Battery Problems

Bought my '82 xs a year ago and have put 5000 miles on it. Don't know how old the battery was when I bought it, but I have always kept it on a tender. Last week it reached the point where it wouldn't turn the starter and wouldn't hold a charge. So I replaced it this morning and all is good. But I have a buddy who tells me that if the stator is good that I should be able to disconnect the negative battery cable while the bike is running and it should stay running. Well mine doesn't stay running. Is my stator bad?

Also, my kick start works great normally, but when the battery is dead it won't start it. Why?

82 with electronic ignition needs a good solid 12 volts to make sparks. The alternator isn't strong enough to carry all the loads till about 1500 RPM. XS650 uses an electromagnet in the alternator you need a battery to juice the electromagnet to start charging so you can't kick or push start an XS650 with a dead battery. You should have a voltmeter hooked up where you can see it. Checking the alternator brushes is a normal maintenance item, if you have never checked them NOW is the time. Do NOT run with a failing battery, it places a heavy load on the alternator causing it to run hot and increasing odds of component failure.

Will check those brushes. Thanks for the explanation and info!