did you ever find a solution to this problem? I have a BV6000 and I am experiencing the very same problem, though I did not even do an upgrade.
Any suggestion on possible workarounds would be hugely appreciated.
TL;DR: So you just pushed an OTA upgrade to thousands of phones, which makes their battery go dead till they won't charge anymore.
Do you realize the implications of it? What a huge cock-up. How are you going to fix this? People need to get back to a working phone somehow.
My experience below.
OTA upgrade to 6.0, phone runs much slower WTF. Drained all the battery with normal usage of the phone - even while charging! It would not power up anymore. Tried leaving it off but plugged to the original charger for an hour: no chance, still dead. So I checked the battery with a tester: 3.29v. My guess, this is too low for the charger circuit to start. So the phone decides the battery is dead for good and gives you the silent finger. I worked that around by charging the battery manually a bit with another charger, till I brought it to 3.8v (which is still well below the maximum of 4.35v). Then I put it back in the phone; plugged the phone to the charger, showed 5%. Left it charging for four hours with the phone off, only went to 13%. What the f*. Clearly fast charging is disabled in this ROM. But it's not even using the full 500mA of the first USB specification - charging is even slower than that.
The phone is unusable. If I power it on, it will discharge too fast. if I leave it off, it will take days to charge fully. And I'm lucky enough I know how to revive a battery even when your charger decides it's too dead to charge. Most peoples' phones will just go dead. Lots of unhappy people with bricked Blackview phones.
NB: Before you say I have a faulty charger: not the case here, my home is charger heaven and I have many others to try from.