I have a Misubishi Canter, it's over 4 years old. I have had problems with the battery draining since new. I had 4 batteries in first 3 1/2 years, the next lasted 4 months, then 2 weeks, then 1 week, now it won't stay charged for a day. I have bought good batteries, had it checked out in many workshops, it's been to electrical specialists and no one can find anything wrong. Anyone got any ideas as to what could be draining the battery?