The construction of the battery

Fig 1: Identifying Battery Components

Courtesy of VOLVO CARS OF NORTH AMERICA.

Battery 

1. Handle (on certain battery models)
2. Plugs (not maintenance-free as well as AGM-batteries)
3. Positive and negative posts.

The battery is in a plastic container and has six internal chambers, one for each cell. These chambers are not connected to each other. This means that the electrolyte level can drop in one chamber without affecting the level in the other chambers. The cells are connected in series by a sealed joint between each chamber.

On the top of some "open" batteries, there is a cover with six plugs, one for each cell. The plugs can be opened to check electrolyte level and for topping up with battery water. Maintenance-free batteries as well as AGM-batteries do not have these plugs and therefore cannot be topped up with battery water.

The battery is filled with electrolyte. The electrolyte consists of diluted sulphuric acid (H₂ SO₄ ) which is a mixture of concentrated sulphuric acid and distilled or deionized water. Electrolyte is often referred to as battery acid. Electrolyte has a density of 1.28 g/cm³ when the battery is fully charged.

A battery is built up of two or more cells connected in series depending on which terminal voltage is desired. An open car battery consists of six cells and the terminal voltage of a fully charged battery is 12.72 V, that is 2.12 V per cell. For AGM-batteries, the max. terminal voltage of a fully charged battery is 12.93 V.

The AGM-batteries' container is provided with a safety valve. The purpose is to protect the battery against too high pressure inside the battery. Too high pressure can occur in case of incorrect charging, e. g., with too high voltage.

Evacuation hose (not sealed batteries) 

Certain batteries (such as genuine Volvo batteries) have an evacuation hose. This applies to car models where the battery is located in the cargo compartment (not in the engine compartment). The function of the evacuation hose is to lead any gases (oxyhydrogens) that build up in the battery during charging away from the cargo compartment out into the open air.