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When your laptop's battery is 100% charged, should you leave it plugged in so any battery power doesn't get used, or will that cause overcharging, overheating, etc.? Should the laptop be unplugged when the battery level is 100%?

I'm asking because my laptop's screen tends to get dim when unplugged, so I don't like to run it on battery.

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You can increase the brightness of the screen when it's running on battery. Differs machine to machine, but most laptops have function keys for screen-brightness –  dbr Jul 25 '09 at 15:23
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See also yesterday's "Is it better to use laptop on battery or on AC power?" at superuser.com/questions/12358/… –  Arjan Jul 26 '09 at 9:08
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If you have a Thinkpad (and probably for other machines also), there is a setting in the Bios for screen dimming on battery. This can be reached when you power the laptop on (on a Thinkpad you press the ThinkVantage button, on another machine it will probably be a Function key). –  Joel in Gö Jan 18 '10 at 9:11
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Possible duplicate: superuser.com/questions/12358/… (probably won't get closed though) –  Hello71 Aug 2 '10 at 0:43
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Can I just say fantastic question, I've always wondered myself but never asked before. –  Robert Massaioli Jan 20 '11 at 1:33

7 Answers 7

up vote 135 down vote accepted

This page has a good answer: "it depends"

The answer is: YES and NO, it depends on the situation.

Having a battery fully charged and the laptop plugged in is not harmful, because as soon as the charge level reaches 100% the battery stops receiving charging energy and this energy is bypassed directly to the power supply system of the laptop.

However there's a disadvantage in keeping the battery in its socket when the laptop is plugged in, but only if it's currently suffering from excessive heating caused by the laptop hardware.

So:

  • In a normal usage, if the laptop doesn't get to hot (CPU and Hard Disk around 40ºC) the battery should remain in the laptop socket;

  • In an intensive usage which leads to a large amount of heat produced (i.e. Games) the battery should be removed from the socket in order to prevent unwanted heating.

The heat, among the fact that it has 100% of charge, is the great enemy of the lithium battery and not the plug, as many might think so.

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+1, nicely explained. BatteryUniversity is one more place for good information. batteryuniversity.com –  nik Jul 25 '09 at 19:41
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Removing the battery is NOT recommended for MacBooks. These actually run slower when you unplug the battery, as the A/C adaptor cannot provide all peeks that may occur in power usage. I guess this may be true for other brands as well, so even though it may increase battery life, I'd not recommend to take the battery out for any brand. See tomshardware.com/news/… and support.apple.com/kb/HT2332 –  Arjan Jul 26 '09 at 9:10
    
Good point, Arjan. BTW: I don't remove the battery on my Macbook Pro, because it dissipates the heat very efficiently. The bottom line is: it's a conundrum - performance against battery life. –  splattne Jul 26 '09 at 9:30
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@Arjan, does this means that the battery is still used sometimes when it's 100% charged? Then the first part of the answer isn't correct either... –  Steven Roose Aug 10 '12 at 11:06
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Nice, but that site doesn't tell the whole truth. You can read here that when storing Li-Ion batteries (and leaving them in the laptop is essentially storing them), their capacity will decrease unrecoverably. You will see that at 100% charge this loss of capacity is very high, whereas at 40% charge it is much lower. Thus, keeping a battery in the laptop when it's not needed is very detrimental to it's life span. Also, it increases the risk of applying "top-up" charges, which again will shorten the life span. –  Stefan Seidel Sep 18 '12 at 21:20

From a view point of your battery's health you should keep the laptop plugged into the wall and unplug the battery from the laptop. Not because of over charging or whatever, but because of the heat generated by the laptop. By removing the battery it will be cooler, which is good for the battery.

If you were to unplug the laptop from the wall, you would start to drain the battery, and would have to recharge it later. Since the limiting factor on laptop batteries (all rechargeable batteries, actually, that I know of) is the number of times they can be charged and drained, any time you can avoid draining the battery will make it last just that much longer.

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Your first 2 sentences don't make sense? –  Click Upvote Jul 25 '09 at 16:19
    
Sorry, i realize now my prepositions were confusing. I edited it to be more specific. –  Mike Cooper Jul 25 '09 at 17:28
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It never crossed my mind that you could use a laptop without the battery. Great! I have a spare laptop whose battery is almost dead. Removing would lighten the weight. –  lamcro Jul 25 '09 at 18:58
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some laptops will not function without the battery. You will know if this is the case because the moment you try to unplug the battery the system will shut off immediately. There are only a handful of laptops that do this however. So an "expedition" is in order. –  Axxmasterr Jul 25 '09 at 19:05
    
More than a handfull these days. More than 50% of the laptops I come across won't boot or stay running when the battery is removed. –  Polynomial Feb 8 '12 at 15:41

This all depends on the type of material the battery is made out of. Each battery material has different ideal characteristics. Look on your battery label and determine the type of battery it is. (i.e. lithium-ion (Li-ion), nickel-metal hydride (NiMH), nickel-cadmium battery (NiCd), etc. Once you determine which type of battery it is simply google the material, and read about the capabilities. This should tell you all and more of what you would like to know.

A note on charging.. Not all charging circuits are created equal, so if you have a deficient charging circuit, you might indeed want to unplug the battery after it has been fully charged like was suggested above.

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trickle chargers or low current chargers, in particular, tend to not cut off when the battery is at 100% charge and can degrade battery capacity over time. –  Jeff Leonard Jul 25 '09 at 22:38

This is simple, and the best way to keep your battery life maintained. Exactly what I do:

  1. Fully charge the battery
  2. Let it discharge to around 40%.
  3. Plug back in the AC.
  4. Unplug the battery.
  5. Wrap the battery in plactic, make sure it is sealed.
  6. Pop it in the fridge until you need it.

As my laptop is a desktop replacement, because I prefer laptops, this is what I do. And it comes with an excellent result.

Be sure to let the battery get to room temperature before recharging!

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Do you have a link describing physics of your approach? –  stim Apr 6 '10 at 13:18
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There is a less simple approach? (-; –  hippietrail Nov 23 '11 at 22:19
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-1 What you have described is merely extending the batteries life. Which is not asked in the OP. –  HaydnWVN Dec 15 '11 at 8:04

Here is my two pennies for what I have learned from a chemical engineer who is a friend of mine:

  • If your battery is a Nickel-Cadmium battery then you must not leave the battery plugged in to the power supply when it has reached 100% of charging,because this will cause the battery to overheat,inflate and eventually explode.

  • If you have a Lithium-Ion or a Lithium-Polymer battery and it is charged 100% and you leave it plugged in to the power supply then this will not affect the health of the battery or make it overheat.The reason why it doesn't affect the battery is because the laptop as a device demands from the charger electric energy to function and electric energy to charge the battery.When the battery is charged or missing then the laptop demands from the charger energy only to function. As you may have already understood when you charge your battery and function the laptop the power consumption is higher compared to the case when you operate the laptop without the battery or when the battery is fully charged.Now in order to use your battery with the most efficient and "healthiest" way you have always to think the concept of "Charging Cycles". Let's say for example that you have charged your battery 100% and you want to leave from a place to go to another and use your laptop there. After the use you have left 20% of battery so you decide to charge your battery. After the charging, this will reduce the charging cycles by one since you charged you battery. Now let's assume that again you want to use your laptop's battery only this time instead of leaving it to discharge to a low point like 10-30% you decide to leave it discharge to a point of 80%. After the use you decide again to charge your laptop. Eventhough it has 80% of battery left because you decided to charge it then this again will reduce the charging cycles by one. So to sum up,it is better to take advantage most of the "juice" of your laptop's battery when you decide to use it without the charger.

THIS OF COURSE IS A GENERAL GUIDANCE FOR BATTERY USE. THIS WILL NOT NEGATE/RESCIND ANY INSTRUCTIONS PROVIDED FROM THE LAPTOP MANUFACTURER OR THE BATTERY MANUFACTURER TO YOU VIA A MANUAL OR ANY OTHER WAY!

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I remember something from Nokia about how leaving mobile phones plugged in after they were fully charged caused an increase in overall energy usage, as compared to unplugging when charged and recharging when needed. This was part of some green initiative. I would imagine that laptops are the same. Therefore, by leaving your laptop plugged in while turned off and fully charged, you may be increasing the size of your electric bill.

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No. Modern power bricks dissipate very little power when inactive. –  Mechanical snail Sep 19 '11 at 20:44
    
They still use more power than when they're unplugged. Also, there's no way to tell which ones are modern (just like NiMH) unless you measure. But in practice it probably is just green lip service. –  XTL Mar 7 '12 at 12:52

Apple has a very handy site about the batteries used in their products. While a fair chunk of it is specific to Apple products, there's a good amount of information in general about modern batteries. For example, do you know how to properly store your battery if it will be unused for some number of weeks?

The short answer to your question is "Almost certainly" keep it plugged in, but ensure there's plenty of airspace to allow proper cooling of your laptop's battery. Bedspreads and carpeting may be comfortable for you, but are far from it for your laptop's battery.

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