I've used a shunt regulator to limit voltage across a battery under
charge, and such might help here (with a Zener/fuse crowbar for
insurance if the regulator fails open). This approach does require a
good heat sink, however; you may have to dump all 23 or 30 watts into it.
Brian N0KZ wrote:
>was disappointed to learn that the W4RT charger doesn't like voltages
>greater than 16 volts, since that pretty much makes solar power
>impractical (required: lead acid battery, solar charge controller).