Incidentally, I just swapped out batteries in our club radio repeaters, and looked at the backup charger that we use. The repeater radios draw 12 volts, and needed a way for the repeater power supply to fail over to battery power. Whoever designed the system installed Samlex BBM-12100 units. This takes the incoming power supply feed, passes it through to the powered unit, and charges the attached batteries at the same time. If the power supply shuts off, the batteries are seamlessly connected to the load.

http://www.samlexamerica.com/customer_support/pdf/Manuals/BBM-12100_Manual_Jun2005.pdf