Is the charging device on a ZD21 an AC or a DC generator? Reason I ask is because when mine is running it does not appear to be charging the battery at a reasonable voltage. I bought it used and the previous owner has some sort of direct wiring from it to the battery. No rectifier. I'm not familiar with how it was originally. I've run some basic tests but I can't determine if it's putting out AC or inadequate DC to keep the battery topped off. Thanks!