The 248 has a single 12V battery inside...two wires connecting the battery to the rest of the preamp.
I'm no electrician (caveat emptor), but pretty sure all you need to do is wire up a connector with a 'parallel' wiring configuration (as opposed to 'series'...electricity 101) and then hook up an external battery that supplies 12V with a connector that has the same polarity as the internal battery.
My thoughts were to either drill a hole in the side of the 248 box and run the wires outside of the box to the connector, or just find a spare location on the exterior of the 248 box to mount the connector onto and wire it up on the inside. The external battery of course would have a connector that mates up to this connector. You'd also need a battery charger...the charger would be wired up with the same connector as 248 (not the battery).