18650 Battery size-dimensions. What do all these numbers really mean? Wild, wild world of Lithium-ion batteries

The Answer:

18 - The first two digits "18" stand for li-ion battery diameter in millimeters. In this case width of the battery is 18mm (millimeters).
65 - The third, and fourth digit stand for length in millimeters. That would be 65mm.

0 ? -  For those who want to know why zero is there, this is how battery manufacturers calculate batt. length: 650 divided by 10 is 65mm.  Since we are simple people we will forget about zero for now.

18650 battery size dimensions
18mm by 65mm - 18650

Metric system (millimeters) is used almost always when we see li-ion battery with numbers like:
Above are the most common li-ion battery sizes. So, next time when you see 18650 battery you will know for sure meaning of all these numbers.

Let's see what we have learned today.

What is 26650?

26mm - diameter
65mm - length

Battery dimensions in a real world can, and will vary.
These measurements are for bare Li-ion cells only (also called "unprotected" or "flat top").
If you see "PROTECTED" printed next to 18650, 16340, 14500, 26650 ...  that means that external protection circuit board is welded to the bottom of the bare cell, and flat wire is added to the side of the cell.
Because of added external protection protected Li-ion batteries are 2.5mm to 3mm longer, and can be 0.5mm wider than bare Li-ion cells.
I will explain what Protected Li-ion battery is in one of my next posts.

By the way, 16340 rechargeable li-ion battery is also called RCR123. It is the same size as CR123A primary (non-rechargeable) cell. First letter "R" means rechargeable.
And again, if word "Protected" is printed next to 16340 (or RCR123), battery will be longer than regular CR123A by 2.5mm to 3mm.
14500 Li-ion is the same size as AA battery, but it should never be used as a replacement for regular (Ni-Mh) AA battery. Why? Well, stay tuned I will keep posting some interesting stuff.

As you can see from this photo battery size will vary slightly. This is one bare unprotected 18650 battery. Flat top.
Protected 18650 (or any 26650, 16340, 16650,14500 ...) lithium-ion batteries will be approximately 2.5-3 mm longer, and 0.5 mm-0.6 mm wider in diameter.


  1. Great, thanks for letting us know !!!!!

    1. Thank you for taking time to read. Hope it helps to better understand Li-ion cells.

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    3. you mean ACID CAR BATTERIES!!!

  2. can any one tell me if this number mean? what is the current charge rate ? for :
    it 18650 battery 3.7 volt what is the rate ? how fast can it charge. thanks

    1. Is it from battery pack? It looks like it is. What's the name of host device?

  3. Good explanation. Thanks

    1. Henry, Thank you. This is just to quickly get some basics about li-ion bateries.

  4. Good explanation. Thank you.

  5. Thanks! Something to begin with. I must admit that these numbers can be confusing when you are new to li-ion rechargeable battery cells. But not that complicated after all.

  6. Sorry, idk if my last comment went through or not. Please respond to this one and ignore the last (if it does go through).

    Thanks a ton for all the info!
    What does the rest of the details on our batteries stand for?



    1. Every manufacturer has its own codes and, marking. Below, you can learn how to "read" Samsung 18650 battery codes.

      INR18650-25R --- cell model name | SAMSUNG SDI --- cell manufacturer | 2D82 --- date code (Capacity ; “2” is over 2.0Ah, Year, Month, Week)

  7. Replies
    1. Thank you. Any Li-ion rechargeable battery related question - feel free to post it here.

  8. Thank you. Great explanation.

    1. Louis, Thank you. Feel free to post your questions about Li-ion 18650 (or any other battery size) here.

  9. This helped me so much! I've been designing a battery array & case for 26650 batteries, since that is what is available, and it took me a long time to finally find the dimensions of the cylinder. Turns out it was so simple!

  10. I may be a little dense but will a cr123 battery take the place of a 18 650

    1. Rippy, you are asking if you can use CR123 instead of 18650? Is that correct?

    2. Anonymous6:58 PM

      takes 2 of the cr123 .but can be u use if vo and am .r same..

  11. This comment has been removed by a blog administrator.

  12. Great post thanks for sharing. I read your complete write up and it look very
    interesting. Looking forward for your next post.


    1. Cherry, Your company looks promising, but what batteries are inside your e-bike battery pack?

  13. FINALLY! All i was searching for was the numbers (18650 for exaple) for an aa battery. This explained that and much more. Thanks

    1. AA batteries are different chemistry and voltage batteries. There is 14500 Li-ion battery that is exact size of AA battery, but voltage is much higher 3.7V (4.2V fully charged)

  14. Finally! All i was searching for were the numbers (18650 for example) for an aa battery. This explained that and much more. Thanks!

    1. Jonathan,
      Thank you.
      Please let me know if you would like to learn more about 18650 battery (or 14500, 16340, 26650...) related topics.
      Any input would be welcome.

  15. Anonymous8:41 AM

    can anyone please tell me how to identify the imprinted PART NUMBER on the 18650 LI-ION cell ? thanks.

    1. 18650 battery can be identified. Please provide:
      1. Device (battery pack) manufacturer name (e.g. DeWalt cordless power drill)
      2. Color of that 18650 battery cell (link to picture with visible part number would be very helpful)?
      3. What is printed on that battery exactly (all printed characters must be included)?

    2. Anonymous8:24 AM

      Oh sure...please go to this link to view the image of the LI-ION CELL to see all the information that you are asking for-
      Also please tell me if i can build a charger myself for charging these LI-ION cells. Regards.

    3. 18650 batteries on that picture are made by LG ( LG Chem ). Battery model: B1. Capacity: 2600mAh. Voltage: 4.35V. It is very important to know that these are not standard 4.2V cells.

  16. Anonymous1:51 AM

    Oh sure, I am providing a link to the image of 18650 LI-ION CELL containing all the characters which are imprinted on the cell for your reference-
    What is the "Amp-Hr capacity" for these cells? Also please guide me if i can use a custom made CHARGER for charging these cells. Thanks for the help.

  17. Anonymous7:15 PM

    I was wondering whether there is a difference between the following markings and what eth difference is

    The OEM batteries that came with my Stanely Fatmax were;

    INR18650-20R M SAMSUNG SDIEM 125

    I purchased a replacement cell; INR18650-20R SAMSUNG SDI 125

    Am thinking there must be a difference are it does not work!!

    1. Please post a URL. What is the source of your new 18650 batteries?

  18. Anonymous6:07 PM

    Thanks for your post. If I have a 18650-32R battery can I use a 18650-25R in its place

  19. Don't know if this blog is still open...battery is from Monster b/t speaker....pack is blue with red, white, and black leads coming from battery pack. On label LLA3-3S1PYK22
    140311M20750. Any help is appreciated...