Different vehicles need different-sized batteries, mostly because of the engine size, space restrictions under the hood, and the number of powered accessories.

Bigger engines take more energy to crank, requiring a physically larger battery. A battery suitable for a compact car probably won’t have the capacity to start a large pickup’s V8 engine.

And on the flip side, that pickup’s battery probably wouldn’t fit into the compact car’s smaller engine compartment. Also, crowded engine bays require shoehorning in different-shaped batteries. Some batteries are placed in the trunk, which brings other concerns into play.

Batteries are grouped by physical size, type and placement of the terminals, and mounting style. Replacing your battery with one from the same group ensures that the battery will fit the tray and that the leads will connect properly. (Check your owner's manual or an in-store fit guide for size recommendations.)

Once you know the size you need (group number), choose from one of two types of batteries:


Batteries once required drivers to periodically top up the water in the electrolyte, the battery’s power source. Modern maintenance-free batteries consume far less water than traditional “flooded cell” batteries. In the past few years maintenance-free ones have crowded the older style off the market. (We no longer test those batteries.)

Low-maintenance batteries retain their fluid for the life of the battery. Caps on maintenance free-models aren’t meant to be removed.


Absorbed glass mat (AGM) batteries contain a very sparing amount of acid electrolyte, have a sealed case, and use a different internal chemistry that reabsorbs loose hydrogen molecules that react back into water. That combination eliminates the need to replenish electrolytes, extending the battery’s life span.

These batteries are more forgiving of “deep discharge,” which happens when you accidentally leave your car’s headliner dome light on, for example—at which point turning the ignition key results in nothing but clicking sounds from under the hood.

A single instance of deep discharge, which coats the lead plate electrodes with sulfate deposits, can reduce the life span of a conventional battery by a third or more. That makes AGMs a good option for absent-minded drivers or for vehicles that regularly sit unused, such as motor homes.

That resilience comes at a price, though, and AGM batteries can cost twice as much as others. Many higher-end vehicles come with an AGM battery; never replace an original-equipment AGM battery with another type, because the toxic gases vented or electrolytes leaked by non-AGM batteries may be dangerous if the battery is mounted in the trunk or passenger compartment.

Editor's Note: This article also appeared in the January 2016 issue of Consumer Reports magazine.