I have a 2005 Honda Odyssey that I changed the battery on about a month ago. I was a little paranoid that I might also have a charging problem so I've been keeping a little voltmeter plugged into the cigarette lighter.
I've observed a really strange pattern. When I start it up, for the first few minutes everything looks as I would expect, 14 Volts or so. Then it seems that the battery voltage intermittent. It will jump between 12.5 V up to 14 Volts, but mostly around 12.5 Volts. Note that it jumps between these two voltage ranges it doesn't gradually move up and down. If the voltage at the cigarette lighter is right it's not charging the battery most of the time. However, if I shift into park or neutral it's back to a steady 14ish Volts. Also, once I get over 65 mph it's a steady 14 Volts. The voltage doesn't really follow rpm, it's vehicle speed. If I manually shift down into a lower gear at, say, 55 mph the voltage stays around 12.5 Volts.
That's with the lights off. With the headlights on I see a steady 14ish Volts at all speeds and in all gears. Any idea what's going on?