Not a problem. "12V AC" is usually 12V RMS - meaning nearly 18V peak. A
full-wave rectifier will out put two nice 18V peaks per cycle that will
charge a 12V
battery just fine.

Lloyd Sumpter
"Far Cove" Catalina 36

The problem might actually be "too much".
These unregulated systems tend to boil the water out of batteries.