What's an acceptable battery power drop on light load?

Question

After an overnight rest in the warm weather (above 16 degC), unlocking my 2008 Jetta 2.5L SE, my OEM battery with the minimal "welcome" electronics -- remote open of the driver door a few seconds earlier; automatic turn on of the electronic speedometer and the foot lights (I keep the dome lights turned off through a switch); open hood -- measures 12.15V under such load through a 10-dollar voltmeter in the 20V mode.

However, a few seconds afterwards, the voltage suddenly jumps directly from 12.15V to about 12.27V, and then slowly climbs to 12.32V or so. I suspect this happens when the foot lights and the speedometer go off.

It would appear that 12.32V would put the battery into the 60% charge according to most of the State of Charge charts.

Is it fair to say that my battery has finally been resurrected? (I haven't had to jump start it in months, but, then again, overnight temperatures have not been much below about 10degC for quite a long time now; I also suspect that the extra hot temperatures in the daytime may have helped to dissolve the crystallisation.)

Answer 1

Almost any amount of load you put on the battery is going to drop the voltage. How much depends on the capacity of the battery as well as the health.

It would appear that 12.32V would put the battery into the 60% charge according to most of the State of Charge charts.

State of charge charts are referring to open circuit voltage (without any load on the battery)

See my answer How to test a car battery for more information on testing the battery, as well as the state of charge.

Answer 2

It very well could be the battery has what is called a surface charge. The battery will have enough power to start the vehicle, but if too much of a load is put onto it, the voltage will drop and not be enough to run the vehicle with or with out a good alternator. The battery will basically be sucking the alternator dry. When this happens, the wipers will start running slower, the lights will dim dramatically, and all heck breaks loose (not literally, I just liked throwing the cliche out there). A battery will behave better in warmer weather than it will in cold weather. Nominally, a good battery should be around 13.1-13.5vdc at rest (no load, fully charged). From the sounds of it, yours is a little under the weather. If I were you, I'd plan on replacing it soon, as it may have issues when you least expect it or can least afford it.