Flashlight is supposed to use 18650 batteries, will 2 - 18350 work?

I have a UltraFire 1600Lm Cree XML T6 18650 Zoomable LED Flashlight. Will using 2 18350 batteries increase output greatly without burning up the led? Just wondering if this would work, but afraid to try it.

I wouldn't...I'm not a flashlight guy and let others chime in,,but don't!

If the flashlight is designed to be powered by one 18650 only, with two 18650 in series you might kill it. If you connect batts in series you increase voltage. 18650 are usually 4.2 V, so you get 8.4 V with serial connection - that is quite a lot and if you do not kill it you might lower its life time significantly. I would not try it and looked for another flashlight for higher output. However you might connect batts in paralel to keep 4.2 V and allow to draw higher current. Maybe it might increase the output as there will be not that large voltage drop under high load.

To give you a reference I have 1700 lumen flashlight for my night biking and laserpainting purposes and it uses four 18650, the battery pack is combined by serial and parallel connection giving out 8.4 V (two parallel conected pairs of serially conected batts). You might check also this flashlight forum:

CandlePowerForums

There you should get more accurate advice regarding flashlights faster.

If you use two 18350's in series you will be inputting 7.4V instead of the 3.7V you need. This is highly likely that it will kill the flashlight, or significantly reduce its lifetime. Also two 18350's will be 5mm longer than one 18650. I recommend you stick to the recommended battery type.
This wouldn't benefit you in any way as even if you had two 18350's in parallel, the capacity would still be less than a single 18650. If we are talking about protected cells that is.

Edit: Was in process of writing this at the same time Radim. Also he wants to use 18350's. Two 18650's would be significantly too long for the host.

CurtisOliver said:

If you use two 18350's in series you will be inputting 7.4V instead of the 3.7V you need. Also two 18350's will be 5mm longer than one 18650. I recommend you stick to the recommended battery type.
This wouldn't benefit you in any way as even if you had two 18350's in parallel the capacity would still be less than a single 18650. If we are talking about protected cells.

Edit: Was in process of writing this at the same time Radim also he wants to use 18350's. Two 18650's would be significantly too long for the host.

Click to expand...

Oh, now I realized I overlooked the OP wants shorter batts. Sorry, I tacitly assumed the OP was going to have some custom made extension or external battery pack. In this case it is as Oliver says. By combining two shorter batts there will be some place wasted between them, which could be used for more capacity of single cell instead. In other words - no reason to do it like OP asks IMO.

No problem Radim. Two 18650's in parallel makes more sense.

Hi,
Sorry I'm late on this good info for this member here. I killed plenty of flashlights using 2 lith-ion's when they required one cell . but it made no difference being i was tearing them down to build . most of those are set in parallel not series .

Rich

I have modded a couple of flashlight with more powerful led/driver combo's I got off eBay fairly cheap. You might want to search eBay for something more powerful that could be made to fit. Cree make some drop in modules that might work. Hope it helps.

Yeah, as has been said by many before you can't just increase your voltage to the LED driver to increase the time between charges. Better to get higher output goon new batteries. I use Panasonic 18650s at 3700 mAh. These give me a long period between charges.

If it would work at all or be damaged depends entirely on the driver circuitry used.

I case it is a buck regulator that can handle 8.4 volts input it would not be a problem, but you woud not get more lifetime either. As pointed out probably a bit less as there is less energy in the two smaller batteries combined due to the gap between them etc.

If it is a linear regulator it could overheat and die, or it coud survive, produce a lot of heat, and cut working time in half or a bit under.

Chances are that a light made for a single 3.7 volt cell would be damaged if powered by 2 shorter cells in series. The only way to find out is to figure out what the driver circuitry actually is. Since the forward voltage of a typical white LED is very close to the cell voltage of a lithium battery this may vary quite a bit.

If it uses some led with multiple dice in series (like the 10/30/50/100 watt bigass ones) it's probably a boost topology and likely to be okay with 7.4 volts input.