the charging requirements needed for each kind of battery. Then select an automatic battery charger for multiple types of batteries that will detect the kind of battery it has been hooked up to and the charging rate that it needs.