is it better to leave MacBook pro plugged in and charging all the time or to empty the battery?

I have a real world comparison that I can make, I left a 15" MBP plugged in on charge in my office in London, then I returned to the USA and had a stroke, 3 years passed and the MBP stayed on charge. Last year I decided to repatriate all my UK Macs, on arrival the MBP was showing 82% battery health so I replaced the battery in it. An identical MBP that stayed here and was used normally showed 96% battery health. Same machine, just not continuously charged.

Csound1 wrote:

I have a real world comparison that I can make, I left a 15" MBP plugged in on charge in my office in London, then I returned to the USA and had a stroke, 3 years passed and the MBP stayed on charge. Last year I decided to repatriate all my UK Macs, on arrival the MBP was showing 82% battery health so I replaced the battery in it. An identical MBP that stayed here and was used normally showed 96% battery health. Same machine, just not continuously charged.

I wouldn't say that it's a great idea to NEVER take it off of the power adapter. Most people will occasionally take it off (even for a "desktop computer replacement") just to "let the electrons flow" as Apple used to say before they scrapped their "battery calibration" recommendation. Your situation was of course atypical. Had the power been cut off it would have likely gone down to 0% in sleep mode in a few weeks, and then gone into deep discharge. Almost 3 years would definitely kill the battery. I've also heard that many of the cases of battery swelling were on machines that almost never got taken off of the power adapter.

So for the OP, if you need to use it that way, go ahead. I'd say it wouldn't hurt to let it run on battery power alone every once in a while.

There is no penalty for leaving it plugged in all the time with TODAY's batteries. Deep discharge has been the enemy of long battery life, but most computers will sleep, then shut off before the danger point.

Be very cautious when reading stuff on the Internet -- a lot of what you read no longer applies to the batteries used in today's MacBook Pro computers.

Increasingly, these batteries are managed by software in the three different micro-controllers involved in powering and charging. Human Intervention is not helpful. Apple has made their recommendations more nebulous and vague, so the best recommendation is, "Use it the way you want to"

Batteries - Maximizing Performance - Apple

I don't know about better, but you shouldn't have too many issues using that way. If you need to use it off of battery power alone, go ahead. The things that destroy battery longevity are excessive heat and allowing it to totally discharge. The complete system should prevent any of those from occurring. When it gets to "0%" and a forced, orderly shutdown occurs, there is actually some reserve capacity left. Leaving it that way (for perhaps months) without charging can result in the state of charge getting so low (through self-discharge) that there will be substantial capacity loss or even a battery that can't charge.

I wouldn't worry about "emptying the battery" if you need to use it, but certainly make sure that it gets charged soon after that.