Hi everyone, this mac does not detect the battery and subsequently does not charge, but when a charged battery is connected, it powers off it although it's still not detected.
Pin 4 [SMBUS_BATT_SCL] on batt connector has 3.4v whilst Pin 6 [SMBUS_BATT_SDA] has 2.3v. I'm suspecting either ISL6529 or SMC but I want your expert advice before I go ahead changing anything.

in first try remove d6950

Yes I have done that already and the voltages are still the same..

Try to change ISL6529.

measure voltages on all pins of Q7050

I have changed ISL6259 and the voltage is now 3.4v for both signals but the battery is still not recognized. I'm thinking that it might be a firmware issue or faulty SMC chip.

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Posted after 7 minutes 39 seconds:

Pins 1-3 has 12.3v and pin 4 also 12.3v whereas pins 5-8 has 0v with empty battery connected.

What about adapter jack LED (with and without the battery)? Try to find and replace on another battery.

Adapter LED is green with and without battery. The macbook works on a charged battery even though battery is still not recognized and the macbook's battery charges in another macbook.

Might be SMC faulty. Try to update firmware.

I have correct firmware but macbook says "this software is not supported on your system".

You should find the reason of SMBUS_BATT_SDA drop out = 2.3v
Resistance must be between 10-13kOhm form SDA, SCL to GND, and voltage 3.4 both.

Check R5280, R5281 resistance in switched off state - it must be 2.0 kOhm both.

R5281 was high and after replacing, both SDA and SCL voltage is 3.4v but battery is still not recognised.

I have the same issue on my A1278, this is SMC faulty. I have replace charger, removed side LED battery bar, replace battery, 3.4v SDA SCL - but no battery connected. This caused only by damaged SMC smbus.

This ruling is mainly of the signal (= SMBUS_CHGR_SCL = SMBUS_CHGR_SDA 3.48V for both) the signals van to SMC, ISL6259 and battery. The damage can be in the SMC and must be replaced, if you even has problems and not detects the bateria, try removing the condensers of the SMBUS linea.

Try to change smc chip, dual digital diode near battery will be fine. I'm think its smc fail.