Macbook Pro 16 losing battery when plugged in

Advanced power/temperature management in the newest MacBook Pro is designed so that If you are requesting intensive operations, your Mac can use the entire power supplied by the Power Adapter and freely "borrow" power from the battery on a moment-by-moment basis. That is the way it was intended to work.

Make sure the Power Adapter is showing connected (typically shows the lighting bolt inside the battery symbol on the menubar).

If an all-night render depleted that battery too low, you may need to give it time doing nothing or sleeping with no Apps running and power adapter connected to re-charge.

If you have not connected the power adapter and left it doing nothing all night long, read the suggested procedure in this recent article, developed after an Engineering investigation into problems with a related MacBook Pro model:

If your MacBook Pro (13-inch, 2019, Two Thunderbolt 3 ports) keeps shutting down - Apple Support

This likely applies to more models as well, but the investigation has not issued procedures for more than the model listed.

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Grant Bennet-Alder wrote:

Advanced power/temperature management in the newest MacBook Pro is designed so that If you are requesting intensive operations, your Mac can use the entire power supplied by the Power Adapter and freely "borrow" power from the battery on a moment-by-moment basis. That is the way it was intended to work.

Hasn't that been the case for all MacBook Pros even as far back as the original ones? No power adapter that Apple has included can supply peak power to a notebook computer. The battery has always been integral to operating at maximum power. I seem to recall that when they operated with a bad battery or even the battery removed, they would throttle the CPU clock to half the frequency to keep the system from overwhelming the external power supply.

y_p_w wrote:

Hasn't that been the case for all MacBook Pros even as far back as the original ones?

Yes.

The difference is that improved temperature management and better fans in these models mean that temperature throttling happens far less often, so you can push them much harder than ever before.

I suggesting checking out what it's doing under the hood with coconutBattery. Also check Activity Monitor in Utilities to see what might be using the most power and what's using current CPU cycles.

https://www.coconut-flavour.com/coconutbattery/

That's rather odd. It's saying there's power being drawn from the battery when the power adapter is connected. And it's a tiny draw. If it's not doing anything heavy duty then the battery draw should be zero.

Unless the design has changed since my 2011 model, the battery is allowed to draw down. At 100% it stops charging, then allows the battery to drop to gently 95% instead of always forcing a charge. At 95% it recharges to 100%. This has kept my batteries healthy for 8 years.

steve359 wrote:

Unless the design has changed since my 2011 model, the battery is allowed to draw down. At 100% it stops charging, then allows the battery to drop to gently 95% instead of always forcing a charge. At 95% it recharges to 100%. This has kept my batteries healthy for 8 years.

Sure. However, it shouldn't be drawing from the battery unless the power needs are more than the power adapter can supply. The charge level is normally going to be dropping from self-discharge as well as from whatever supplemental current is needed.

However, I re-read the OP's situation, and if it's doing a high power operation for hours on end, it probably isn't going to be drawing down on the battery. I would also expect something like that to cause the fans to to run fast enough to notice.

Not sure what the deal is, but earlier I didn't proofread and I would expect that it would draw down on the battery.

You can look up the specs though. The available processors are rated for a max 45 watts, although constantly using that amount of power could cause some serious overheating. The GPU would draw quite a bit of power too. There should also be other components drawing power Add conversion losses.

For the most part I wouldn't expect that any Mac notebook computer should be operating at high power for more than just brief spurts. It should back off before destroying itself or going into thermal shutdown (which I've seen before).

Are you using the correct Apple Power Adapter for that Mac?

Find the right power adapter and cable for your Mac notebook - Apple Support

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