AC 18650 Replacement?

[Dick.Sonderegger]

I have one of the new LED "Tactical" flashlights. It's powered by an 18650 3.7V Li-Ion cell. I want to build this into a sculpture and find some way to power the flashlight by an external AC power source so I don't have to worry about replacing cells from time to time. Any thoughts?

 

[Chemelec]

First you need to determine How Much Current the Flashlight Requires.

 

[Dick.Sonderegger]

First you need to determine How Much Current the Flashlight Requires.

Agreed. The cell that came with the flashlight says "5500mAh", but that doesn't tell me how much current the LED chip draws, steady state. I have found several articles saying the LED chips perform best at 20mA. I have also found articles that claim if your device states it requires a certain voltage, it probably already has current-limiting built in. The 18650 cell that came with the flashlight says 3.7V, so I presume the circuit in the flashlight is set for 3.7V

 

[WHONOES]

A simple way would be to measure the current drawn by the LED from your battery and go from there.

 

[Dick.Sonderegger]

Agreed. The cell that came with the flashlight says "5500mAh", but that doesn't tell me how much current the LED chip draws, steady state. I have found several articles saying the LED chips perform best at 20mA. I have also found articles that claim if your device states it requires a certain voltage, it probably already has current-limiting built in. The 18650 cell that came with the flashlight says 3.7V, so I presume the circuit in the flashlight is set for 3.7V

Aha! I just found that this flashlight uses a CREE XM-L2 chip. The spec sheet for these chips states "DC Forward Current = 3000mA". The sheet also states "Forward voltage (@ 3000 mA, 85 °C) = 3.3V"

 

[Dick.Sonderegger]

Aha! I just found that this flashlight uses a CREE XM-L2 chip. The spec sheet for these chips states "DC Forward Current = 3000mA". The sheet also states "Forward voltage (@ 3000 mA, 85 °C) = 3.3V"

I'm fairly sure I can come up with a power supply that takes in 120Vac and pumps out 3000ma at 3.3Vdc. My real question is "How do I get that power into the flashlight?" Is there some readily available 18650 cell substitute that is wired to to accept external power?

 

[WHONOES]

How is the connection from battery to LED achieved?

 

[Dick.Sonderegger]

How is the connection from battery to LED achieved?

The 18650 cell that came with the flashlight is a "button top" cell. It has a protruding button on the positive end of the cell. This button presses against a spring on the underside of the holder for the LED chip. The negative end of the 18650 cell is flat. There is a protruding button in the tail cap of the flashlight that presses against the flat surface on the negative end of the 18650 cell. The switch is a push-button in the tail cap of the flashlight. I don't know (yet) what carries the current to complete the circuit. I don't know if the aluminum body of the flashlight carries that current of there is some other path.

 

[Chemelec]

Yes the Aluminum Body carries the Return current.

It doesn't matter if our supply delivers 3000 mA.
That 3000 mA is Reserve Current so it will last for a Reasonable Time.
Even 1000 mA would probably be Ok.

Most Common LED's Draw 20mA.
But these LED's are Probably Higher Power types, so they will draw More Current.
You Need to Measure the Current Draw when using one of those 18650 Batteries.

 

[Audioguru]

The 3000mA rating of the LED is the absolute maximum allowed current when it is cooled with liquid nitrogen or something so that its metal base is cooled to 85 degrees C. How does the flashlight cool it? Maybe 700mA will cook it. You need to measure its current in the flashlight to know.

 

[kellys_eye]

The LED may be driven in its constant current mode.

 

[Chemelec]

The LED may be driven in its constant current mode.
IF IT HAS THAT?

 

[Bluejets]

You could use just about any dc plug pack ( anything over 6v)that can handle maybe 2 amps and add a down converter off Ebay. These usually handle around 2 amp and adjustment for your 3v3 level. I doubt the LED would draw that much current anyhow.
As far as any current control or whatever, the cell is just a bare cell being replaced with a power unit so same thing really.

 

[Audioguru]

An 18650 lithium cell is 4.2V when fully charged and can provide at least 30A to blow that 3A LED to smithereens if nothing limits the current. The flashlight must already have a resistor or something to limit the current. But it will be difficult to find a power supply that provides 3.7V to 4.2VDC. Maybe a regulated 5V supply feeding a Schottky rectifier in series with a silicon rectifier will provide about 3.8VDC.

 

[Chemelec]

Audioguru, If you look at the LED's they use: "CREE XM-L2", they have a CUSTOM Made, Built in Current Limit Chip.

 

[Bluejets]

But it will be difficult to find a power supply that provides 3.7V to 4.2VDC

That's why I suggested a common 6v supply and use a converter to adjust down to the required level.
These converters are readily available and cheap from Ebay.

Op would still need to do a current check as Chemelec suggested earlier to ensure any such converter would be of sufficient size. Perhaps err on the higher side would be a good move.

Edit...here is a 2Amp unit ( actually 4 units )
https://www.ebay.com.au/itm/4pcs-DC...309959?hash=item417dac2d07:g:ZekAAOxy3HJTGyaL

3Amp one here....
https://www.ebay.com.au/itm/LM2596S...245111?hash=item2edf0690f7:g:T2wAAOSwWHBaQtcL

8 amp but need higher input voltage ( 7 to 32)
https://www.ebay.com.au/itm/DC-DC-C...489335?hash=item2cbaad6ef7:g:i80AAOSweWVXc9wk

 

[Audioguru]

Audioguru, If you look at the LED's they use: "CREE XM-L2", they have a CUSTOM Made, Built in Current Limit Chip.

The datasheet for the Cree CREE XM-L2 LED does not have a current limiting circuit. Whoever made the flashlight added a current limiting circuit and a heatsink but we know nothing about it.

 

[(*steve*)]

I think it's fairly safe to assume that the flashlight has already been used at least once and did not explode.

Hence, one must assume that a constant voltage source of about 3.6V is going to safely power it.

The remaining question is there required current. If the OP can't or won't measure it, then we can guess that it will be less than 3A (probably significantly lower, but we don't know).

The problem with a buck converter is that the OP needs to measure the voltage as it is adjusted to approx 3.6V. if the OP can't or won't, that's a problem.

A 2A 5V adapter would not be sufficient for the worst case 3A LED current, but may be worth trying anyway.

 

[kellys_eye]

Datasheets that I've seen show the device running at an 'average' of 1500mA.

It's maximum dissipation is 10W.

How hard it is driven is entirely down to the method of cooling employed but without that information we can't make any determination for current consumption at all. If the OP doesn't want (or can't) to measure the actual in-operation current then we're wasting time 'guessing'.

Personally I'd slap in a 5V PSU that can handle 3A and throw in a current limiting resistor(s) on the assumption of 1A, 2A and 3A operation and see what happens.

BEST method would be to dismantle the thing and see what's actually being used - CC or CV - determining the efficiency of the heatsink won't be easy.......

 

[WHONOES]

See datasheet link for more info,.
http://www.cree.com/led-components/media/documents/XLampXML2.pdf