18650 li-on battery works in my 4 x AAA flashight - is it safe to use? (setup, LED)
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Just bought a harbor freight tactical flashight that runs on 4 aaa batteries and stuck a 18650 battery in it and it works much to my surprise. The flashlight isnt made to use 18650 battery so i want to know if it's likely to damage the light or not. Because i was thinking of using the 18650 to see if run time would be longer than 4 aaa eneloop rechargeables which last about 1 hr or so.
Last edited by Live-life; 10-11-2023 at 01:54 PM..
Just bought a harbor freight tactical flashight that runs on 4 aaa batteries and stuck a 18650 battery in it and it works much to my surprise. The flashlight isnt made to use 18650 battery so i want to know if it's likely to damage the light or not. Because i was thinking of using the 18650 to see if run time would be longer than 4 aaa eneloop rechargeables which last about 1 hr or so.
I am trying to figure how you managed to have the flashlight working using only one AAA lithium battery.
Anyway, the nominal voltage of the standard Alkaline AA and AAA battery = 1.5-V, While a non-rechargeable Lithium AA and AAA = 3V (3.5 volts for the rechargeable one).
Four new Alkaline AAA batteries inside the flashlight = 6 volts (4 batteries x 1.5 volts each).
Four 18650 AAA batteries in the flashlight = 12 volts (4 batteries x 3 volts each).
Three alkaline AAA batteries in your flashlight + one 18650 lithium battery = 7.5 volts.
The LED may illuminate a little brighter, but you are risk damaging the flashlight unless the LED can handle the voltage increase (from 6 volts to 7.5 volts).
The flashlight does feel a little warmer with the 18650 than with 4 AAA's but not much. It's a $10 flashlight from harbor freight so I guess if I damage it using the unintended battery just go pick up another one. What I really want to see is if runtime will be noticeably longer with one 18650 compared to 4 AAA' s since I'm using it as a bike headlight and trying to get longer run time if possible. But yeah I'm surprised the flashlight even works with that battery.
Last edited by Live-life; 10-12-2023 at 09:27 AM..
Well, if you still have enough space you could replace 18650 with 21700 battery for even longer life.
You should buy protected batteries. Unprotected batteries do work, but pose a fire or burst risk if the device, charger and/or use overdraws or overcharges the battery.
A Protected battery has a tiny Internal PCB (protection circuit board). The PCB prevents under-voltage during DRAW and over-voltage during CHARGE. Use unprotected batteries only in a device that has its own overdraw and overcharge.
The flashlight does feel a little warmer with the 18650 than with 4 AAA's but not much. It's a $10 flashlight from harbor freight so I guess if I damage it using the unintended battery just go pick up another one. What I really want to see is if runtime will be noticeably longer with one 18650 compared to 4 AAA' s since I'm using it as a bike headlight and trying to get longer run time if possible. But yeah I'm surprised the flashlight even works with that battery.
Yes, the LED will run hotter at 7.5 volts than the 6 volts it was designed for. Also, you should be aware that Lithium does not react well to high temperatures.
I watched a video where the host was explain how his pistol tactical flashlight blew-up when shooting his pistol at night. Tactical flashlight manufacturers include a warning relating to the specific battery types that can be used with their gun-mounted tactical flashlights.
That is true. The rechargeable version's voltage is 3.7 (volts), but for a non-rechargeable lithium battery's voltage is 3.0 (V). The OP put three AAA 1.5-volt batteries (= 4.5 volts), plus one 3.7-volt 18650. Total voltage = 8.3 volts. It means that he's running the flashlight's LED by over 2 volts (out of the flashlight design). That flashlight can only get hotter since the LED is being overloaded. It will be slightly brighter, but will get hotter sooner.
The flashlights designed for the use of Lithium batteries need to have a housing that aids dissipate the heat produced by the LED. It makes no sense to use batteries the are outside of the flashlight specifications. Even a regular LED flashlight that uses Alkaline batteries can get hot in one's pocket if one forgets to turn it off, or if left turned on for several minutes at a time.
I'll update after using flashlight with 18650 on bike ride to gauge brightness + battery life compared to the 4 aaa eneloop rechargeables i been using. In the house 18650 seems brighter but be able to tell better on dark roads.
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