Charging a battery backwards, discharging it, and recharging properly can do a little permanent damage to the cell groups, but since you caught it quickly, maybe things will be OK. There's a sneaky way to charge a 6V battery with a 12V charger, but it also requires a 12V battery with removable cell covers. It's definitely not a trick for a rookie to attempt, so I'm hesitant to post the procedure. PM me for more information.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!