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There are many powerful flashlights, some of them have just one 18650 li-ion cell, 10W LED and they are very small.

I have read few tests, I watched few video tests on YouTube and looks like some flashlights really work at 10W power for few minutes and some of them can operate 1 hour at 10W.

For example - there is Armytek Wizard ~1000lm flashlight prototype test here: link

Test results for maximum (about 1000lm) mode:

  1. 5 minutes into the runtime test the headlamp is uncomfortable to hold. At the 15 minute mark it is unbearable to hold.

  2. At 56 minutes the headlamp flashed 3 times signaling that the voltage is in the “Warning Level”. This was followed by the ON and OFF switch indicator light flashing once every second in a green yellow orange combination. According to Armytek the “Warning Level” is triggered when the battery voltage is 3.1 V. At this point the light level has not dropped. There is also a “Critical Level”. According to Armytek the “Critical Level” is triggered when the battery voltage is 2.9 V.

  3. At 1 hour and 8 minutes light output drops to firefly mode 2. At this point the runtime test was concluded.

  4. Immediately after the test was finished the battery was removed and checked for voltage and temperature. The voltage was 2.93 V and the temperature was 132°F (for the battery). After about a 60 minute rest the battery voltage was about 3.0 V.

  5. This headlamp has thermal protection built-in which is supposed to be triggered at 80°C or 176°F. When the thermal protection reaches what Armytek calls the “Critical Level” (80°C or 176°F ) the light output drops to 100 lm. During the whole runtime test the thermal protection did not activate.

132°F is about 55°C

I have tried to find that in Panasonic NCR-18650 datasheet, but there is nothing about temperature range exept that discharge characteristics for 60°C exists, so I guess 55°C is ok for discharging. But how high that temperature can be?

What is safe discharge temperature range for (high quality) Li-Ion 18650 battery?

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  • \$\begingroup\$ Interesting. A few years back, manufacturers like Panasonic would sell separate Li-ion cells only to battery pack manufacturers. The latter would include a battery protection circuit into battery packs. The protection circuit did over-temperature protection among others. At that time, Panasonic would not sell the individual Li-ion cells to mere mortals. \$\endgroup\$
    – Nick Alexeev
    Dec 7, 2014 at 23:12
  • \$\begingroup\$ The headlamp is getting hot. Not the battery. LEDs burn easily 60% of the energy through them as heat, the rest goes into luminous output. The battery is probably fine to touch if you were to take it out immediately after/during those tests. \$\endgroup\$
    – KyranF
    Dec 8, 2014 at 6:30
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    \$\begingroup\$ @NickAlexeev I know that standard (not "protected") LiIon cells are not for "end users", but here where I live - I can buy unprotected cells as... battery pack. \$\endgroup\$
    – Kamil
    Dec 8, 2014 at 7:55
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    \$\begingroup\$ @KyranF Battery must be hot too, because flashlight body is made of pretty thick aluminium and it is very small. \$\endgroup\$
    – Kamil
    Dec 8, 2014 at 7:59
  • \$\begingroup\$ Updated this to recommend that you check the specifications of your precise make and model of 18650 for which the manufacturer has a complete specification document, unless it's a dodgy company. They are OEM so companies kindof insure them for sale to each other for safety within a range, because their business depends on reliability. \$\endgroup\$
    – bandybabboon
    Mar 26, 2019 at 10:15

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Maker and model specifications for your particular battery will state the range and the cutoff temperature for the thermal fuse which is mandatory in all 18650s sold in the US, that is probably the ultimate limit and I think it's 87-95'C. Today some batteries like Panasonic have very good PTC, you can short them from 4.4 volts and they are messed up but in one piece.

Battery shops generally specify a discharge temperature of -10 to 60'C and a charge of 0-45'C for 18650's, although charging under 10 degrees does not help lifespan, and will kill a bad battery.

different batteries get too hot at different output currents. Some batteries have temperature protection which switches off the battery above a certain temperature.

Some crazy people test their batteries to see the output performance, and dangerous temperatures:

from https://www.e-cigarette-forum.com/forum/threads/panasonic-ncr18650pf-10a-2680mah-18650-bench-test-results-a-great-battery-beats-mh1-and-mj1.692438/

http://www.diytrade.com/china/pd/12734942/Panasonic_original_18650_NCR18650PF_2900mAh_10A_3_7V_batteries_with_Flat_top.html

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