If the open circuit voltage of the solar panel under the present illumination is greater than the battery voltage, then the panel will provide as much current as it can at the battery voltage. The problem would be in finding a ~30V panel, although you might connect several in series to get that.
This will not harm, or charge, the battery because of the battery diode. You just have to be sure that the absolute maximum voltage the panel(s) can produce at high noon on a clear day on the summer solstice is not more than the mower circuits can tolerate.
I don't know what happens if you use a boost circuit on the panel output. It might work, but I'm not sure.