When comparing car batteries, there is generally only one reason to compare them, and that is when your battery needs replacing. You are going to want to replace the battery with one that is equal to or better than the one being replaced. To determine which battery is better, you will need to perform a series of measurements and do a little reading.
The battery has a tie-down strap that bolts it to the battery pan on your vehicle. You need to remove this before you can remove the battery. Additionally, the battery will be connected to the vehicle's wiring harness at the battery terminals. Disconnect the negative (- or black) wire from the battery, and make sure it does not touch anything. Then remove the positive (+ or red) wire from the battery, again making sure that it will not touch anything. Lift the battery from the vehicle and set it on the ground.
Now that your battery is fully accessible, take your measuring tape and record measurements for the height, width and length of the battery. This tells you that a battery of the same size or smaller can fit in your battery pan. Generally you want the battery to be as large as possible, so try to get a new battery that is identical in size. Now when you removed the battery terminal cables, you should have noticed whether they were removed from the top or from the side of the battery. Your new battery will need to have the same type of connection as your last one.
Vehicles require batteries to meet certain criteria in order to be able to start an engine. The specifications written on the battery will tell you what this criteria is. Stock batteries are a good indicator of the minimum criteria for correct operation. Mostly you are looking for Cold Cranking Amps (CCA) and Cranking Amps (CA). When you compare the two batteries, you need to make sure that the replacement battery's specifications are equal to or greater than the battery being replaced.
Batteries come in many varieties. Deep cycle batteries are ideal for marine environments but not as good for general driving. Sealed batteries are more expensive but can be turned upside down without spilling acid. Read the labels to determine what type of batteries you have for your comparison, and choose similar types of batteries.
If you need to compare the state of two older batteries, hook them up to a battery charger until fully charged, and then hook them up to a load tester to determine how much power they can still generate. Less amperage means a lower quality battery.
