04' briggs ELS charging issues

ive got a huskee lawn tractor, my grass cutter. it is powered by an 18 horse brigs ELS single cylinder. i got it for free back in the spring. it ran with a jump start, and the muffler had fallen off. i fixed the muffler, and just kept jumping it with my other mower, a toro Proline walkbehind. this was getting old, so i broke down and bought a battery. not a fancy high dollar interstate or diehard or anything, just a plain old napa lawn & garden battery. i was mowing with it a few weeks back, and i noticed the battery light on the dash was flashing, and the voltave was showing in the info center.. 20+ volts!!! so i shut it down, let it set for a few hours, went back and it fired right up, finshed mowing, and it was still overcharging to about 18 volts! i hadent touched it since then up until about a week ago, to haul some brush around in a dump cart. usually i use my mudder (bolens G-10) to tow with, but i need to get a belt for that. anyway, it cranked over like nobodys buisness. i ran it for over an hour at about 3/4 throttle, and some time at full (i got stuck, perhaps on purpose ) i noticed it was staying about 13-14 volts. so i dont know what happened. now, yesterday, i went to mow with it again, and the battery was stone cold dead.. not an ounce of crank. so i jumped it, and it was fine. charges up to about 13.5 volts, but drops off to 5 volts when the mower shuts down. i jumped it again, and put a 12 volt light on it (a small headlight) and the voltage drops off and the engine dies. i dont know what happened, but something is frigged over. if yall have any ideas, it would be greatly appreciated.. thanks -will

I know we talked about the overcharging before, and discussed some ideas. I don't remember the outcome. Now, however, it would appear that the constant and severe overcharge boiled the battery, and now it's fried. I believe it's time for a new battery, but it is going to start trying to overcharge again?

i guess i'll get one from napa again, theyre pretty cheap, so its not a big deal if it cooks again. i might wire up an ammeter and a tie in a good volt meter somewhere so i can tell whats going on and keep an eye on things. if it keeps doing it, i think it might be time to pull the flywheel and inspect the magnets and what not.. i remember the proline did it once, (20 horse kohler command twin) and it broke a magnet. these ones might just be rusty or something from sitting. idk, im not strong with electrical