Weird symptoms so linked to video https://www.reddit.com/r/MotorcycleMechanics/s/SMrzTB174x

Yup. Weak battery.  Probably sat on the shelf too long after manufacturing and before selling.

Take it to auto parts store and have them test it for a dead cell.  If it’s ok, they usually offer to charge it for you for free if you don’t have a charger.  A battery can read 12.6+ volts after a fresh charge but still have a dead/bad cell.

I like to see 13.2+ volts fresh off the charger.  The voltage relaxes to something lower than that after about 20-30 minutes off the charger.  Then I like to see it read somewhere around 12.8+ volts.  I’m picky.  It it can’t stay above 12.6v, I start shopping for a new one.

These cheap Amazon batteries have served me well on a few project S40s.  They even have a warranty!  Not sure how the warranty would work with these mail order Amazon specials.  I haven’t needed to take them up on the warranty offer.  I always put them of the automatic trickle charger over night when I first get ‘em.  No problems.  Work well.

I know some people like to spend $200+ on a lithium battery, but that ain’t me.

https://www.amazon.com/dp/B008BTMJ4M/?coliid=I12H950UYFLUUQ&colid=1CBXBA6JVIT...

I want to mention something else that experts out here pointed out to me some time back.  You might get what feels like a strong crank from your battery (not the case in this video), but the bike still doesn't start.  What happens is the battery has enough power to crank the bike but is just short of providing enough spark for the ignition.  I remember not being able to start and taking somebody's advice to charge the battery and try again.  That worked!  Another tip:  If you jump start from a car, do NOT have the car running.  You can ruin the voltage regulator or who knows what else.  I did this on my Sportster