On 5/3/15 3:45 AM, sms wrote:

As to voltage, a fully charged lead acid battery, with no load, will be
2.3V per cell x 6 cells = 13.8V. Once it's under load, it's 2.1V/cell or
12.6V. If you have a shorted cell (or more than one shorted cell) it's
easy to check because under noload the voltage will be lowered by the
number of shorted cells x 2.3V.

An alternator/rectifier/voltage regulator, or battery charger, needs to
put out 14.4-14.8V to properly charge a car battery. While people call
car batteries "12 volts," they are not 12 volts except under load where
they are between 12V and 12.6V.

An article entitled is "Car Batteries Are Not 12 Volts" is available at
http://www.landiss.com/battery.htm.


A battery that has just been on a charger or come in from driving may
well read 13.8 volts, but it will settle down over a period of hours. A
brief load helps it settle faster.

The article points out that it's hard to know the best voltage for an
automobile regulator. They all seem to be temperature compensated these
days, providing more voltage for a cooler battery. That helps.

A battery used for for a couple of 10-minute drives a day would probably
last longer with more charging voltage. A battery on the interstate 10
hours a day would probably last longer with less voltage. How can a car
manufacturer predict your driving habits?

I once owned a charger with a switch for conventional or
maintenance-free batteries. The maintenance-free does better with more
charging voltage.