Im willing to let the past go as long as he has changed like he said. Everyone deserves a second chance until they prove otherwise. With that being said here are few things If the battery is 100% dead, i.e. Shot, no amount of charging will help. 10,20, even 50amp engine start will not harm a battery as long as its not left on for an extended period and you have no other wiring issues. Are you sure you hooked up the leads correctly. Charging in reverse will smoke a battery in a heartbeat. FYI, I use 10amp all the time and even 50amp start if I need to.