I recently asked about a mystery component on the {lead acid} battery box of my ancient Powabyke . On the outside of the box is a large screw head of the type you would unscrew with a coin . This is not mentioned in the instruction book , but it appears in photos of many early models including the shopper . Yesterday , I unscrewed it . It is actually a circular cover , and underneath is a four pin socket . Its purpose remains a mystery . Is it for plugging in some kind of diagnosis equipment ?