IT-Tech Online

This is a 13” MacBook Pro 2015 with retina screen. The laptop works fine when a  charger is connected but shuts down immediately when the charge is removed. A “service battery” message is displayed on the notification section and there is an orange light on the charger connector. The customer said the laptop battery had been replaced by another computer shop but the laptop is still not charging. Initial inspection shows no evidence of water damage or corrosion.

Connect the MacBook to our metered power supply. The current drawn from the power supply is 0.027A in standby mode. Replace the battery with a good battery (the battery must be not fully charged, prefer around 50% charged). The current drawn is still 0.027A. This confirms that the problem is in the logic board. How do we fix it?

We remove the logic board from the case and inspect it under a microscope. There is no evidence of liquid damaged at all. The schematics for this MacBook Pro is 820-4924A.

1. Measure battery connector J7050. The figures of pin 13 and 14  SMBUS clock line and data line are 0.508 and 0.509 respectively in diode mode. It means the battery communication bus to SMC should be fine. Measure pin 15 battery detection SYS_DETECT_L. The figure is 9.8 KΩ. It matches the R7050 figure on the schematics. So the battery detection circuit should also be fine.
2. Inspect U7100 charging circuit PMIC (Power Management Integrated Circuit). There is no burned marks or burned components. Measure the value of the resistors between U7100 pin 27 and 28 CHGR_CSI_R. The figure is 22Ω, ok. Measure the value of the resistors between pin 17 and 18  CHGR_CSO. The figure is 2.9Ω, ok. The charging circuit seems ok.
3. Measure Q7155  MOSFET in diode mode. Pin 5 to 3 is 0.472, ok. Pin 3 to 5 is an open line, very good. Both pin 5 and pin 3 to pin 1 are all open line, excellent. All these measurements indicate the Q7155 is working fine.
4. There is no obvious faulty component found. This logic board is not easy to fix. We have to rely on intelligent guess based on past experience – try and error approach. As the charging circuit power management, IC U7100 is the first suspect. Let’s replace it. The battery is still not charging after replacing U7100. We have just killed an innocent charging PMIC.
5. The second suspect is Q7155. This MOSFET is used to control battery charging current. Let’s replace it. The problem remains after replacing Q7155. We have just killed another innocent chip. This guess-by-experience method does not work in this case. We have to stop this problem-solving approach
6. It is time to rethink our strategies. Obviously, it is hard to get the charging circuit to charge the battery in this case. But we may work the other way around. We may use a good battery alone to power the laptop without connecting to the power adaptor, so we can follow the battery discharging current flow to pinpoint the faulty components.
7. Connect the logic board to a good working order battery and measure the voltage of Q7155. The voltage on pin 5 and pin 3 drops from 12.4V to 0V and keep repeating. The voltage on pin 4 drops from 5.2V to 0V and repeats in a circle. There is no shorted circuit on pin 5, pin 3 and pin 4. This measurement result confirms that the Q7155 MOSFET is working perfectly. For some reason, it opens for 1 or 2 seconds then closes. We have found the problem now, but why the Q7155 did not remain open?
8. U7155 pin 1 connects to the battery positive terminal directly. Dropping voltage in pin 1 means the battery is cutting the output voltage by itself. If the battery is good, the only thing that can make the battery to shut off by itself is the battery detection circuit. This circuit is very simple, consisting of only one 10KΩ resistor R7050.
9. We have measured the resistor R7050 an hour ago and the figure was 9.8KΩ. Let’s measure it again even though it still looking good with no sign of damage. This time the figure is 5.9KΩ. This resistor has an intermittent fault!
10. Replace R7050 with a good 10KΩ resistor and we get a working laptop.

Why the faulty R7050 caused MacBook battery not charging? – Understand the principles

In order to answer this question, we need to understand the principle of the Apple MacBook charging circuit.

1. When a battery is connected to the logic board, a 10KΩ resistor R7050 is connected to the battery connector J7050 pin 15. The smart battery internal circuit detects the presence of the 10KΩ resistor, then opens the battery internal MOSFET to output the battery voltage. In other words, the battery output will remain close unless a 10KΩ resistor is detected. This is the safety feature of the smart battery.
2. The battery voltage PPVBAT_G3H_CONN is about 10-12.6V depending on the battery is fully charged or not. PPVBAT_G3H_CONN passes the internal diode of Q7155, charging current sensing resistor R7150, fuse F7140 and becomes PPBUS_G3H power rail.
3. Now we have the PPBUS_G3H power rail. Bear in mind that this voltage is passed from the diode inside the MOSFET Q7155. Q7155 is not open yet. So at this moment, this temperate PPBUS_G3H is a “weak” power rail and only can provide very small amount current. This is a great safety feature of Apple logic board design. It prevents the battery from exposing due to overheating if there is a short on the logic board. This temperate power rail is powerful enough to power a PMIC to create an always-on power rail PP3V42_G3H.
4. The PP3V42_G3H power rail will power up System Management Controller (SMC) and charging PMIC U7100. SMC will read the code stored in the chip inside the battery via a pair of SMBus: SMBUS_BATT_SCL and SMBUS_BATT_SDA. This code contains information such as battery voltage, charging status, battery capacity, charging circle counts, temperature and so on.
5. If the battery check is ok and it has enough power to power the system, SMC will communicate with the charging PMIC U7100 via a pair of SMBUS: SMBUS SMBUS_CHGR_SCL and SMBUS_CHGR_SDA. Then U7100 will output CHGR_BGATE to open MOSFET Q7155. Now we have a stable, “full power” PPBUS_G3H power rail ready for use by the laptop.
6. When we press the power button on the keyboard, the SMC will power up the Apple laptop using the battery along.
7. When a power adaptor is connected to the MacBook, U7100 will create its own PPBUS_G3H from the power adaptor. U7100 outputs CHGR_BGATE to control the Q7155 to achieve battery charging or discharging according to the battery status and the applications we are running on the laptop. All these functions are governed by the SMC.