Refilling laptop batteries!

The Li-Ion battery in my IBM Thinkpad X31 laptop has been getting a bit long in the tooth. Rated at a 4.4 AH capacity new, /proc/acpi/battery/BAT0/info told me that it was only holding 1.8 AH now that it was a few years old. (This is normal for Li-Ion batteries, which degrade over time, even without multiple charge-discharge cycles.)

X31 laptop battery

 A brand new IBM (or Lenovo now) battery costs over a hundred dollars, but by shopping around I was able to find a "compatible" battery for as low as $50. It was only rated at 4.4AH, but that is relatively close to the 2nd generation 4.8AH batteries that IBM/Lenovo sell for twice as much. I started to wonder if it might be cheaper to buy  OEM li-ion cells and simply replace the cells (keeping the case, and charge/discharge electronics). The first step would be to determine what type of Li-Ion cells I'd need to buy, so I decided to open up my old battery.

X31 battery disassembled

As you can see, the standard X31 battery has six cells, in three parallel groups of 2. Cells are nominally 3.6volts, so this adds up to 3 x 3.6 or 10.8 volts. In the photo I have removed the shrink wrapped packaging from one cell to view the markings. Note the relatively complicated PCB along the back side of the cells that handles charging and discharging. If you zoom into the photo, you can see that the controller PCB is connected to each end of every pair of cells (orange and black wires to the far ends, silver metal tab connections to the middle two). This allows the controller to charge each parallel pair of cells at a different rate. The controller PCB is also connected to a thermocouple that is resting between the two middle cells. This gives the controller a temperature reading on the cells during charging and discharging. If the PCB detects that the temperature is too high, it can lower the charging rate, or shut down the power draw (and laptop).  Also note the heat fuse (small white block in series with the power line between the two leftmost cells) that is designed to open the circuit if the charge/discharge controller for some reason fails to maintain a safe temperature. All of these safeguards are designed to keep your laptop battery from igniting, and will be very important to maintain in any "re-manufactured" batteries.

I actually determined what type of li-ion cells were used by measuring them and then looking for li-ion cells of a similar size. They are about 2.5" high by 11/16" diameter, or very close to the 64.9mm x 18.3m diameter size of a 18650 style cell that I found on

My battery is rated at 4.4AH, or 4400mAH. As it has three sets of parallel cells, each set of two cells must have a 4400mAH capacity (because they are in series, you add the voltage, not the amperage), so each cell must have a 2200mAH capacity. 

From a mAH per dollar standpoint,'s 2000mAH cells are the best value, but I decided that since the total cost difference was only six dollars, I could afford purchasing the 2200mAH cells. These are slightly lower than the 2400 mAH capacity of the newcells that Ibm/Lenovo now use, and I'll end up with a 4.4AH battery (just like the original part number, before IBM/Lenovo upgraded it to 4.8AH).

Just the cells cost $34.20, but I chose to purchase them with solder tabs attached (an extra $1.50) because I figure the people at are better than me at attaching tabs, plus having extra tabs to work with (those that come on the cells, plus those I salvage from the original cells) will make my life easier. (Besides, the $7 of shipping is the largest extra expense….if the 2600mAH cells weren't almost twice as expensive as the 2200mAH cells I'd have gotten them just get a 5.2AH capacity battery!) The total cost was $43.42. A week later my batteries arrived. (Thanks UPS!)

Important safety note!

You should never replace li-ion cells with cells that have a lower capacity rating, or charge/discharge rate rating. The electronics in laptop batteries are programmed to prevent the cells from overheating and catching fire, and do a very good job as long as the replacement cells in the battery are of equal or higher rating than the original cells. 


215 thoughts on “Refilling laptop batteries!

  1. I have dell xps l502x laptop. The orginal battery died a few days ago. so instead of buying a compatible or genuine battery pack i decieded to rebuild it .I opened the battery case & found that there were 18650 lithium ion cells from sanyo 1000mah each . I serached on numerous websites and finally got panasonic 18650 3100mah battery. I installed them & then charged for nearly 2 hours (till the charging led switch off) but when i use it on battery it drains really fast just the same back up that i got with old cells…..any idea / suggestions please

    • It may be that your BMS system hasn’t figured out that you have a new cells. So the percentage battery life goes down very quickly until it hits zero. BUT, after that, your battery actually still has 70-90% of it’s charge, and the laptop will continue to work for hours while the BMS keeps reporting 0% charge left. (this happened on my laptop).

      If you keep running the laptop down until the battery actually dies and then recharge it and do it a few more times the BMS should hopefully “re-calibrate” itself.

  2. Jay thanks a ton for answering my query. One more thing i want to ask is that as i installed cells with (3100mah) more than double the capacity installed before (1000mah) will the BMS system able to charge these cells upto 3100mah or it will cut down charging after say 1000mah charging for each cell is done…?

    • Each BMS system is different, but in general they will use voltage to recognize when the cells are “full” so larger capacity cells will be charged until they are full. (It could take longer to charge them than the original cells, as the BMS may limit the total input current based upon it’s assumption that you have smaller cells in the battery still.)

  3. Hello Jay, great work you have done with your battery pack.

    I have a dead IBM T43p 9cell battery with 7200mAh capacity. I recently cracked open it and decided to follow your procedure. I will definitly dodge the Ultrafire cr@p and buy decent ones. My question is the following: Obviously I should be looking for 2400mAh batteries and buy 9 of them. I know I can buy 2600mAh batteries and have them installed, but what the highest mAh rating can I buy and install? Can the battery electronic board hold for example 5000mAh batteries?


    • Your batteries circuit board should have no problem with you increasing the capacity of the cells (the voltage says the same). It will of course take longer to charge the cells (and longer to discharge, which is the point). Your main limitation will be how much capacity you can fit in the space, as you are limited to cells of the same physical dimensions as your original cells. I doubt you can find 18650 cells that actually have 5000mAh capacity, regardless of what their packaging says.

  4. Hello Jay, thank you for your reply.

    Yes my bad, I was just googling the reviews on the 18650 batteries and I understand now that the TRUE highest available is 3100mAh.

    Need to show you a picture of my battery core and ask you about something related to it. Tried to email you but didn’t get through. How can I do that?


    • You should be able to send an email to me at the email address on my main contact page (firstname at, make sure you spell my last name correctly (without an “i”). If you are unable to attach the image, you can use a free image hosting service such as and send the url instead. (Or, you could do that and just submit your question as a comment here…)

    • After having done it, yes, I agree ;> The only situation where the labor is worth it is if you simply can not find replacement batteries for sale on the market, or you can’t find replacement batteries using the largest capacity 18650 cells and you absolutely need/want the best cells in your battery.

  5. Has anyone heard of someone who will take your old laptop battery pack and do this for you? There must be someone.. Seems that would be an excellent business opportunity, I have tried the aftermarket crap and its pretty poor, using the manufacturers controller appears to be the hot setup.

  6. This site is very inspirational, but something is really boring me.
    OK I understand some battery pack when you unsolder the cells from the battery pack it is not guarantied to the chip to kill him self or not.
    But I ask myself is it the same thing when:
    1. the cells was dead and have a nice 0V voltage, and
    2. I unsolder the cells from the battery pack?
    I understood one thing, the chip to be working must have some voltage and some current, when all batteries are dead, there is no voltage and no current and for the chip is absolutely the same thing you do the one way or a hard way.
    Another thing, If you want to parallel two batteries one dead and one good replacement, it is not always possible to get up voltage to the dead batteries close enough to the new one. In my experience which is not absolutely big, I tried something with dead and not dead old batteries 18650.
    I tried to charge them and to shock them like they suggest with NiMH when they are dead. The voltage was 5V, 12V, and 18V with some resistors of 2,7 Ohms and 5.6 Ohms 7W. The measured currents for the two batteries in parallel (both dead) was something like 400mA, 1.2A and 2.4A. Because I don’t know for sure what is the allowed current to charge and beside thing i was wanted to shock them I started with higher voltage and current from 4.2 suggested. And for the big current I was charged for a 5-10 seconds.
    And I was afraid of boom explosion, but nothing happens not just worm up nothing cold like I didn’t charge at all. But The voltage was enough and after one week the old good batteries was OK good and the dead was but but better one good, but not enough to be like old one good 1.5V or 180mV, yes they was bad OK.
    The opening battery pack: believe me it is not for everybody the hand is on a thin ice to be sliced from the screw driver or pierced and must have a strong hand both to control battery pack not to slip out from the hand. It is not for everybody.
    Another battery pack I opened up the new one laptop (sorry for a both I don’t wrote the laptop model number) all batteries have a voltage 10mV or a zero.
    And you suggest me to charge them up to voltage close enough to the 3.7V. OK But How? I think If original charger board failed to charge them up How can I will succeed ? OK I was unsolder them battery cells, guided with my logic that the chip is already dead not because i unsolder the cells but because of low voltage of the cells. OK I tried the same thing with the first one old batteries to charge them up. But This time success was failed completely. The current was almost 0mA whatever voltage I started and it was the end of the test. Please I want from you to confirm or to explain me am I in right with logic’s outlined hear or not.
    Because I have my opinion. Thanks in advance.

    • If your existing batteries are already at 0 volts, the BMS system is probably shut off already. In that case, I don’t see any benefit in trying to bring them up to the voltage of your replacement batteries and soldering them in parallel. You can probably just cut them out and then solder in the new cells.

  7. Jay,

    My laptop battery meter shows the following. Design Capacity – 48840, Last Full Charge – 5740 and wear level at 85%. It is not charging to the full extent. So I am trying to replace my sanyo as09c31 battery with new cells. I opened up the casing and checked the current voltage in the cells and its shows 10.8 V. Is it a battery issue or something else is not correct ?

    • 10.8 volts sounds like a 3 cell battery (or 6 cells, 3 sets of 2 parallel cells, or 9 cells, 3 sets of 3 parallel cells) That voltage is normal for a set of 3 LiIon cells in series. It sounds like your cells are just old, but still provide full voltage, even if their capacity is only 15% of new. You can probably swap them out for new cells and get a lot of your capacity back, although you may need to jumper in the new cells before you remove the old cells to keep the charge controller chip from realizing that you removed the old cells. (Some chips disable themselves when/if they detect zero voltage.)

    • Set up your new cells in a 3 set of 2 parallel cells as well. Make sure the new cells are also at 10.8 volts (same as the old cells) so that not much current will flow when you connect them. Then, connect the new cells in parallel with the old cells, including any connections between the 3 sets of cells. (The BMS uses connections into the middle of the pack to monitor voltage and charge levels for each set of 2 parallel cells individually.)

      Once you have the new cells electrically connected, you can disconnect the old cells, and the BMS module will never know that you replaced the cells….except that the battery cells will miraculously start holding more and more capacity than they used to ;>

  8. Jay, HP technical support told me that if I replace my orignial 4 Ah battery by even say +/-.5 i.e. 3.5 or 4.5 Ah then it may possibly crash laptop. I guess its not correct and I should be able to replace with 8800mAh battery -or do you I should better prefer buying 4.4 Ah, since original 4 Ah battery is not available and I may not feel easy replacing cells, specially soldering etc. Another question, does compatible batteries are safe to buy ?

    • HP Engineers would be able to tell you for sure, as I don’t know much/anything about the specific HP laptop you have. However, I doubt that your tech support contact knew what they were talking about. Unless the software/firmware on the laptop was badly miswritten, it will work with a higher capacity battery. It is possible that a particular BMS chip in a specific battery is programmed to expect cells of a certain AH capacity, but at least with the battery that I used (which was a generic battery, not an OEM battery) the BMS adjusted to the higher capacity cells.

      Buying a replacement battery will always be easier and more cost effective (if you consider your time worth money) than replacing cells. Many replacement/generic batteries are as good as the OEM equiptment, but some brands are made with sub-standard cells and are not as good. Read reviews, try to avoid deals that look too good to be true, etc…

      Perhaps you want to buy a generic laptop battery and use that for another year or two, and if you need another one, replace the cells in it with higher capacity cells. In my experiance, the OEM batteries have more restrictive programming on their BMS chips, which in some cases mean that if you remove the original cells the BMS refuses to charge new cells. (If it detects that a cell has “died” by going to zero voltage, it refuses to charge again.)

  9. Hello Jay, am using Lenovo 3000 Y500 as my laptop battery dead in stage. It contained 10.8 volts, 4.8AH (6 cells, 3 sets of 2 parallel cells). As I saw the reviews on internet regarding refilling of Laptop battery, I had been working on it. I had opened the battery pack safely, and replac the old one with new one as they are before. After that I connected to my laptop. It is showing ” 0% available (plugged in,not charging)”.
    Can you help me regarding this problem….
    Thank you…

    • I’m afraid I don’t have any specific solution to your problem. I recommend that you shut down your laptop completely, remove the battery, and re-insert it, and then leave it plugged in overnight. That may re-set the BMS chip. Alternatively, it may be that the BMS chip will never allow you to charge the battery as it detected that the cells “died” and doesn’t realize that they were replaced.

  10. I’m experiencing the same than #137 with my 3×2 CGR18650CG cells powering a HannsNote 10″, when the + current is not passing thorugh the PCB. I discharged the bateries by using a lamp and charged them later with the notebook, without problems (well it took something like 9hrs for a full charge). Anyway the full charged set was not feeding/recogniced by the notebook. Any improvements within this time?

  11. hi there , can you please advice the precautions to be thought about the “old battery memory stored in BMS chip and the connection of the new cell jumper with old….. something like this” my is sony vaio -VPCEG 6 cell, i know every steps to well handle the assembly, just herd the old memory and BMS totally dying while removing the old cell. kindly please help any one will be great help tnx

    • To convince your BMS chip that you have never removed the cells, you need to always have working cells providing power to the BMS chip. To do this, you need to solder the new cells to your BMS chip before removing the old cells. Be careful when you do this, as if the voltage of the new cells and old cells are differnet, current will flow into the lower voltage cells from the higher voltage cells, potentially causing an overheating situation depending upon the voltage difference. To prevent this, make sure your old cells are charged up, and the voltage on the old cells is very close to that of your new cells before attaching them.

      • thank you jay for quick reply,
        my is sony viao VPCEG series it have 4 connections of wire coming out from cells( 2cellsx3 . total 6 cells) in parell to BMS chip,two are B+ and B- other two are B1 and B1 , . what i have plan is to take 6 same identical new (2200mah was old now i will use 2600mah new cell 3.6v) cells and do the welding same as old and then connect the four (B+,B-B1,B2) , after sucessful connection i will cut the old one , is it ok?
        i just want make sure as you told the old batteries must be fully charged to maintain 3.5v? and what about the new batteries do it also need to be charged fully. the old battery connections are already supplying 3.5v if i connect the new one at same time will it be doubled? like 3.5×2=7 or its in parell remain 3.5v .. thanks , ravi

        • The cells in parallel remain at 3.5 volts, but double the capacity. So your entire battery is 3.5v * 3 packs of 2 cells, or 10.5 volts.

          Your plan to put two sets of cells connected in the same way to the BMS (and then cut out the old cells) is sound. Just make sure that both the old cells and the new cells are at 3.5 volts and you should be fine.

          Note that your BMS will probably report that the new cells are “running out” well before they actually do run out, as it will think you are still using the old cells. So your OS’s battery remaining indicator may hit 0% around 1 hour, but the cells will continue to power your laptop for quite a bit longer.

          If the BMS is good, it will gradually re-calibrate to the new cells.

      • my old cells are still giving power for almost a bit more than a hour but i need more backup so plan to changed with new cells , they are in good condition, so i guess there won;t be any problem of lower volt, none of old cells are damaged , should i recharge the old batteries fully before connecting new cells . at present the battery meter don’t move up remain at 92% all time and and says plugged in charging charging.since i bought i haven’t refill the cells and i have used this battery pack for almost 3 years

        • As long as the new cells and old cells are both at the same (or very close to the same) voltage you can replace them in parallel. The actual voltage doesn’t matter much, as long as they match.

          • bingo. it worked every thing is fine and working perfect , thank you very much jay for your help, just a small problem with battery meter calibration don’t shows actual readings.

          • Glad to hear it worked for you! If you are lucky, your battery meter will self calibrate itself after a few full discharge cycles. If not, you’ll have the extra capacity, but not be able to know exactly how much is left….

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