AO Silver Member
Join Date: Mar 2003
Re: Quick electrical question
Some slight dimming is normal but anything more than that could point to the fact the battery needs replacing.
An alternator charges the battery at 14v to 14.75v at anything above 1,000 revs. The higher the voltage the brighter the head-lights will look. At idle the alternator will still be charging the battery slightly but not sufficient to power a lot of the electrical devices as well.
At idle, a battery produces about 12.5v with no electrical load on it (anything drawing a significant current). Under high load conditions, plus during any very cold weather, this can drop to 10.5v to 11v. The amount the voltage can drop by depends on the condition of the battery (basically how old it is) and if it was under-specified (capacity in ampere-hours) in the first place. It's quite possible for the voltage to drop to only 9v under these conditions.
If your battery is more than 3 to 4 years old I would get it checked and think about getting it replaced.
'A business absolutely devoted to service will have only one worry about profits. They will be embarrassingly large.' - Henry Ford.