This video shows how to change / replace the clock battery in a Vespa ET2 / ET4 with a regular 1.5 Volt AA Mignon battery (or any other common 1.5 V battery). Solution also possible for other Vespas e.g. LX, GTS
Have tried this out a year.back. It's a great idea, as long as the cable is thin. The battery is only 1.5 volt, so do not use a too thicker one that''s used on 12volt car wiring. It will drain very fast
Very helpful video but the one improvement I'd suggest is this. At the bottom of the black left hand knee pad in front of the rider is a Phillips screw. Undo that and pull the knee pad off, there are a couple of fuses in a holder in here but also enough room to place an AA battery holder, and a hole through which the wires will pass. It would just keep it a bit neater and less intrusive.