What battery voltages should you see

[BillGrissom]

This question arises often - either "is my battery low?" or "is it over-charging?". The answer according to a voltage monitor ($7 HR, on sale so bought 2):

Engine Off: >12.6 V "fully charged", <12.1 V "almost dead", <11.8 V "dead".

Engine Running: 13.4 to 14.8 V "normal", >14.8 V "over-charging".

The LED's show the above ideal values in my 96 Voyager. Previously, I had an Eqquis digital voltmeter ($15 Amazon), which also plugs in the cigarette lighter. As I recall, it showed 12.6 V engine off, 14.3 V engine running. Had no instructions on what is proper.

The HF one appears a bit strange since when your car is "normal" some green, yellow, and red LED's are all lit. Best to plug it in an outlet that is only powered from the key, so it won't run down your battery overnight. Since most don't have that in an A-body, maybe plug in only for diagnostics. I like a monitor on trips so I don't get surprised at a rest stop or some podunk town with no stores. I should give one to my boss since I measured 7.8 V across his battery before jump-starting his SUV last week. He said he suspected his battery was going south.

 

[67Dart273]

14.8??

 

[BillGrissom]

14.8 V does sounds a bit high, but I am no expert on battery chemistry. My guess is that the 2.2 V above a fully charged battery (12.6 V) does not cause more current flow into it, which would "boil off" the water (convert H2O into H2 and O2 gases). The electro-chemistry must be similar to a diode effect. If you went higher in alternator output, you would start to dissociate the water.

I suspect most voltage regulators are set to control to ~14.3 V. Of course they control to the voltage drop they sense, which is where most A-body owner's problems arise since the sensed voltage is lower than true battery voltage, due to drops in corroded wiring.

 

[RedFish]

I suspect the 14.8 was put there to prevent users from panicing when they did see brief high end reports. Newer vehicles with headlight delay, security systems, etc.. will use up some battery charge after the charging system stopped. So if you run out there and fire it up, turn on, heat defrost and rear defrost, and seat heaters, and on and on, a high voltage could be seen for some length of time.

 

[KitCarlson]

It depends on battery temperature and charge time. The voltage can be higher when cold, but if charged cold, if it warms up it will be overcharged. In general automotive systems are not optimal, they are cheap. When was the last time you found a battery temperature sensor, or others charge strategies found on stationary batteries.