Metal, eh? sounds like a battery killer to me. The plates are arranged +-+-+-+-+-, and lead acid batteries eventually die because as the lead gets plated back onto the plates, it tends to form needle like crystals that eventually grow long enough to short out the cell. You just dropped in a piece of loose metal probably plenty big enough to short it out by itself. If it didn't settle into "the right spot" already, it is just waiting to get shaken into that spot.
The problem is, if you can't see it, you can't retrieve it either. Sooner or later it'll make contact, and when that happens, the battery dies.
On the assumption that that washer was steel, it is also likely being eaten away by the acid. This is both good and bad, because of course, it could get eaten completely by the acid before it shorts out the cell. But that's bad, because it will mess up the chemistry of the cell, too.
Sigh. All I can say is, "WE'RE ALL GONNA DIE!"