Battery
The battery does not actually store electricity-it stores energy chemically. The battery contains a number of negative and positive lead plates, along with a liquid solution of sulfuric acid and water called the electrolyte. When an electrical load is placed across the battery, the sulfuric acid combines with the lead in the plates to produce lead sulfate and water. A great number of electrons are released as electrical current flow by this chemical process. Charging the battery reverses the reaction-lead sulfate is released from the plates and sulfuric acid is formed.
Charging the battery does not remove all of the sulfate from the plates. The battery will gradually become sulfated and will no longer hold a sufficient enough charge to start the car. Replacement of the battery is the only remedy. Temperature also affects the efficiency of the battery. The current-producing capacity of a battery at 5°F (-15°C) is only half its capacity at 68°F (20°C), and partly-discharged batteries can freeze due to the higher proportion of water in the electrolyte. A frozen battery produces no current, but can usually be restored when thawed.
Both conventional and low-maintenance batteries have been supplied as original equipment in the Mk2 Golfs and Jettas cars. Conventional batteries have vented filler caps and may require distilled water to be added periodically to maintain the electrolyte level.
Low-maintenance batteries have sealed caps. Under normal operation the battery is maintenance-free, the electrolyte does not need to be periodically checked, and normal charging can be done without removing the caps.
An apparent faulty battery could be the result or a faulty alternator so it's always good to test the system.
Testing Battery
Battery testing determines the state of battery charge. On conventional or low-maintenance batteries the most common method of testing the battery is that of checking the specific gravity of the electrolyte using a hydrometer. On maintenance-free batteries, because of the sealed case, an open-circuit voltage test is used.
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