On Wed, 18 Feb 2009 00:42:34 +0000 (UTC), David Lesher
wrote:

Then use it to measure the battery voltage ON the battery terminals.
With the master off, etc. it should be at least 13.5 volts.

Unlikely to reach 13.5 volts unless the engine is running, or the battery
is connected to a charger. (Or it is very cold).

For a lead-acid battery, fully charged at 78°F, voltage should read about
2.12VPC (volts per cell) * 6 cells = 12.72V.

An AGM style battery might be slightly higher, but I don't think it's
higher than 12.8V.
--ron