V40 L4-1.9L Turbo VIN 29 B4204T3 (2002)
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 sulfuric
acid (H2SO4) which is a mixture of concentrated sulfuric acid and distilled or deionized water.
Battery types
In principle, lead batteries can be divided into two different categories:
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Open batteries
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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:
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Gel batteries
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AGM-batteries
Gel batteries
Hint: This battery type is not found in Volvo's range and is only described briefly in this section.
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 fiberglass 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 fiberglass 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.
