The best results I've had is with LARGE capacitance banks and only charging 2-3 volts above the battery voltage....around 14-16 or so. What is the largest capacitance you can come up with with the caps you have? You can probably find some 33,000 uf 60v's on ebay pretty cheap. I found some years ago on ebay for about $5 each.

With too high of voltage pulsing too quick, there isn't enough current to get the batts in real charging mode.