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Temp effect on alternator output and batter voltage at rest

My battery is about 5 months old, and this morning I was surprised to see that it was flickering between Middle and Full on my 12V LED Digital Battery Tester. When I started the engine, I noticed the alternator output jumped around between 13.7 to 14V which seem a little higher than during summer when it was about 13.4 to 13.6V. Is it normal for the alternator output to go up a little as the weather starts to cool down? When I drive the car for about 3 minutes, and turned off the engine, and then the battery voltage at rest was about 12.46V. Is this normal?

Somehow I have managed to live without a device such as a Digital battery tester. It seems to provide a way to look for problems for no reason. If the vehicle started like it should just drive on.

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Well, I went out to check one more time after having breakfast for about 30min, and the battery resting voltage was down to 12.36V from earlier 12.46V. Should I start considering buying new battery before the real winter arrives?

I’m with Volvo on this one

Sounds to me like everything is in order

Take the vehicle to a parts store that tests batteries/charging systems.

A five month old battery should hold a static charge of about 12.6 volts.

So either the battery isn’t holding a charge, or there’s a current draw on the battery.


The quiescent battery voltage (first thing in the am with engine & everything else turned off) changes a little with temperature, and different manufacturers of batteries use slightly different chemistries, and that can affect the voltage too. So measuring that voltage isn’t very meaningful. To test a battery’s condition, that’s done w/ a load or conductance test. The voltage output of the alternator does depend on ambient temperature too, but mostly it depends on how close to fully charged the battery is at the time. If the battery cranks the engine robustly and consistently on the first start of the day, that’s nearly as good as doing a battery load test to determine the battery is good. It also shows the alternator is doing its job too.

lead-acid batteries per-cell floating voltage varies with temperature.

if you want to read more:

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