The only time you probably think about your vehicle's battery is when you try to start your car ... and it doesn't, notes Consumer Reports.

If you've ever had that sinking feeling — in your empty office parking lot or when you need to rush the kids to school — you'll remember how important it is not to take the battery's health for granted.

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 V-8 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.

Different groups

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.


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

• Low-Maintenance/Maintenance-Free. 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, they have crowded the older style off the market. Low-maintenance batteries retain their fluid for the life of the battery.

• AGM. AGM (absorbed glass mat) 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 such as motor homes that regularly sit unused. That resilience comes at a price, though, and AGM batteries can cost twice as much as others.

Battery buying tips

When shopping for a battery, Consumer Reports suggests the following:

• Don't buy an old battery. Look for a date code printed on a sticker on the side of the battery. Always check to make sure you get the newest one on the shelf. When batteries sit, they can start losing their charge. Here's how to decipher the code: A battery made in October 2015 will have a numeric code of 10-5 or an alphanumeric code of K-5. "A" is for January, "B" is for February, and so on (the letter "I" is skipped). Consumer Reports instructs its shoppers to buy batteries that are no older than six months — preferably three months or newer.

• Take your old battery to the store so that it can be recycled. Don't just throw it in the trash! Batteries contain lead and must be disposed of properly. If you don't have your old battery with you at time of purchase, you may be charged a "core charge," or a deposit of $5 to $25.

• Don't focus too much on cold-cranking-amp (CCA) claims. Modern cars with fuel-injected engines controlled by computers take no more than a few seconds to start. They don't need the highly inflated CCA numbers that manufacturers like to put on the packaging and marketing materials.

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