It's not bad to leave your laptop plugged in

There is one point that should be noted for macbook pros (and probably many other varieties of laptop).

> When plugged in, the battery is not in use. The laptop’s power circuitry bypasses the battery unless it’s needed. Depending on how smart the charger is, it may occasionally poll and “top off” the battery if its charge decreases to a certain threshold below 100%, but this is rarely needed in practice.

Not strictly true.

If you plug in your macbook, your battery is still being used. Evidently the power supply actually cannot meet the peak power demand of the system, so the battery needs to be there to provide for that [1] [2]. Apple also feels that the conditioning the battery receives while it interacts with a powered system increases its longevity.

Generally the power adapter provides more power than your machine is using, so obviously it can charge. But if you remove your battery and benchmark your system, you'll find that your system is limiting itself back about 20-30% to account for the lack of battery.

References: [1] http://support.apple.com/kb/HT2332 [2] http://www.tomshardware.com/news/apple-macbook-pro-battery-b...

> When Apple decides whether a battery is defective or has been worn out normally, the “special utility” they run is System Profiler. You can run it, too. Check the Power section, and it’ll tell you your battery’s cycle count, the intended capacity at manufacture, and how much capacity per cycle remains. Apple technicians compare the cycle count to the capacity loss. If your battery has lost a lot of capacity in its first year but hasn’t performed enough cycles to reasonably correlate to the capacity loss, they’ll replace it under warranty.

Apple actually does have a special application that they run to determine if the battery should be replaced and if its under warranty or not for that replacement. I recently got a new battery from them and the genius pulled out an iPod Mini and booted from it and ran what looked like a Carbon app, it was certainly unpolished and reminded me of the OS 9 days. It pulled data from what I assume was the battery and plot it on a 2-D axis, where depending on what quadrant it was in they make a determination on replacement and warranty. That all being said, it might just be an easy way for them to make a determination instead of going to system profiler, but it'd make the most sense to have a standard tool for the entire company.

This article focuses entirely on cycling as a rational for leaving the laptop plugged in, but heat and charge state are staggeringly more important to absolute capacity.

A fully charged lithium battery loses absolutely capacity much more quickly than a partially charged battery. Even worse, a warm, fully charged battery will lose absolute capacity at an astounding rate. A battery at 140F will be down to 60% of its initial capacity after 3 months if it is fully charged during that time.[1] If the battery could be maintained at 40% charge during that time it would still have 85% of its capacity after a full year of use, even at the same temperature.

This means that leaving a laptop plugged in will kill the battery in a very short time. The alternative is to let the battery continually cycle down and then charge it again. This isn't a good solution either because the extra heat very well may offset any benefit of keeping the battery at a lower charged state.

All in all, I think it means that a laptop is not meant to be on all the time; unless you are willing to replace the battery at fairly regular intervals.

[1] http://www.batteryuniversity.com/parttwo-34.htm

I have a 1st gen unibody MacBook, about 15 months old. It's spent almost its entire life plugged in, but with occasional excursions away from power for the day.

These are the figures iStat gives me for the battery at the moment: 100% Health and 133 cycles. It still gives me 5+ hours battery life, enough to mean I don't usually take the power cord with me.

Really, the manufacturer should manage the battery for me when the notebook is connected to a power outlet. I don't want to have to worry about that kind of thing. Like, if it should run on battery once per month, just make it do so automatically. Is that too much to ask? Maybe some physical law prevents that kind of thing?

He's wrong on the "leave your laptop plugged in always" point, but presents a bit of a false dichotomy.

Your battery will last longer than three years if you keep it below 80% State of Charge. A battery below 80% has nearly no loss of capacity over time. If you have a good laptop with a removable battery, taking it out once it reaches 80% will extend its life by years.

http://www.slate.com/id/2234009/

http://www.batteryuniversity.com/print-partone-19.htm

I just assume I need to buy a new battery once a year. The life of the battery seems to decrease over time and budgeting for a new battery each year seems to work for me.

I usually keep a laptop as my primary for 2 years, so it is only one additional batter per machine.

iFixit wrote a great article behind maintaining your battery back in November, covers a lot of similar data as the two battling arguments on HN today.

http://www.ifixit.com/blog/?p=1897

The folks at the Verizon store the other day were telling me that it's best to let a phone battery run to near-depletion before replenishing, because they only have a finite number of depletion-replenishment cycles, and running the battery to, say 50% capacity then replenishing uses up one of these cycles.

I have no idea whether or not this is true.

Well, I've tried to worry about this stuff, but it's too much of a hassle. Instead, I replace the cells in my battery: http://www.youtube.com/watch?v=BtqRvAu71Gw

Works for my laptop battery and is super cheap, though caution please.

From the apple website article linked in the posting: "Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing."

I don't know if its still true or not, but the chemistry in a lithium battery breaks down over time, reducing its capacity to store charge (even when not in use). This is greatly accelerated by both heat and charge state of the battery.

The best bet for long term storage was "40/40". Discharge the battery to 40% and store it at 40 degs F. You could go as high as 60 before getting into the steep part of the deterioration curve.

The bottom of a running laptop seems the exact opposite of this ideal environment. Your laptop is going to hold it at 100% charge and heat it to 90-100 degs. If you're going to leave your laptop plugged in for a long time, it might be best to remove the battery and place it on a shelf nearby.

Somehow, When it comes to this subject I tend to trust someone working on battery technology at Lawrence Berkeley more than I do a webapp developer.

That's what I thought: Deep-cycling Li-ion is much worse than keeping them charged up.

I beg to differ. I used my last laptop 10+hours/day always plugged and the battery wore off after 6 month. My current laptop, exactly the same. I only used it plugged in and now, after 10 month, the battery lasts 5 minutes.

while it is ertainly not good idea to intentionaly deep-cycle Lithium based batteried, there is also another good advice: remove battery when you intend to use laptop for long time (more than sy 3 hours) on mains power. Althought lithium-based batteries tend to lose capacity with age, this effect is strongly correlated with temperature and running laptop produces heat that will increase battery's temperature. (this requires wall suply that is able to cover laptop's peak power usage, which is nowadays not always the case)

AFAIK if you don't need the battery, it would be best to store it in a cool place, fully charged. Li-ion batteries don't like heat, and the bottom of a laptop is a quite warm place.

Thanks for this!