My house was hit by lightning. At that moment, I had two cars hooked up to a Deltran dual battery tender. The circuit that the battery tender was on flipped the breaker, but not before melting the back on the GFI plug that was in that circuit and tripping the inline fuse on the battery tender that was hooked up to one of the cars. The line to my 69 Charger did not blow its inline fuse.

After the hit, the resto appearing turbostart AGM battery in the Charger wouldn't take a charge. Figuring it was damaged, I swapped it out for a new one. I also have a new battery tender (and three new TV's, new Time Warner equipment, and a new mother board on the furnace after all of this). The car runs fine, starts fine, charges fine.

BUT...and here is the question...the new battery on the Charger never seems to get to full charge. Measured at the battery terminals, volts from the battery tender are at about 13.5, from the car running on it's own at about 13.9, right around 12 at the battery when unhooked to the car or anything. Is it possible that I have a short somewhere in the Charger that is keeping a low, steady, drain on the battery? Where would I even look? Visually, all seems OK.

I do not think it is a short in the Charger. To test it, remove both battery connections and run the maintainer straight across the battery and measure the voltage. If it does come up, then there is indeed something in the Charger pulling it down. The first place I would look there would be the alternator. If one of the diodes is leaking, it could bring the voltage down.
If you have the same voltage results, still being low, then your battery is bad. Not uncommon to find two of them in a row bad.
Craig

You can also take the negative terminal off of the battery. Then put a test light from the negative cable to the negative post so the circuit is complete.. The light should be out. To check open a door and watch the light. it should light brightly. Kevin

Does your car have a clock inside it? If so that will draw a tiny bit of current making the battery slowly discharge over time If not it shouldn't have any current flow with the ignition switch off and the key removed unless you had a security system installed in it.

If I understand correctly, with the tender on you measure 13.5 volts. If you pull it off you measure 12.0 volts soon after with batt terminals removed? I assume your meter is calibrated or close and reads 12.5+ on other battery's.

If that is it, sounds like it could be several things (easy things first) 1) new battery is not yet fully charged. Leave it on the tender and try again or better yet use a known good conventional slow rate battery charger and measure again. 2) Battery terminals have high resistance... poor connection or dirty 3) Battery is bad.

End result, at least for now. I was probably just impatient. The battery now reads fully charged. I took the car out a few times and it starts well, the in dash Alt gage settles down like it should.

This lightning strike has been pretty interesting. No physical damage to the house, so we were lucky. But we keep finding mostly electronic things that are damaged. We have a in-ground pool, we are now up to the mother board, the control panel board and the board in the heating unit.