2006 Hawkeye 300 2×4.

The battery has been bad since I got it a couple months ago and I have just been using the pull start with no problem. My daughter was riding around the neighborhood and I thought I would pull the battery, charge it up and check the voltage and what-not. She pulled in the driveway, still running I pulled the battery and then she took off down the street. She got about 100 yards out and it sounded like it backfired and died. I went down there and couldn’t get it started so I pushed it back to the garage. It wouldn’t start and smelled like something electrical was hot or burned. The noise Im not sure if it was backfire or maybe an electrical pop now. I put the battery back in and still wouldn’t start. I put the battery on the tender and it was around 6v this morning when I checked it. Do these have to have the battery (good or bad) in to run correctly? What should I check to find what went wrong and isn’t running now? I have a battery from my Seadoo that I could wire up for temporary. Any ideas?