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- it happened after I left the bike sitting for 6 months and I have replaced the battery twice since - both of which are now totally dead after a few days each.
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Was the battery left on a "trickle charger" during that 6 months, to maintain the battery condition/charge. They don't "charge" constantly, just monitor the level and periodically condition it plus maintain an optimum voltage.
Not running any other little gadgets on the bike hidden away, connected direct to the battery, that might have inadvertently drained it - lights / security / etc?
A multimeter really will help you narrow down the potential problem here... get poking.
Grounded wire or bad switch perhaps.