First, 13.8v is the correct voltage to float charge a gell cell. But
reading an open circuit charging lead might not be the way to check it,
as the regulators got kinda smart a decade and a half ago. I would
look for the current going to the battery and read the voltage with the
charger connected. The higher the voltage of the the battery, the less
current should be flowing, with a max of 250 ma.
Re: the flashing LED issue, the low battery threshold that usually trips
the sensors is about 11.5v. There might be something else causing the
LED to flash, such as a blown fuse. Take a voltmeter reading across
each fuse. Al should read zero volts (no IR drop across a dead short).
You might also read to "do something" to "reset" the flashing LED such
as arm/disarm, press # or something else. CADDX like arming disarming
to clear issues.
Hope that helps,
John Sowden
American Sentry Systems, Inc.