The Solargizers work a little differently from what you might think. They require battery voltage to operate their electroncs! So, that 24 v unit might not work on a 12v battery (what you are doing if the solargizer is hooked up to only one 12v battery). Or, you could have a bad Solargizer. I have found dead ones.
The solar panel puts out only around 6 volts, as I recall, and the electronics build a charge then discharge across the battery, raising the battery voltage with a high voltage spike added to the battery voltage. That's probably what your voltmeter is seeing. Modern voltmeters tend to be peak-readers on short pulses.
Not much energy is transferred from the Solargizer to the battery so almost zip net charging takes place. The purpose is to break up sulfation.
Somewhere on here is a description of some testing I did years ago, o'scope and all, but I don't find it.
[the above is a SWAG based on actual testing. It's my opinion and I'm sticking to it
]
Found the earlier post: Good info on Solargizers:
https://www.steelsoldiers.com/showthread.php?10126-Solargizer/page6