The Standard Capacity For Car Batteries
Determining the Standard Capacity For Car Batteries
The number and size of the lead plates in each cell determine the capacity of a battery. In practical terms, capacity indicates the battery’s ability to provide current.
For instance, a battery with a high capacity will still provide starter current if the side lights have been left on all night, while a low capacity battery would have no current left in it under the same condition.
The capacity of a battery can be difficult to judge, but physical size is usually a good indication. Stamped or printed on most batteries would be figures like 33 or 38 for instance, indicating its capacity.
Sometimes the figures are followed by the letters Ah (ampere-hours), the unit of measuring capacity. The manufacturer determines the ampere-hour capacity of a type of battery by testing samples.
If it is capable of providing 3.5amps for 10hours before becoming discharged, it is rated as 35Ah i.e. (3.5amps X 10hours), and in the battery specification, it will have its capacity shown as 35Ah at the 10hour rate.
Because battery capacity is affected by the rate at which it is discharged, the same battery will be capable of providing 2amps for 20 hours and so its capacity could also be quoted as (2amps x 20hours)or 40Ah at the 20hour rate.
Therefore, when making comparisons, it is important to find out at which hour rate the Ah capacity figure was determined.
In practice, the capacity will reduce as the battery grows older and loses some of the active materials from its plates. Provided the battery fitted to a vehicle is replaced with one of the same capacity, no problem should be experienced.
Determining the battery lifespan
The life of most batteries is two to three years, but this depends to a large extent on their operating environment, including temperature, vibration, charging system performance, and on how well they are maintained.
When a battery is discharged, the plates in its cells are charged to lead sulfate. Unless it is recharged without delay the lead sulfate will become hard and leave parts of the plate beyond recovery thereby reducing the overall capacity of the battery.
If the battery is left discharged for a day or two, crystals will start to form. These expand and force active materials from the plates into and through the separators, causing internal short-circuits.
Once a cell has been damaged in this way, no amount of recharging will bring it to life. It is useless and cannot be repaired.
Older batteries which have areas of non-reversible lead sulfate on their plates can be damaged by indiscriminate fast or boost charging. The plate gets hot and lumps of lead sulfate fall off them, taking areas of useful material with them.
This not only reduces the battery capacity, but the material which has fallen off can jam into the separators and cause short circuits between the plates.
The water used for topping up is also important. If it contains impurities such as lime or chlorine, the plates will be damaged or become coated. Coated plates prevent the electrolyte from making contact with the active material.
Distilled or de-ionized water is cheap and should always be used for topping up. Rainwater or melted ice from the freezer compartment of a refrigerator is not suitable as both are likely to be contaminated.
If the battery is thought to be faulty, it can be tested by an auto-electrician but first, make sure that it is fully charged because a discharged battery will always appear to be faulty.
A battery is fully charged when, while it is on a charge, three hourly checks with a hydrometer show no change in the specific gravity of the electrolyte in any of its cells.
Never add anything other than distilled or de-ionized water to the battery. The acid will be lost only by the spillage of electrolyte often from over-filling.
