Part 1

Design and function

Introduction
The battery type used in most cars is usually know as a lead accumulator. These batteries are so named because the electrodes are lead and the role of the battery is to store (accumulate) energy.
A key characteristic of lead accumulators is that they can meet the extreme physical requirements for charging and discharging. Lead accumulators provide optimal performance as both starter and reserve power batteries. Lead accumulators are relatively cheap to produce and can store large amounts of energy. As a result, lead accumulators make up in excess of 50% of the entire battery market.

Principle




A battery is a unit which stores chemical energy which can then be converted into electrical energy as required. When the battery is connected to an external source of consumption (the car starter motor for example), the stored chemical energy is converted into electrical energy and current flows through the circuit.
The start battery in a car, the lead accumulator, is rechargeable. Such a battery can be recharged repeatedly, whereby electrical energy is converted to chemical energy.

Function




The function of the battery in a car is to supply the car with a stable voltage in all situations. One of the tasks of the battery is to provide a current when starting the engine. Other tasks are to provide a current to components (such as the parking lamps) which require current when the engine is not running (i.e. when the generator (GEN) is not charging). The battery also operates as an energy reserve when the capacity of the generator (GEN) is insufficient and needs to be supplemented.

A battery works as follows
The battery consists of several cells. Schematically, a battery can be said to be made up of two electrodes, one positive and one negative. These electrodes are different materials, which are submerged in a container filled with electrolyte, often called battery acid. As a result of the different electrode materials, there is an electrical charge between the positive and negative electrodes. There is a chemical reaction between the electrodes and the electrolyte causing electrical energy to be generated. If the positive and the negative poles of the electrodes are connected to form a closed circuit, a current flows through the circuit.
In a lead accumulator, the positive electrode consists of lead oxide and the negative electrode of porous lead. The electrolyte consists of diluted sulphuric acid (H2SO4) which is a mixture of concentrated sulphuric acid and distilled or deionized water.

Battery types
In principle, lead batteries can be divided into two different categories:
- Open batteries
- Sealed batteries

Open batteries
In principle, open batteries have free-hanging plates in the battery box, most often insulated from each other by the negative plate being located in a small separator bag of polyethylene. The electrolyte is highly viscous and can flow freely between the plates.
A maintenance-free battery is normally an open battery where the design has reduced the gas and also has made the battery box so airtight that any gases stay in place. A lot of acid means that the fluid will last for its service life without topping up.

Sealed batteries (recombination batteries)
For sealed batteries the battery box is designed as a pressure vessel with relief valves and go under the name VRLA (Valve Regulated Lead Acid), that is, valve-controlled lead-acid batteries.
If you shut in the acid and hydrogen gas then these can react with each other and be reformed to water. This is called recombination and is a good way to almost completely eliminate fluid losses. It is not possible to shut in and recycle everything, but it can be reduced significantly. The sealed batteries can be divided into two groups:
- Gel batteries
- AGM-batteries
Gel batteries
A gel battery has a few substances, often silicone compounds, added in the electrolyte so that it gels. Thus there is no free-floating acid that can run out of the battery.
Gel batteries have good capacity, but due to the slightly higher resistance in the low viscosity acid, they may be limited as start batteries. They can handle deep cycle discharges very well, that is, when you slowly discharge the batteries a lot several times.
Gel batteries are very rugged and are often used, e.g., in cleaning machines and golf carts.
The small volume of electrolyte in the battery and that it is low viscosity makes the battery sensitive to dehydration that an overcharge with resulting gas generation could entail.
AGM-batteries
In AGM-batteries the electrolyte is kept in place by the separator paper, consisting of a fibreglass mat (AGM = Absorbed Glass Mat), working as a "sponge". The capillary forces in the separator means that electrolyte ends up in the right place. The batteries can be built with very thin separators, which keeps down the internal resistance. This means that that you can get high power from a small volume, making it perfect as start battery.
The fibreglass mat is quite thin, which makes for a small electrolyte volume in the battery. This makes the battery sensitive to dehydration that an overcharge with resulting gas generation could entail.






The construction of the battery





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 (H2SO4) 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.

Note! The battery is damaged if the safety valve has been activated.