This is a follow-on to the "electrical problems make my head hurt" thread. I did some testing, and realized that I had posted some erroneous information in that thread, so I want to set the record straight.
This test is of a healthy 11AH AGM battery after charging, with a voltmeter across the battery terminals. Static voltage (nothing on) is 12.8 volts. When I turn on the ignition, the voltage drops to 12.3, but then immediately climbs back up to 12.5. (This is apparently the result of the ignition module energizing the coil and then de-energizing when no signal is received from the trigger mechanism.) Now I turn on the headlight, which is a sealed-beam automotive headlamp, I believe 45-55 Watts. Note, the taillight is also on, and of course the ignition is still on. Voltage drops to and remains steady at 12.1.
When everything is disconnected (turned off), battery voltage recovers to 12.7.
An unhealthy battery will read lower when loaded, to a varying degree, and won't recover into the high 12's when unloaded.