Hi all - thanks for your responses so far.
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Trackten - the info below is from an associated thread - 12 V CASATRONIC ISSUES -
viewtopic.php?f=10&t=8406I bought a burglar alarm 12 v, 2.1a battery . I made a bracket and secured the battery with Velcro straps.
I then wired the battery charge output from the rectifier, to the battery positive terminal via a 1.5A in line fuse.
I wired the battery negative terminal to a earth wire attached to the earthing point on the frame.
I then ran a wire to the headset via another 1.5A fuse from the battery positive terminal
I was given an old ignition switch (turns out its actually a Beedspeed ACDC switch) I used a multimeter to work out the switch positions, connected the positive battery wire as the power input, and the wire that corresponds to fully clockwise as the switched power feed to the SIP power unit.
I also connected the green kill wire to the switch so I can turn off the engine when the key position is "off", as I have replaced the button with the switch.
So I turn the key fully clock wise, the speedo fires up powered by the battery, and is unaffected by lights or low revs.
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@ Burnside - I did have a USB adapter connected via a MODCHARGER, that has a LED in it, hidden in the toolbox. I have disconnected the adapter completely now and hope that was the cause - I will update this thread in a couple of weeks.
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@Sticky - If only you posted that article a month ago! Its much more elegant, simpler and cheaper than the battery + ACDC switch + fuses holder option ( total approx. £60)