I hate internal batteries arranged so you have to slacken the strings to change them. And I hate cutting holes in the ribs of an instrument to fit an externally accessible control panel or battery box. So here’s how I arrange for a standard internal preamp to use an external battery.
Acoustic electric instruments usually connect using a stereo jack socket into which you plug a mono jack. The socket tip is the signal, the sleeve, as usual, is ground. The ring – the extra connector a stereo or balanced jack has between the signal and ground – connects to the battery negative.
tip = signal
ring = internal battery -ve
sleeve = ground
Inside the instrument the battery positive is connected to the preamp. Inserting a standard mono jack connects the battery negative (ring on the socket) to ground and switches the preamp on because the jack plug ground bridges the ring and sleeve connectors on the socket.
Now if you remove the instrument’s internal battery and short out the battery connector inside the instrument the stereo socket has the following connections:
tip = signal
ring = external battery +ve
sleeve = ground, battery -ve
It is then easy to wire up an external battery box to power the internal preamp.
However one problem is that you if you fit the battery box with a stereo jack to connect the instrument and by accident insert a mono jack this will connect the ring to the sleeve and the battery will be short circuited. To avoid this I wire up my box with flying leads, a stereo jack to go to the instrument and a mono jack to go to the amp or DI box etc.. The screens of the two cables and the battery negative are soldered together and insulated with shrink wrap. The signal wires are joined together and the battery positive is connected to the wire that goes to the ring of the stereo lead. The cables are tie wrapped together and the whole assembly slips into the battery pouch that came with the preamp.
Inside the instrument I snip off the battery connector and solder the wires together but if you don’t want to do anything irreversible you could easily make up a battery shorting connector. By the way, the extra set of wires in the photo – black / red / yellow – are for the B-Band volume control.
Caveats. This isn’t the kind of thing to attempt if you don’t have some understanding and experience of electronics and wiring. I’ve done this to several instruments but you need to check that the wiring is as I describe, specifically that the “ring” terminal goes to the battery negative. I’ve never seen a different arrangement but that doesn’t mean they don’t exist! The preamp shown is a B-Band with the preamp mounted in the jack socket.