Anotehr advantage of GFI, if it trips you need to clean your batteries ASAP. Not only are you leaking voltage while charging, but your battery pack is no longer isolated from your frame.If the GFI fails a grounded frame will still protect you. It will pass back all the ground leakage current, and if its really bad (a short) the service breaker will blow from the current.
When the pack is completely isolated you can touch ANY battery terminal (one at a time) with no danger. Once you get leakage current, this is no longer true. The GFI will warn you of this situation.
