How to choose the right battery for a car.
To find the right car battery we must know the mechanical properties such as length, width, height, polarity, terminal configuration, hold down style, and body style. We also consider electrical properties, mainly cold cranking amps and reserve capacity.
A simple number, called the group size, describes all of the physical characteristics. Car batteries conform to specifications set by a trade association known as the
Battery Council International (BCI) and each battery has a group number which determines its physical dimensions. For example, the Group 24 battery has a length of 10 1/4"
width of 6 13/16" and height of 8 7/8". The group number also encodes:
- Polarity: whether the positively charged terminal stems from the left or right side of the battery. An otherwise identical battery will not work if the positive and negative terminals do not match the leads from the car. To discourage polarity related mistakes during battery installation, leads and terminals often have color codes and labels: red + for positive, black - for negative, and batteries with top post terminals often have different diameters for the positive and negative posts.
- Terminal Configuration: Some batteries connect to the car through posts protruding from the top, some from threaded sockets mounted on the front, and some with nuts and bolts that screw onto or into the battery.
- Hold Down Style: Without a battery hold down mechanism, batteries bounce freely around the battery compartment risking damage from rough terrain, sudden stops, speed bumps, tight turns, and sudden changes in speed. To remain safely anchored to the car, some cars clamp their batteries in place with bars that stretch over the top while others hook onto protrusions on the bottom. Even if a battery has the same polarity, terminal type, and physical dimensions, it may not have a hold down style compatible with the car in question.
- Body Style: The most profound differences in battery shape include the top style: flat, acid access capped, and/or euro top. In cases like group 65 batteries, it also describes pronounced indentations on the bottom of the battery.
The main electrical properties of car batteries include voltage measured in volts and capacity measured in amps. Because modern car batteries have long since adopted a 12 volt standard, we focus on capacity requirements. In climates with consistent and moderate temperatures, we need only consider cranking amps denoted: CA. Cranking amps refer to the amount of current a battery can deliver all at once in order to start an engine. In colder climates we focus on cold cranking amps because low temperatures significantly reduce battery effectiveness. Cold cranking amps measure the number of cranking amps a battery can deliver when surrounding temperatures reach 0 degrees Fahrenheit.
We also consider reserve capacity because car manufacturers continually increase the number of built in electrical systems such as on board computers, touch screen displays, sound systems, and USB ports. Because the power needs of these electrics differs from those of the ignition system, we use a different way to measure the battery's capacity. Instead of relying on the battery for a short burst of intense power, the electric peripherals steadily draw lower amounts of power over longer time periods. In cars with a lot of built in electronics, we have to consider the deep cycle capabilities of potential replacement batteries. We measure deep cycle battery capacity in terms of reserve capacity minutes or in amp hours.