In addition to battery charger, you need to get
multimeter for testing and troubleshooting. You
CANNOT run battery for more than 2-3 seconds at time. Total 30-seconds
MAXIMUM. Otherwise you'll drain it beyond 50% and this will destroy battery. Seems you've discharged it to 0% multiple times already, so that battery is most likely bad and you'll need to buy new battery.
Any time you're having difficulty starting bike beyond 30-seconds, use jumper-cables from car's battery (with car
OFF). Otherwise you'll destroy VRM - voltage regulator.
Multimeter will let you test and troubleshoot parts on bike to determine if they're good or bad, based upon numbers you measure. Sure you can randomly start replacing perfectly-working parts with brand-new perfectly working parts, but nothing will change, your bike will still not start and your battery can't get recharged unless bike is running for 15-minutes or more.
If you don't want to test and measure and learn to troubleshoot, but would rather just randomly replace parts. Then just sell bike and buy one that's working. Will end up costing you A LOT less and waste A LOT less time.
There's
nothing wrong with parts on your bike. Just need to test and measure and re-adjust things. But you can't determine what needs adjustments until you measure and come up with some
numbers. Get multimeter and do those tests outlined above. And this one as well:
1. measure voltage of battery at rest before cranking.
2. measure voltage of battery when cranking.