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So with the battery, when the machine is on and I unplug the magsafe, the computer will go off like it's going into sleep or hibernation. Fans will keep spinning. I'm assuming that's because it cannot detect the charge in the battery. The battery symbol from the status bar is also completely gone, if I try and enable it, it'll disable itself immediately
DO NOT use anything but 85 watt OEM Magsafe 2 chargers on 2015 machines anything else causes issues. What voltage do you currently have on PPBUS_G3H without the battery?
So no battery detected on System Report too?
Did you try known good battery?
Even old one (no hold charge), just for test.
Post exact voltage for PPBUS_G3H with charger and battery, both connected.
Check resistance to ground at SCL/SDA lines of battery bus (diode mode); also check voltage there.
8.2 volts means the SMC isn't running, and your SCL line is low which is no good. Resistance to ground on the SCL data line? Next thing would be try new ISL then next is SMC on that line if no short.
"diode mode both are 1.5 (red probe ground, black probe pin)"
This is really strange.
Are you sure to check in diode mode, with red probe at ground???
You should get aprox 0.500...
SCL line is blocked; could fail SMC, or U7100.