A friend complained to me that after only two years, she had to replace her laptop's battery because it wouldn't hold a charge. I found myself telling her that she shouldn't keep her notebook computer plugged in continuously, because it would kill the battery faster. Then I stopped myself: Was this just outdated geek lore rendered obsolete by modern batteries?
Yes and no. It depends, of course, on what kind of battery you have. Battery technology has come a long way over the years, and surely in 2009 you don't have to worry about how long your laptop's been plugged in. However, one major notebook manufacturer (which ships Lithium-ion batteries) thinks you should, and suggests adding a reminder to your calendar to deplete and recharge your battery once a month. To quote: "Apple does not recommend leaving your portable plugged in all the time."
My friend, however, has a two-year-old Dell. Cursory Googling for her model didn't turn up the equivalent of Apple's definitive statement, only lots of opinions which ranged from "it's a non-issue" to "yes, it kills batteries!" Dell.com's battery recommendations page doesn't say anything about not keeping your notebook plugged in. HP's battery tips page doesn't answer the question, either. I pored through my wife's ASUS Eee PC user guide and didn't find any warning about continuous charging. A non-mention might make you think it's a non-problem, but if this is an issue for Apple notebook batteries, it is for PC notebooks with lithium-based batteries too. When I asked, my Twitter followers returned mixed replies, but many notebook users (both Mac and PC) DID report anecdotal battery problems when the machine was plugged in constantly.
Other folks more educated about the differences in battery types than I am dropped knowledge about which ones are problematic and which aren't. Here's what they had to say about their notebook batteries.
I asked: Is it true that keeping your laptop plugged in continuously degrades battery life? GOOG turns up conflicting reports, need authoritative source.
wilw said, "Anecdotally, I can tell you from personal experience that it does. I've replaced 2 MBP batteries for that reason."
kevinmarks said, "it depends on the kind of battery you have in the laptop."
howtogeek said, "A bunch of good info on the topic"
YourGoToGuy said, "No research. But my Dell battery just went dead after a couple of years for that reason."
ethnicomm said, "guess it depends on if the battery has *memory* - I thought it was best to run it down to almost nil before recharging."
cfarivar said, "if so I'm screwed!"
aloncarmel said, "asking the same. Apple answer about Mbp is that it doesn't harm the batt. I dunno. I still drain it."
CoreyHarris said, "I have always heard the same thing, But I don't like to run off battery if I don't need to."
room214 said, "I have set my battery to only start charging when below 88% and stop at 99%. It should help increase the life of the battery."
brandon_wirtz said, "depends on what kind of battery you have. Lead Acid (heavy short life like your car has) Nope. Just LI yes. Li polymer no."
brandon_wirtz said, "Some Battery types get a memory, some don't some only charge so many times some don't"
brandon_wirtz said, "Also depends on if your laptop has a good Voltage regulator for charging. Faster charging laptop is more likely to wear battery"
bonsai said, "With modern LiPoly or Ni-MH batteries, it's all about the charger rather than batt. chem. so it actually depends on the laptop."
andrevr said, "Pretty sure that's not the case anymore. Smart charging circuits and battery tech have kilt that one."
andrevr said, "Here's some real data for you to consider. My Air has spent ALOT of it's life connected."
FrankRamblings said, "absolutely true. I completely killed a thinkpad battery in 9 months that way. To the point where total batt life was 12 mins."
scorpusmaximus said, "I'm no expert, but I agree with Ray Maxwell on the battery question. Charging and partial charges degrade them quicker."
bwhalley said, "it does, for fact. if it's a desktop that's portable, only put the battery in when you need to move it."
alexknight said, "This happens with Lithium Ion batteries. Apple has a kb article that explains you should drain your battery once a month."
dariomartinezb said, "PLS, share your findings. I keep my laptops connected all the time... I've found batt life to be really short for some of them!"
GBendinelli said, "that was true for Nickel Cadmium batteries, but isn't really true anymore. It's good to deplete it 100% after every 25 charges."
davidleary said, "depends on whether or not you Laptop manufacturer is in the Laptop Battery replacement business too"
jtimberman said, "Depends on the battery type. Apple's info for Macs"
diesh said, "for lithium batteries this is a good heads-up from Apple."
thompsonpaul said, "recommendation is to remove battery when under AC power for long periods."
thompsonpaul said, "Yup, it's true, at least for Lithium Ion, as stated in ASUS's laptop usage guidelines. Will try to find you online src"
thompsonpaul said, "Worst condition is keeping charged batt @ elevated temperatures, which is case w/ running laptop batts. http://cli.gs/BrvPnY"
revtristy said, "It's true for me. I kept my laptop plugged in most of the time and now my battery shuts off randomly when not plugged in. Sad!"
ChrisKubica said, "If u have a Mac, this app helps u see info re yer battery's life. A pic"
chrisbarber86 said, "Yeah it does, ideally you need to either remove the battery, or charge it up full, run it down 99% flat, then charge again etc"
tchachra said, "I have a laptop at home on my desk always plugged in. Never had a battery issue. Mac's and or PC's."
haselhurst said, "i believe this is true. my battery was fine, then left laptop plugged in continuously for 2 years, no buggered."
Cocodmonkey said, "experience says yes."
GitEmSteveDave said, "I pull the batteries out of mine occasionally."
emilsit said, "My bet? Depends how smart your charger is (and what battery technology)."
gdarklighter said, "no. Modern power supplies are smart."
biggsjm said, "According to Apple you need to cycle once a month. Run down to empty and then recharge."
joshbrez said, "My first MBP battery ran down enough w/ it always plugged in for Apple to replace it for me"
yedmart said, "sure does."
UnreadZigmund said, "I've also researched this after buying a netbook but couldn't see a consensus."
sameersama said, "well as far as personal experience is concerned then it does mess up the battery."
theogor said, "Yes and No."
puneetsarda said, "yes it does My laptop's battery life has fallen to 35-40mins"
robert_wheeler said, "from experience supporting many laptops yes leaving them plugged in degrades battery life but it can take months to happen"
jaskirat said, "No way. Using your laptop on battery all the time degrades battery!"
guscuddy said, "I'm no expert, but my last laptop battery died after a little over a year (and all of a sudden) because I kept it plugged in."
emhs said, "At the Shack, we have a policy to not keep the battery in demo laptops. Not quite authoritative, but close."
Update: This Bit About Batteries (see the "Calendar Life" section) explains details of Li-Ion battery life under different conditions. Thanks @ChrisKubica!
Of course, the best way to find out is to RTFM for your notebook and battery type to be sure,. My primary notebook is a MacBook Pro, and as per Apple's suggestion, I don't keep it plugged in continuously at my desk. (Because I'm on my second battery already, I'm paranoid and pull the plug a couple times a week.) Image courtesy of andrevr.