My friend has a 1983 Yamaha Maxim 550, its his first bike and he got it just 2 weeks ago.
Well, he forgot to shut off the key so his battery went dead.

We jumped the bike, got it running for a few minutes. But when I took the cables off, the bike stayed on for 30 sec more, then just died.
We tried jumping it again, with the same result. Once the jumper cables are off, the bike would just die.

Does anyone know what would cause this?
Im guessing some relay or electronic component. However, mechanics would be my area of expertise, and electronics my Achilles heel. So, truth be told.. i dont have a clue

The extent of my knowledge being that once running, if it where only the battery, it should stay running until the key or kill switch is turned.

Any help would be very much appreciated,

hes eager to get back on the road

Battery is toast,you can try charging for a day and see if will come back but mostlikely not.

If the battery go flat line it can mess with the voltage regulator,put a good battery in and check to make sure the charging system is good,anything above 13 v is ok at @2000rpm unless it goes over 15v.

Good luck let us know how you make out.