Old iPhone 5 that suffered what seemed to be a classic tristar fault, would not charge if battery got too low, but would resume charging as soon as a bit more charge was put in to the pack.
Turns out, after all the running around, looks like the PMIC isn't driving the Q4 gate properly (maybe from a damaged PMIC, maybe from failed sensing). Checking SWI line from the battery and it's fine, though we're unable to test the points under the PMIC and CPU.
In the end, after this video, removing Q4 and shorting D-S generally resolves the issue but of course it's no longer a perfectly functioning phone.
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