Today I acquired an adapter for electrical motorcycle accessories, which has a cigarette lighter socket, two USB outlets, and a voltage gage which was the main reason for the purchase.
Its a nice tidy little waterproof rated unit, has handle bar and flat mounting plates, and an on/off switch. The voltage gage even has a clear display window in the rubber cover plate. The 140cm cable is fused and complete with terminal ends. Cost less than £10.oo delivered.
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Take care that the voltage gauge doesn't run all the time as it will drain your battery!