Fortunately Deltran has this nice page that introduces you to charging algorithms:
And we can see that the float voltage is 13.2 or maybe 13.3 or maybe 13.4 or something along those lines. Oddly they delve in to this information w/o giving actual specs.
Since Deltran is reluctant to publish its actual float voltage we turn to Battery Stuff who reports 13.3 float (not guaranteed)
Therefore your goal should be to find an old technology flooded cell battery that is compatible with 13.3 float.
Comforting quotes from the world of Deltran(exact quote):
" For example, the Battery Tender? Plus (part of a different product line) switches out of absorption mode when the charge current falls below 100 milliamps (or 0.1 amp) or when the absorption mode has lasted for 8 hours. With properly set timers or charge current switch thresholds, the battery should be charged to more than 95% at the end of absorption mode. Ideally it would be at 100%, but there are some practical limitations that usually prevent full recharge."
Yes on their FAQ page they insert a ?. Attention Whovians, step right up and plug in your Tardis.