The shumacher chargers seem to have the highest voltage of them all,....over 16 volts on my larger one, and well over 15 volts on the smallest maintainer ones.
I try to avoid using the big 16v one, as no battery needs over 15 volts.
If you want long battery life, and very little water consumption, connect to a PD converter, and forget it.
If you got suckered into the hi voltage boxes, and believing you have to babysit and sleep w/ your batteries, good luck.
13 yrs out of a pair of batteries ??? Very possible, but you can bet they have never been fried w/ hi voltage boxes that were never designed to be battery chargers.