Monday, February 22, 2010

Pull the plug. Your battery will thank you.

One question that I have been asked frequently is “do laptop batteries fade faster if they are continuously plugged in?” A reader asked this question in the blog and I thought it would be good to get into battery failure using this specific question.

Batteries failure depends on the chemistry and can be broadly classified as mechanical failure and chemical failure. When I say mechanical, think cracking, breaking, and shedding of the electrode. Chemical means reactions, like corrosion, that alter the state of the battery for the worse. In lithium batteries both kinds of failure can happen. For example, people have shown that the electrode particles can break, especially when you fast charge the battery.

But the question that was posed regarding failure when you plug-in the battery is specifically chemical in nature.

First some basics. For chemical stability, the battery should be operated within the stability window of the electrolyte. For water-based batteries, the stability window is 1.2V. Go above this window and you split water and make hydrogen gas and oxygen gas. This is about the time you should be wondering how lead acid batteries even work considering that their voltage is ~2 V, but that is outside of the scope of this post.

But getting back to our laptop, the stability window is ~3.2V. Meaning that when you operate the battery above this the electrolyte is oxidized on the positive electrode and reduced on the negative electrode. Remember that we only want to oxidize and reduce the “active” materials and don’t want to do anything else. All these reactions other than the ones we want are called “side reactions” and these are really bad for the battery. The nominal voltage of a laptop battery is 3.7 V which means that something bad wants to happen as we use the battery. Just because things
want to happen does not mean that they actually do (for example, I want to buy a Tesla or a Volt, but...).

So long story short, stuff (e.g., passive layers and poor kinetics of reactions) happens and things are not as bad as they seem and you can increase the voltage up to 4.2V without bad things really happening. All chargers for Li-ion cells today cut the battery off when it reaches 4.2V. What you have to realize is that at 4.2V, these side reactions are present in finite amounts and start to chemically kill the battery, but its not that dramatic.

Operating to 4.1V makes things better and extends the life, 4.0 V is even better and so on. So why don’t battery manufacturers cut the voltage off at, say, 4 V to get better battery life? Because every time you cut this voltage down you decrease the capacity of the battery and its run time. The 4.2V cutoff is a compromise between good run time and decent (read “not pathetic”) life.

Were you supposed to understand all that? Not really, I just wanted you to know that I’ve really thought about these things. What you
do need to know is that if you keep your laptop plugged in, you force your battery to remain at 4.2V continuously and these side reactions continue to happen and slowly kill the battery.

On the other hand, if you charge the battery and then pull the plug (so to speak), the battery discharges some, the voltage drops, and these reactions become less of a problem and your battery life goes up. So the best things you can do is to charge the laptop (or cell phone, camera etc.) and once its charged, pull the plug. Your battery will thank you for it.

As a matter of fact, if you own a Lenovo Thinkpad, you can actually change the state of charge to which you charge the battery using the “Battery Maintenance” utility. You can change this from charging to 100% state (where the voltage is 4.2V) to 90% so that your voltage is less. You lose some energy is doing that, but atleast you can change it to 100% when you need battery power and put it back down to 90% when you can plug in. I wish my Mac has the same feature.

This problem has implications for PHEVs and EVs. Lets say you have a 15 kWh PHEV pack. You come home after a 40 mile commute and you plug it in at 6:00 PM. Let’s say you have a 120V, 15A outlet, so that you can put out 1.8 kW of power. So the battery is going to charge in 8 hours.

By 2 AM you have a fully charged battery. If you leave your house at 8:00 AM, your battery is going to be sitting at 4.2V for 6 hours in any 24 hour period. This is not going to be good for the battery. It gets worse if you decide to bump the amp/volts on your house to charge it faster. So we need to get these batteries charged faster, but we also want to make sure to have smart chargers that don’t do what I’ve described above. Something to think about.

What does this mean for researchers? If someone can find an electrolyte that has a wide voltage window of stability, then this problem goes away. Or you can try to use materials that work within this window (For example
A123 Systems battery does this on the positive side). But this means the battery has a lower voltage, which means it has lower energy and less run time. We don't want that, do we? Finally, we can try to isolate the electrode and the electrolyte and see if we can kinetically hinder these reactions. In the Battery Program at Berkeley we are actively working on this problem so that we can get more energy and better life.

In the mean time, remember to pull the plug.



  1. I have the answer, put a gasoline tank in the car the same size of the battery! No more charging problems. And when the "battery" is empty you can walk to a recharging (gas) station and carry some energy back in a can.

    I'm not trying to get off the topic of batteries but feel like we are trying to use the wrong tool (Li-ion cells) for the job.

    Can we squeeze the gasoline equivalent electrical energy in a battery the size of a gas tank? Gas has 33.7KWH per gallon so a 20 gallon tank has 674KWH! Even a 5 gallon tank has 168KWH.

    My fear is that huge amounts of capital and time are being invested in a dead end (for cars) technology.

    Who decided that Li-ion was the way to go for cars?

    We are talking a 10x increase of capacity to make EV's practical. Oh, and a multiple decrease in cost.

    Is this doable in the near future or is it all "pie in the sky" dreaming?

  2. Anonymous- :-) You are right. Batteries are expensive (as my post on PHEVs was mapping out), dont last as long as gasoline cars, have much lower energy density, and take a long time to recharge. I dont think there is any debate on all these. The 10x seems a bit (maybe more than a bit!) off, but the basic premise is worth thinking about.
    If climate change were not an issue and if gas prices remain at present levels, these alternatives dont matter. However, if there is a push toward clean alternatives, then batteries are an option (as are fuel cells and bio fuels). In the realm of batteries, Li-ion has great scope because of the higher energy compared to other batteries.
    Now, can we make the necessary improvements in the energy (and improve cost, life and safety) of these batteries? As my post on Moore's law was pointing out, its not easy, but I think there is a lot of hope. There are ways to improve energy by a factor of 2 and maybe even 3. These are not easy, but they are doable. I will try to expand on this in my next post. "pie in the sky" is fusion energy. Improved batteries are in many ways a much much more feasible option. The real question is can we make them good enough to be able to replace what we drive today without any change in our way of life. This remains to be seen.


  3. As I am sure you must know, the Chevy Volt only allows the battery to operate between 30% and 80% SOC (state of charge) to avoid the problems you mentioned. It also has a 150,000 mile 10 year warranty.
    People shouldn't be afraid of this issue.

  4. You hit the nail on the head when you wrote "change in our way of life". Trying to push battery technology for cars so we can continue to live a urbanized life is like putting solar cells on a house with single pane windows, illogical at best.

    Let's say the battery is improved to the point where a 4 passenger car can go 100 miles on a charge. Then we have to increase the grid capacity, build more power plants and then shovel in more coal. Or maybe even build a 10 billion dollar nuclear plant or 100 of them. All for this obsession with the car.

    I love all the talk of batteries, it's soooo cool. But unless we charge the batteries with solar cells at our home/work locations the whole thing seems pointless. And I like to drive on overcast days as well as clear days so I don't know how that would work out.

    Jock Stucki


    Good Story at the intersection of electric vehicles and smart grid.

  6. JohnCBriggs- You are indeed correct about the Volt. THis is one of the tricks we need to play. But, if we can extend the window further, we should be able to use this extra 20% without any life penalty and can decrease the cost.
    Jock- Very good points. There was a study taht showed that for coal-generated electricity, its better to drive an HEV rather than a PHEV! This surprised me a bit, but it does give us an indication on the complexity of the problem.

  7. Interesting article. I just want to know whether the electric grid in U.S. (or anywhere in the world) are capable to handle the demand and load if say 50% of the population drive a clean HEV/PHEV vehicles today? I'm guessing upgrading the electric grid could be as troublesome as finding better batteries.

  8. All to keep on with auto as usual. I too am sceptical of the universality of Moores Law. However Chris Anderson opines that it now applies to atoms and machine industry as well. To a degree, but not enought to justify refering to Morre at all!

    My critique is mainly about "cars as usual" and our reluctance to venture outside the old comfort zone. I can envision transport sans many autos. And I can do so without the battery of the future involved. Downsize the testosterone (top speed, acceleration...) and so many problems disappear. But that's psychology (social engineering?), not machine engineering! Verbotten no doubt or at least not an option in a me,me,me world.

  9. Thanks for the laptop details Venkat! I've tried to explain this to people for years in layman's terms and can now point to something specific they can read if they want.


    Kevin Donnelly
    SF, CA

  10. Hi Venkat
    This is Neelam, graduate student at Rice University and m working on Li-Oxygen battery. I am following your blog from last week (got the information about it from my junior Prakash Venkateshan).
    First, I would like to say it is a nice blog to get some research updates and problems related to the Li-ion batteries. A good platform to discuss various issues and challenges related to energy storage devices.
    Well, I completely agree that operating voltage and electrolyte stability window is one of the problem for the safety of Li-ion batteries.
    But here I would like to point out on the operating temperature of the Li-ion batteries too. According to the studies the capacity loss is dependent on it and is always integrated with it even in most favorable conditions.
    So far, the best operating temperature for these kind of batteries is room temperature, they can maximum stand from 45-60 degree C. And generally for a laptop battery which is always plugged in to an AC power source, we create a operating temperature that goes more than 45C (considering the present cooling systems in the battery packs)
    A battery operating at 25C with 40% of charge will loose c.a.4% of capacity in a year, whereas at the same state of charge but an elevated temperature of 40C and 60C it will loose c.a. 15% and 25% of its capacity, respectively; due to fast kinetics of side reactions( mentioned by you) at high temperature and faster degradation of electrolyte.
    {ref. Fuel Cells Bulletin
    Volume 2007, Issue 10, October 2007, Pages 12-15}
    Also, I agree with the point that the state of charge at which battery is kept is a major factor to decide the life of the battery.
    SO, overall apart from voltage limits the operating temperature also affects the stability of electrolytes and rate of side reactions.
    Moral of the story: do not put your laptop always on AC power, it kills your battery :)

  11. Hi,

    even for the MacBook Pro do you think is it necessary to pull the plug when the battery is charged?

    In the positive case, when should I recharged it?


  12. Gabriel- I've never measured the potential on my mac battery, but I suspect that you would want to pull the plug.
    I dont understand your second part of the post. As you asking when you should start recharging it? I typically use the battery for a while (say 1/2 hour to 1 hour), then plug it in and wait to fully charge it, then I pull the plug and use it again for 1/2 hour to 1h and then I repeat this. Takes some getting used to and I forget to do this, but I try.

  13. In response to the above post comparing the amount of energy gas and batteries contain, there are important additional considerations.
    One thing I've never heard mentioned is health, specifically cancer. The odor emitted from gasoline has changed since lead was removed from gas. It was replaced by benzene. Benzene is highly carcinogenic. The latest statistic is that one in every three females in the United States will get cancer in their life time, and one in every two males.

  14. Thanks for all the information. occasionally, I used to pull the plug of my Laptop Charger after full charge . From now on, I have a solid reason to do it.
    Rajagopalan C , Cochin, South India

  15. They told me this at the Apple Store in Bangalore but I didnt believe them! Now I'll start doing it. Thank you for the scientific explanation, of which I understood very little, but has somehow engendered a lot of trust in my reptillian brain ...


  16. I always thought that to keep topping up your battery (as opposed to fully charging and discharging it) was supposed to reduce the battery life. Isn't that true any more?

  17. Niel- you are thinking of Ni-MH or Ni-Cd batteries and the memory effect.
    In general for your car lead acid batteries you have to keep them topped off (to prevent sulfation). Ni-Cd batteries should be discharged all the way (there is some debate as to memory in Ni-MH, but does appear to exist) Li-ion works better if you keep is below top of charge. No need to fully discharge it. As a matter of fact, if you keep the Li-ion battery between, say, 80% and 20% state you will do very well (the 20% is not as critical, you can even let it discharge completely)

  18. "we are trying to use the wrong tool (Li-ion cells) for the job"

    Gasoline has been the wrong tool for decades. Li-on and other sources are finally a step in the right direction.

    Nice blog Venkat and keep up the great work.

  19. Hmm. Tesla recommended I leave my car always plugged in and in "normal" mode. Normal mode is, I think, 90% charge. Since I rarely drive far (<50mi/day), I'm wondering if I should always leave it in "storage" mode and only charge up if I plan of taking a longer trip...

  20. What are your thoughts about the Bloombox?

  21. It seems like this is an interesting problem for software to solve. If having a battery sit at a higher voltage is good for capacity and bad for life, being able to predict when the user is going to pull the plug and how much capacity that they will want would be useful. You stated that lenovo laptops can let the user make this decision, but the user really shouldn't need to. Instead, the device should track the user's history keeping track of:

    - Timestamps of plugging in
    - Timestamps of unplugging
    - Timestamps of running out of juice
    - Meta data like day of week, time of day, ip block, GPS location (if available), applications run, user that logged in, etc.

    Then the device (be it a car, laptop, cell phone, whatever) should do some machine learning to predict a user's habits. If a user always has their laptop plugged in except for weekend trips, then you can go for a lower voltage until saturday morning. If the car is used for short trips to/from work, but runs the battery to empty visiting buddies for Friday night happy hour in the next city over, you can keep the voltage high for happy hour and low for the commute. If the device is seeing a completely new scenario, you just keep the voltage high until you have enough data to predict that scenario as well.

    This approach maximizes both life and capacity, while keeping the user blissfully unaware.

  22. A couple of questions:

    1) Does this apply to other battery types?
    2) Do you unplugging for minutes or for hours? Hours is rather impractical since I do a lot of processor-intensive work on my laptop.

  23. Thanks much for the informations, very useful.
    What do you think about this article? ( I found it a couple of weeks ago and I'm a bit confused.

    Kind regards,

  24. Dan- What you suggest would be a good idea. Would help prolong life . You will need it to help you extend the time before you have to shell out another $25k :-)
    Greg- Very interesting idea. Would surely make a difference. Wonder if the Sony's of the world would like it though (less life means more batteries sold :-))
    For the last two anonymous posts on battery usage: How you use the battery depends on the type. Over the weekend, I will write up a blog post on this with Lead-acid, Ni-MH and Li-ion as the three systems. When you use processor intensive stuff, it may not make sense to remove the plug anyway because in some laptops the performance can decrease (unless you change the default settings). Maybe in this situation, you keep it plugged out for say 15 mins when you get a chance. Sounds a bit painful; I dont know if I would have the discipline to do it.
    I read the article in marco and went to the link to apple and the part about keeping the juices flowing is a bit made up! The part about charging it atleast one a month makes some sense. Depending on your chemistry, you could have some bad stuff happen if the voltage of the battery gets way too low. Keeping it discharged at all time can slowly get it to these potentials. Hopefully my next post will address this issue.
    Hope this helps. Please keep the questions coming.

  25. On the Bloom box- I'm not a solid oxide fuel cell guy and would hate to say something wrong. I suppose no one know what to make out of them because its not clear what their breakthrough is. Will have to wait and see. I was intrigued by their claim to be able to take solar electricity, convert to hydrogen, feed that to the Bloom box to make electricity. The efficiency is obviously down in the dumps with this, but I have heard a bunch of such ideas and I have not yet found one which is more economically than just using a battery! Not sure why anyone would consider this. Maybe a topic for a future blog post.

  26. Laptop battery myths

  27. Don't the laptop batteries have a limited number of "Charge cycles". Won't constant discharging and recharging of the battery use up these charge cycles thereby reducing its life time?

  28. Prashant- Yes and No. Yes in that cycling will decrease life. No in that the depth to which you cycle has a huge impact on life. THe less depth you cycle to, more the life. I will make a post over the weekend on this.
    I read through the and it seems to be adding to the body of myths out there.

  29. Thanks for a very informative post.

    For the scenario you present about charging a car, wouldn't plugging the battery charger into a simple timer (less than $10) and set that timer for the max time it takes to charge the battery (or a little less if you don't need the full charge for your daily commute) greatly extend battery life?

    For your laptop, a power strip would make the plugging/unplugging a lot easier.

  30. Interesting. Thank you for sharing your knowledge and research results in layperson's terms.

  31. Now i have my answer, :) . now i know why i should unplug the charger. Thanks a lot Venkat.


  32. Venkat SrinivasanMarch 9, 2010 at 5:58 AM

    On using a timer: Oh yes.. I agree that its very easy to make sure that we don't keep it plugged in. Now all you have to do is remember to turn on the timer :-)

  33. Hi senior,
    This is omkumar (CECRI 2004-08 batch). This blog of yours is very useful in many ways and are really application oriented. Based on the above topic one question i like to raise is:
    With the advancement of electronics and electrical can't we design a circuit which opens at a particular voltage say 4.2V and at the least current (as current drops during CV charging). This can reduce the unwanted reactions which are bound to take place at that higher voltage. Instead of pulling out the plug, this circuit can break during charging which puts the power directly to the laptop and on the battery when power is switched off. Hope i made my point clear...waiting for your reply

  34. Thank you, this was very helpful!!!

  35. Hi Venkat,
    This is a well explained answer to a question I was asked a few days ago. I thought it was such a good question that I linked this article from my blog, I hope you don't mind. You can check it out here (

    Please feel free to have a look at my article's and comment. I will take a look at more of yours.

    Thanks for sharing!
    - Ben

  36. I really like to use my laptop to the extent of over clocking it - using it for around 14 hours a day - in some cases, the whole 20 hours (without taking out the plug). I'm surprised my laptop survived till today. Is it just me or is it because of Dell? It's an old model. But it's pretty durable given that this laptop has been with me since bygone times.

  37. @Venkat: You mentioned that if you have a Lenovo thinkpad you can actually stop the battery charging at 90% which will extend it's life.

    How easy would this be to implement in other Laptops? Is it a simple as a software / driver update or does it require modification to the hardware also?

    It would be a nice idea if more laptop manufacturers adopted a similar practice!

    Interesting article, I will now run my laptop from battery wherever possible and unplug from the mains!

  38. @ David Ross - I think you will find that it is actually fairly easy to implement a reduced battery charge in most laptops.

    I have found with Dell batteries for example that it is generally just a driver issue, and that if you want to you can change the amount in the settings.

    Generally a call to their support services will set you on the right track.

  39. absolutely marvelous. I wonder when will the next Lotus Award be held, wishing to attend one myself if possible.

  40. Marvelous. I’ve been looking for a complete explanation about how to save the battery lifetime and now I found it. Thank you so much for posting it.

  41. Amazing tips and this will be helpful and appreciate your approach.

  42. I read so many different articles about this subject because I really want to know. I always kept my Sony plugged in and now the battery lasts for 10 minutes. So I assume you are correct.

  43. I don't think there is any debate on all these. If climate change were not an issue and if gas prices remain at present levels, these alternatives there will be no effect. However, if there is a push toward clean alternatives, then batteries are an option. Thanks for sharing.

  44. I like you discussion very great topic.Interesting post and thanks for sharing. Some things in here I have not thought about before.Thanks for making such a cool post which is really very well written.will be referring a lot of friends about this. apartment for sale

  45. thanks a lot...!


    Good Story at the intersection of electric vehicles and smart grid.

    Another idea that I don't see many discussing for electric cars is companies making the batteries easily removable and maybe even possibly all cars using the same type of battery. Then we can all pull in to a full- service station, exchange our battery, pay for the charge and be on our merry way.

  47. Hello Vinkat, i just got my new laptop its an hp probook 4530s and its my first machine i am really nervous about charging it. should i plug it in at all times i heard that when a laptop is plugged in it runs on ac power and doesn't drain the battery i almost always have the "plugging in" option available but sometimes i do run it on battery but not for more than 1 to 2 hrs i immediately plug it in as soon as its possible most of the times i am running it as plugged in and the battery is full, i take a break for 15 mints and then again plug it in the battery again charges to full and so on....kindly provide a detailed reply i would be so grateful...i'e been searching the stuff on internet for like a month or so....

  48. Here is a reason..

    No, modern batteries are lithium ion and you don't need to drain them down to 0% like the old lead-acid or nickel cadmium batteries to keep them healthy. In fact, the more you run the computer on battery power, the more you will shorten its life. The less you use the battery, the longer the battery will last. You don't need to take it out of your machine because your machine will stop charging it if it senses that it has a full charge.

    I work at Fry's in the computer repairs department and get asked this damn question about 20 times a day.

  49. Thank you for the information. I'm wondering, if I'm using a laptop as a desktop replacement, is it better to just take the battery out when I don't have the need to use the battery? Thanks in advance.

  50. Thanks Venkat! I was really confused since I the time I bought a Macbook pro. People kept saying that one should use laptops always plugged in because discharging and recharging it would reduce its life. I think from your post its pretty much clear that once the laptop is charged upto 100% I must disconnect it from the power cord. Hope my understanding of your article is right? If not, please correct me :)

    Many Thanks,

  51. hi, this method of plugging & unplugging constantly strikes me as extremely unintuitive and disciplined usage. How do we know manufacturers have not already implemented a 90% charge in their circuit while telling the user that it's 100% charged? That seems far more possible and would subject no behavioural change on the user for a silly battery. Battery life rarely gets the attention it deserves in reviews so it seems that's very doable without affecting sales.