Common cause for your car or truck battery to go dead either while driving or sitting overnight.

Battery Diagnosis
If the battery tests good but still fails to perform well, the following are some of the more common causes:

A vehicle accessory was left on overnight.
The driving speeds have been slow with frequent stops.
The electrical load has exceeded the generator output (particularly with the addition of aftermarket equipment).


The existing conditions in the charging system include the following possibilities:

An electrical short
A slipping belt
A faulty generator
A faulty generator voltage regulator

The battery has not been properly maintained, including the following situations:

A failure to keep the terminals tight
A failure to keep the terminals clean
A loose battery hold-down retainer

A power failure caused by an existing mechanical condition in the electrical system, such as a short or pinched wire.
Extended vehicle storage.
Extended cranking periods due to a possible fuel or ignition system problem.
Incorrect interpretation of the battery's built-in hydrometer.
There has been an insufficient ampere-hour charge rate for a discharged battery.
There is a continuous current draw on the battery through excessive parasitic drain.
The battery is old.

Self-discharge is always occurring as a result of internal chemical reactions, even when the battery is not connected. In hot weather this chemical reaction is increased dramatically. This is why the number of discharged batteries will increase in very hot weather.

Electrolyte Freezing
The freezing point of electrolyte depends on its specific gravity. A fully charged battery will not freeze until the ambient temperature gets below -54°C (-65°F). However, a battery with a low state of charge may freeze at temperatures as high as -7°C (20°F). Since freezing may ruin a battery, the battery should be protected against freezing by keeping it properly charged. As long as the green eye is visible in the hydrometer, the freezing point of the battery will be somewhere below -32°C (-25°F).

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