The following circuits will allow you to charge a backup battery from your 30A power supply,
AND allow for automatic changeover when the AC mains fail!
This first circuit is built with individual diodes:
This second circuit is built with a 30A Bridge Rectifier:
(FUSE ALL POSITIVE LEADS AT LEAST!)
This is the same circuit as above, with a pilot light added to show when charging:
(FUSE ALL POSITIVE LEADS AT LEAST!)
BUILD & USE AT YOUR OWN RISK!
GOOD CONSTRUCTION AND OPERATION PRACTICES REQUIRED!