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18650 Battery Length Variation

ralphjy

I just committed an inadvertent blunder that I thought I'd share in case it might help others.

 

I ordered a couple of 18650 Lithium Ion batteries for a robot vehicle that I'm building.  I saw that these batteries were "protected" at the point that I ordered them, but did not consider the implication - after all an 18650 has a standard dimension as the name suggests, right?  Well, it turns out that the protection circuit increases the length of the battery - in this instance by slightly more than 3 mm and I'd probably have to destroy the battery holder to get them to fit.  Oh well, I can probably use these in a flashlight image.  I'll need to order new unprotected ones for the robot.  I'm a little surprised that the clips in the battery holder don't give a little more.

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  • Most Li-Ion batteries come in several types -

    - Large Button - these cells have large and wide contact areas which protrude from the positive side of the cell, usually only by a small amount. This allows the cells to be stacked.

    - Small Button - these cells have smaller contact areas on the positive side, looking similar to the "nipple" on an AA battery. These also allow the cell to be stacked but also ensures it clears physical reverse polarity protection mechanisms in some devices (e.g. flashlights).

    - Flat Top - true flat tops are "flush" to the top of the cell and thus won't make contact when stacked. These cells have the shortest (usually nominal) length.

    - Protected - as you discovered, usually has a protection circuit either near the positive or negative terminal that prevents overcurrent, overdischarge and overcharge. This allows the cells to be used in series applications safely with no risk of a single cell becoming overdischarged in use, but it does add length. The length actually depends on the design of the protection circuit, so the length could range from ~68mm to 71mm.

     

    - Gough

  • in reply to Gough Lui

    Thanks, that's helpful info.  I haven't purchased individual 18650 batteries before - I've bought a few already installed in enclosures.  I just opened a battery pack I use with M5Sticks and that battery is a REALLY tight fit in the holder!  And it is a flat top and measures slightly less than 65 mm in length. 

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  • in reply to Gough Lui

    Thanks, that's helpful info.  I haven't purchased individual 18650 batteries before - I've bought a few already installed in enclosures.  I just opened a battery pack I use with M5Sticks and that battery is a REALLY tight fit in the holder!  And it is a flat top and measures slightly less than 65 mm in length. 

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