The battery is a common source of breakdown on the road. A battery that fails will prevent the owner from cranking and starting the car. You should perform a visual inspection on the battery anytime you have the hood up.

CAUTION: Always wear eye protection when working on a battery. Protect yourself from electrolyte splash, which can injure skin and eyes. Never smoke or create any spark around a battery or it might explode.

The first step in inspecting a battery is to look for obvious damage.
You should look for:

Visually inspecting a battery.

Any physical damage to the battery indicates it must be replaced. Broken or damaged cables should also be replaced. Corrosion can be cleaned off the post and terminals, as explained later.

Inspect the top of the battery for dirt or electrolyte. Too much electrolyte on the top of the battery may be the result of overfilling. If the top of the battery is not clean, current can flow across the foreign material. This will cause the battery to discharge by itself when the vehicle is idle. Check the battery hold-down to be sure it is tight. A loose hold-down could mean the active material has vibrated off the plates.

The electrolyte level should be checked monthly on older style non-maintenance-free batteries. During warm weather driving, these batteries lose water out of the cells. The higher the battery temperature and the higher the charge rate, the more gas that is developed and the faster the water loss. Long trips in hot weather subject the cells to high temperatures and high charging rates. These conditions mean the electrolyte level must be checked often. If the water drops too low, the battery will fail to function.

Older style batteries have cell vent caps. Low maintenance batteries have a pry-off cell cover. Remove the cell vent caps or use a screwdriver to pry off the cell cover to inspect the electrolyte level.

The electrolyte level should be well above the plates. Most manufacturers have a guide ring built into the top of the case above the cell. The electrolyte level should be filled to the level of the guide ring. If the electrolyte level is low, add water to the cell. Because regular tap water may have a high salt and mineral content, you should use distilled water. The water should be added with a plastic or rubber tool made for battery filling, not a metal funnel. A metal object could cause a short between the plates. Be sure not to overfill the cell. This would dilute the electrolyte strength and cause an acid buildup on the outside of the battery.