Rob -- Yes, that's true but the reverse polarity and 'all technology' comments are totally wrong. The real point is that this is not such new technology and problems have been in the field for a long time, in non-aviation applications. Knowing how much testing goes into aircraft components, these battery systems must have been cycled hundreds of thousands of times during development (not each battery, of course!). Knowing that the survival of the entire type could be in jeopardy they must have looked as all possible reasons, and this will have been overseen by senior engineers, specialists in the field of batteries and charging systems. If I was Boeing I'd be spending 30% of the investigation on the possibilities of sabotage - don't know how, where or when but it has to be a possibility.
Otherwise:
Note that the problems have been on the ground where risk to life and health is lower - a saboteur might draw the line at that. Otherwise, batteries will usually draw the highest charging current when charging starts and rapidly fall-off (though high-charge rate designs often charge to a maximum internal temperature rather than simply terminal volts). If the batteries are fully-charged the charging current is usually low.
Is there a large load in the powered-down condition? When cleaners and maintenance are on board?