I had to do this about 2 months ago… friend of mine who had been riding bikes for years suggested it and was kind enough to push… unfortunately the battery was so far gone that it didn’t help… but it sure was interesting the tachometer went haywire and would point straight down (which is way below 0). I think that the reason it wouldn’t stay started was because the headlight was sucking up all the juice that would otherwise be charging the depleted battery… I am thinking of adding a switch somewhere to be able to completely turn off the headlight.
if you are jump starting from a car (which does work), don’t turn the car on…. the car battery has enough juice to jump start the bike… but the car alternator will overpower the tiny motorcycle alternator and fry it (also, it just dumps excess power into your chassis, no point heating it up)
I had to come back the next day and take the battery out and plug it in… thankfully I work where there is a lab… definitely got some weird looks when I hooked up a battery to a power supply (not ideal, but if you apply the floating voltage to a lead acid battery it will charge)