tl;dr After word salad
Technically two batteries revived
I refused to believe my Switch Pro Controller battery took a permanent death. I like many let it sit unused for a long time only one day to pickup a lifeless controller that wouldn’t even take a charge. I let it sit on charge for days and just nothing. No orange light, not a thing. Sync button trick did nothing. So, I was rummaging around and stumbled upon my old Wii U Pro-Controller. I wasn’t sure of the battery it used and took it apart to find out, waalaa it’s the same battery as the Switch Pro Controller. I swapped its battery into the Switch Pro Controller and… Still nothing. No orange light, nothing. My doubts further increased about both of these batteries actually not able to be revived, let alone even take a charge at all. I’ve never through numerous devices ever seen a Li Ion battery just up and refuse to take any kind of charge at all.
So I found my Wii-U Pro-Controller USB cord and put each battery in a Wii-U controller. Both were indeed dead, but to my amazement after plugging in the USB cable the orange light lit up and BOTH were taking a charge. Now my suspicions grew even further that the Switch Pro Controller had a safety that if the battery has no voltage at all it will prevent any attempt to throw a charge at the battery. I let one battery sit on charge for about an about an hour (time may vary) in the Wii-U controller then popped it back in my Switch Pro Controller and… WE HAVE LIFE AGAIN!
tl;dr –
The Switch Pro Controller has a safety that will prevent a 100% dead battery from taking a charge. ie: No orange charging light.
Remove and charge your Switch Pro Controller battery in a Wii-U Pro Controller (A 3DS might even work) for an hour (may take longer) and return it to the Switch Pro Controller which should now light up the orange charge light to complete the charge.
Two “dead” batteries in the Switch Pro Controller
https://preview.redd.it/b4fcxzy4wqhb1.jpg?width=4032&format=pjpg&auto=webp&s=572834c49681d92d05d0a46708d16561ecbf80c0
https://preview.redd.it/6ghz9zy4wqhb1.jpg?width=4032&format=pjpg&auto=webp&s=2a102e5c5e2d3ec19bed7a665e67cf78c4d718b1
But in the Wii-U controller they take a charge
https://preview.redd.it/jjz8tcs9wqhb1.jpg?width=4032&format=pjpg&auto=webp&s=db29f5f1facca7d10cab31352ee5e105f76d73e1
After roughly an hour charging in the Wii-U Pro Controller and popping it back in the Switch Pro Controller we have life again.
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https://preview.redd.it/o95di9sewqhb1.jpg?width=4032&format=pjpg&auto=webp&s=a31e7075afdb157eb9373f6c4e2c7a90869ce618