@lusher00
Stop, rewind. iPhone 6+ series (6+ and 6S+) have such huge batteries that it's almost never the original battery that's failing unless you damage it say by prying at it, or from liquid ingress. I have yet to see a bad 6+ battery that just dies on its own, and 6S+ are more recent.
On the other hand, bad replacement batteries are THE RULE, not the exception. In addition, the symptoms you are experiencing are exactly those of a bad battery: reporting wrong percentage, and boot-looping (restarting).
So I hope you did not damage or throw away the old battery and I would definitely put it back if I were you, or get a replacement one from iFixit.com (they are my go-to supplier for good quality batteries and if a bad one slips through their QC, you're covered by their warranty. IMO it is way cheaper to pay a bit more, install once, than ruin the phone and repeat the job a few times.
Second, Tristar symptoms, among others, are: no or spotty connection between computer and iPhone; slow charging + fast depleting; no charging or saying it is charging but no increase in battery %, and sometimes decrease in %; phone works, charges if battery is partially charged, but unable to charge a battery from 0% and restart the phone.
And finally two words of advice, from someone who's been into iPhone board repair for a couple of years:
- Never ever replace anything in a phone that's not proven bad, you may get yourself in trouble just like now.
- If you're intend on trying micro-soldering and micro-BGA rework, train on scrap boards of the same configuration and the same thermal mass as the device you want to fix because you're otherwise likely to shift components, tear pads, and in general render your device useless. I've seen horror stories from so-called trained technicians. I dare not imagine what an enthusiastic amateur will do on his first ever trial.