So, it sucks to say this on the iFixit forum but… I got a battery off Amazon and it seems to be working fine. Got 3 hrs of editing on it and it went just fine. No sudden drops etc. Havent tested internet browsing etc but all seems regular. Will report back in a bit with more testing. All in all I can say that it was not a motherboard issue, it wasn’t related to any repair mishaps or else. Simply a bad battery (well.. two fo them).
Now, I don’t think it is an iFixit issue and their customer service is stellar. I think that given the age of the machine, some of the batteries may be “dated”. Maybe the Late 2013 machine is just old and the batteries that are around now were meant to be purchased long ago… don’t know.
But again, the new battery works fine. I won’t post the link out of respect for ifixit but a quick search on amazon (i got mine from amazon italy) will show a range of batteries around 50 Euros. I got the one with the best reviews…
good luck everybody. BTW the Late 2013 MBP is still a solid machine, I have done all kinds of video editing and color etc and i works great. yes, slower than my new top specs MBP 2019 but hey…6 years old and still competing…
Long live the Late 2013 mbp (with a new battery)