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How do you know if the replacement 12V battery is a good one?

The 000 915 105 DE one I got for my 2008 Jetta 2.5L SE appears to have the "9/16" sticker on it, as well as "3216" inscription (which may correspond to 2016-08-08/14), and it's 2016-10-01 now, but it would only test about 12.37V under the minimal load of just the key being on when it was just installed. It would appear that that may correspond to about 70% charge.

As the battery was already connected, and the car was off with the key removed, the dealership tested the battery with their VAS 6161 tool per my request, and it then read:

    GOOD BATTERY

VOLTAGE        12.45V
MEASURED   436 A(DIN)
RATED      330 A(DIN)
TEMPERATURE      35°C

Should I be concerned, or is it within the spec to not have the brand new battery read 12.7V to 13.1V upon installation and/or with minimal load of just the key being on (ac/stereo/lights off)?

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I would expect the battery to read around 12.6 V (2.1 V x 6 cells) if it was fully, and recently, charged – say after a drive of an hour or so. If the battery has been sitting for a while, either in or out of the car the voltage will drop. Batteries self-discharge and most cars have a small current draw even when off.

The charging rate of a battery tapers off as it approaches a full charge, so the last little bit takes much longer than the part in the middle. So, after a short drive the battery may not have recovered completely from the load of starting the engine. If you find that a new battery never takes a full charge it would be worth checking with the manufacturer or the dealer and asking for a new one – or at least asking them to note that the battery did not appear to take a full charge when new so that you have a better chance at a full value replacement if you need it.

Also keep in mind that the reading your get on a meter is only approximate unless the meter has been calibrated or at least checked against a known voltage.

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