I run three old triumphs one positive earth 12 volts with a alternator ,coil ignition and was originally 6 volts when it left triumphs 1957 (lead acid 5 years old and counting) the other two are mag/dyno system and are both negative earth 6 volts (one gel and the other a AGM type) and I just leave them on the bike if there flat I just charge them and carry on as normal and one bike is left outside in all the year around and as for long term leaving petrol in the tank I put some two stroke oil or paraffin or any thing oily have at hand. I have drained the shadow tank dry as it stored indoors (fumes) Batteries are cheap enough these days so why fuss and they made cheaply as well and do not last as long as they did in the past and you can buy AGM battery now for under £20 and they fit inside a old battery box with lots of amp hours