The vehicle battery is used as a source of energy in the vehicle when the engine, and hence the alternator, is not running. The battery has a number of requirements, which are listed below broadly in order of importance.
To provide power storage and be able to supply it quickly enough to operate the vehicle starter motor.
- To allow the use of parking lights for a reasonable time.
- To allow the operation of accessories when the engine is not running.
- To act as a swamp to damp out fluctuations of system voltage.
- To allow dynamic memory and alarm systems to remain active when the vehicle is left for a period of time.
The first two of the above list are arguably the most important and form a major part of the criteria used to determine the most suitable battery for a given application. The lead-acid battery, in various similar forms, has to date proved to be the most suitable choice for vehicle use. This is particularly so when the cost of the battery is taken into account.
The final requirement of the vehicle battery is that it must be able to carry out all the above-listed functions over a wide temperature range. This can be in the region of 30 to 70 ° C. This is intended to cover very cold starting conditions as well as potentially high under-bonnet temperatures.
Positioning the vehicle battery
As usual, these issues will vary with the type of vehicle, intended use, average operating temperature, and so on. Extreme temperature conditions may require either a battery heater or a cooling fan. The potential build-up of gases from the battery may also be a consideration.
Even after well over 100 years of development and much promising research into other techniques of energy storage, the lead-acid battery is still the best choice for motor vehicle use. This is particularly so when cost and energy density are taken into account.
Incremental changes over the years have made the sealed and maintenance-free battery now in common use very reliable and long-lasting. This may not always appear to be the case to some end-users, but note that quality is often related to the price the customer pays.
Many bottom-of-the-range cheap batteries, with a 12-month guarantee, will last for 13 months
The basic construction of a nominal 12 V lead-acid battery consists of six cells connected in series. Each cell, producing about 2 V, is housed in an individual compartment within a polypropylene, or similar, case.
shows a cut-away battery showing the main component parts. The active material is held in grids or baskets to form the positive and negative plates. Separators made from microporous plastic insulate these plates from each other.
The grids, connecting strips, and battery posts are made from a lead alloy. For many years this was lead-antimony (PbSb) but this has now been largely replaced by lead calcium (PBC). The newer materials cause less gassing of the electrolyte when the battery is fully charged. This has been one of the main reasons why sealed batteries became feasible, as water loss is considerably reduced.
However, even modern batteries described as sealed do still have a small vent to stop the pressure build-up due to the very small amount of gassing. A further requirement of sealed batteries is accurate control of charging voltage.