batteries - Backup battery charger

I'm trying to design some kind of backup battery charger - please see the image:

Normally, the DC supply is ON, and provides current for load and also charges the battery. Battery should be charged to 27V or more, and maximum charging current must be limited to 4 A (when battery is fully charged up the current should automatically decrease to 0).

When DC supply is turned OFF, the battery provides current for load instead, with maximum allowed current.

I'd like to ask You for any references, circuit ideas or even some keywords to look for on internet and other sources.

Thanks.