The Max-Tow Voltmeter I installed on my 1500 normally reads 12.9 -13.1 volts if you've just started it and it is recharging and then settles in around 13.8 going down the road. It has an undersized AGM battery in it, correct width and height but I have a piece of 1x6 board tucked in behind the battery to keep it from sliding around in the battery box.
I suspect the battery may be going out, It was in the bike when I bought it almost 4 years ago, who knows how old it is... a few weeks ago I went to start the bike and found it dead. Not "flat", though, voltmeter read about 11 volts on "ACC" but when switched to "ON" sunk to 8 volts. I put my 2/10/50 amp battery booster/starter on it for a few minutes on the "Start" setting and was shortly on my way.
Surprised to find the dead battery, it had only been a week since the bike was last ridden. I seldom bother with the battery tender in the summer when the bike gets ridden at least once a week. But I have it on the tender now. There's no parasitic draw on the battery beyond the OEM load, so the battery should be able to stand many weeks without battery tender (it has in the past) and still start the bike. The electronic voltmeter has a "sense" lead connected directly to the battery, but that draws nothing, the meter gets it operating power from the ACC terminals.
Since this incident the battery still starts the bike with gusto and vigor, so I guess it's OK for now, and I'm using the Battery Tender when parked. But going down like that in only a week leads me to think it's days are numbered....