XTAR 21700 Li-Ion Cell Review


XTAR sent these 21700 cells for testing. I like 21700 cells (and still really like lights that use them, like the EC65!)  I’m happy to put this light through the paces (and further refine my cell testing format.) As always, click for bigger images!!!

Official Specs

I don’t have a pdf spec sheet, but here’s an image with the same data!


Best I can tell, these aren’t available bare from XTAR just yet.

Short Review

These are very capable cells, and should easily work in any flashlight that fits 21700 cells.  Also with a rated 35A continuous (and a successfully tested 20A continuous), these will likely be good in electronic cigarettes, too.

Long Review


These cells arrived in individual white, XTAR printed boxes.  They aren’t in secondary wrappers, and they didn’t come with a plastic storage box.

Build Quality

zeroair_reviews_xtar_21700_liion_02.jpg These are nicely wrapped cells.  Not much to report here.

The positive terminal is of the three-prong variety.


They’re 21700 cells, which means they should be 21mm in diameter, and 70mm long (and the last “0” means “cylindrical).  I measure them at 21.5mm x 70.6mm.

A 21700 BOSS?  Now that’d be a thing to behold.  😀


I’ve tried to keep the scales similar, so over time the charts will be generally comparable.  Recently I modified the legend so that it has more (and actual) data on some (most?) of the charts.

Discharge tests

If you read the last cell review I did, of the “Liitokala 30Q’s” – compare that review to this one across the board.  Look at the consistency with which these two cells performed. I threw in some temperature data in the background on this chart (but it’ll have its own moment to shine later).  If you know of a better way to display this, I’m all ears.


Again, note the consistency!  Also note the legend on this chart – the actual Ah is listed for each individual run.  Took a little bit of excel magic, but I think it’s worth it.  



“Bounce back” is what the cell voltage does when the cell rests after a discharge. Interestingly, after heavy discharge rates, the cell bounces back higher.  This corresponds to a discharge amount of less energy, and does mean that there’s energy left in the cell.  So if I selected the cell with the highest bounce back (ie the cell that was discharged at the highest current), then discharged it to 2.8V at 0.2A, I’d still find that there was a lot of energy still in the cell. And I have finally figured out what I think it so interesting about “bounce.”  A poorly performing cell will bounce back higher on high discharges.  That’s because the IR is higher, and because the cell performs much worse under high loads.  So a good performing cell (like this one) will bounce back much less because it’s much more capable of high discharge.  And we can see that here.  This cell is capable well past the tested 20A, and so at 20A much more of the energy is used from the cell.  Hence less bounce.  I don’t really have anyone else telling me that, so I could be grossly wrong.  Or maybe I just proposed a new metric for cell quality.  ¯\_(ツ)_/¯

Charge test

Power, Constant

Internal Resistance


Here’s the temperature data.  You should be asking why the temp (on both tests) dips after discharge completion, and then rises again.  I can answer that.  For a few minutes after the discharge at these high currents, my discharge tester has active cooling.  It’s not cooling the cell per se, but the cell is of course near the tester.  So for a few minutes, the fan is running, moving air around the cell, cooling it coincidentally.  But once the tester reaches an acceptable temperature, the fan shuts off.  The cell no longer has the air movement around it, and the temp creeps up because the center of the cell is still not cooled completely.


These are fantastic cells, and I’m happy to be able to put them to use in the few 21700 lights I have.  Maybe I should buy an e-cig for 21700 use?

Up Next

I hope to wrap a couple of Olights for tomorrow, and Thursday I have another cell review!  I have a ton of stuff in the queue!  Stay tuned!  And I’m thinking once I get all the new site stuff set up, I might do a giveaway.  Anyone into a giveaway?  Comment here!


  • These cells were provided by XTAR for review. I was not paid to write this review.
  • This content originally appeared at zeroair.org.  Please visit there for the best experience!
  • Whether or not I have a coupon for these cells, I do have a bunch of coupons!!  Have a look at my spreadsheet for those coupons. It’s possible to subscribe and get notifications when the sheet is edited!!

Author: zeroair

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