96W Apple charger delivers only 92W, draining battery

[sbrunner]

I ran a couple of VM loads yesterday, and noticed that my third party 100W power supply maxes out at 85W, draining the battery in the process. The 96W Apple power supply maxes out at 92W. I did some testing with original Apple power supply, third party power supply and various cables. I gained some interesting insights:

- The max current draw of the new MBP 16" is 4.66A, regardless of voltage, cable, or power supply used.

- The original charger compensates for resistive loss with various cables well. 92W is gained with 20.3V and the original cable. The max I was able to get out of the power supply is 93W with a very short TB3 cable, rated for 100W, the worth was 91W with an active 2m TB3 cable. This is in all scenarios short of the 96W promised by Apple. With a hard limit of 4.66A, it requires a voltage of 20.6V. The specs of the Apple power supply say 20.5V and 4.7A.

- The CPU (2.4 GHz 8-core) sustains > 80W power draw between 3.7 GHz and 4GHz for hours - until battery drained, bc system uses more than power supply can provide.

- I was able to push the system to 117W for several hours which shows impressive cooling.

- If multiple power supplies are connected, it only uses one, the one that reports with the highest wattage, regardless whether it is able to deliver. I had the Apple and third party PS connected. Bc third party experienced voltage drop, it delivered less than the original Apple charger. But the MBP will always prefer that one, regardless of ports, sides, or cables used, probably because it reports as 100W power supply, over Apple's 96W power supply .

The bottom line is that have heavy loads run for extended periods of time will drain the battery, even if connected to power.

Tested with MBP 16" 2019 and 15.2 Catalina. Use reports from iStats. (I am not disclosing the third party, bc I am working with the vendor to resolve.)

 

[chrfr]

It's not new to the 16" that under full load a MacBook Pro will use more power than the AC adapter can provide. The Touch Bar 15" models will at least do that, if not even older models as well.

 

[sbrunner]

The more interesting point is that the Apple 96W power supply is not able to deliver 96W, and for that matter my 100W power supply maxes out at 85ish Watts. The culprit appears to be that max current draw is limited by the MBP, and hence there is no compensation for voltage drop due to cable loss or power supply. If the max current draw would be at 5A, voltage could drop down to 19.2V. For power supply designers, this means they have to pay extra attention to voltage regulation, and feed with slightly higher voltage to account for cable loss.

 

[bsbeamer]

Sonnet eGFX 650 OC eGPU delivers (and reports) 100W PD and have NOT had any battery drain during intensive video renders or overnight. Most PSUs take a hit before they supply the laptop, including any docks - those require 10-15W from whatever charger being used.

 

[sbrunner]

How did you measure that? I used iStats, and I can see that it does not provide more than 87W. Talking to the vendor, and one possibility could be that the it utilizes the 87W PD profile. (It reports as 100W in System Reports, though.) The new MBP 16" may not have a 100W profile, while the 96W power profile is so new that not many PS support it, yet, if any. One explanation therefore is that PS fall back to the largest common power profile, which in my case, and probably your case, is 87W.

On a side note, you are absolutely correct on the docks. Some consume as much as 50W. One of my findings is that PD in most peripherals such as monitors and docks is utterly useless, as the MBP latches on to whatever PS reports the highest. and then to whatever is the highest common power profile. On another interesting side note, that also works with 5V power supplies - for giggles I connect the MBP to one that I use for charging phones, and fair enough, it charges with 12W, as reported by iStats.
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Also, different topic, but do you have issues with your eGPU? My MBP 16" on Catalina does not boot 3 out of 4 times with eGPU connected, and lately reports a kernel panic on AMD Vega 10 timeout when computer is left to idle. It runs rock solid while I work on it. I have also 650 OC with Vega FE - stock voltage.

 

[s66]

I've used my 13" MBP (4port 2016) quite a bit in the car on a Belkin usb-c charger that only has 35W of power available under pretty decent load - it'll not drain it's battery, but charging anything into the battery takes forever. That system came with a 60W supply from Apple.
Even hooking that one up on a USB-C port just capable of delivering just 15W keeps it from draining it's battery under normal use.

The 100W or 96W is probably more due to limitations of USB-PD: 20V max and 5A max. ==> 100W max.