I had a customer on a 50' Swan sloop ask me aboard. "See this wire" he instructed "It stays hot whether the battery switch is off or on. To illustrate the point he went below and removed the master negative feed able from his Lifeline bank.
"Now watch"
He disconnected the 6 gauge power cable at the nav station dedicated 50 amp breaker and struck it against the side of the negative bus bar.
Hot blue flashes. Again and again.
"There are no other batteries on the boat" he explained. "This cannot be happening".
Laying on my back on the teak, I slid open a floor-level cupboard. Strapped to the cubby overhead, six three-farad capacitors had been fastened. The previous owner of the boat must have been an audiophile.