max voltage for 3x CREE LED lamps?

[02gf74]

I have a set of 3x CREE LED lamps, like those below. They have a 4x 18650 rechargeable battery pack to givre 4.2 V; so are two pairs wired in parallel.

Can I connect the lamp to a pack with 4 cells wired in series, i.e. 8.4 V? (there are CREE LED lamps with such packs).

I do not know what is inside the lamps and I don't think it will be easy to dismantle them - but I have made my own LED lamps using a LED driver which takes a variable voltage (up to 18V or so) to provide the correct current.

In theory these should have drivers so should take a variable input voltage but how high can it go before they go pop, if indeed they do go pop?

 

[JamesM]

One lithium cell is 4.2v (3.7v really though). The 4 cell packs that come with those lights are normally 2 cells in parallel in series with another 2 cells in parallel, giving 8.4v (7.4v really though). Why do you think yours is 4.2V? What voltage does it say on the charger?

 

[02gf74]

ok smary pants, my bad - voltages I quoted is that written on the charger

18650 battery is 3.7 V so I have 4 in parrallel or 2p2s (2 in parrallel and 2 in series).


..... but that it neither here nor there - what would happen if I plug in a lamp for use on 3.7 V to 2x the voltage?

 

[JamesM]

18650 cells are 4.2V when fully charged and with no load however in practice they generally considered by most to be 3.7v as they will quickly drop to this voltage once in use. The chinese like to use the bigger number though.

If it says 4.2V on the charger then t must be 4 in parallel. 2p2s would give 8.4V.

I'd was under the impression that the 4 cell pack that came with these lights were all 8.4v. If your battery pack is 4.2v as you said then I'd have suspected it to be faulty hence why I asked what the ouptut voltage of the charger was.

So if it a 4.2v pack/charger/light then its anybody's guess as to what would happen if you subjected to light to twice that voltage. It very much depends on the type of driver and it's input voltage range and the only way to find that out is to find the specs of said driver or try it and risk damaging the light.

 

[legrandefromage]

I accidentally bought the wrong higher voltage battery pack after I lost mine.

boy was it bright accompanied by a smell of burning as the circuit popped. Lamp still works but theres no on-off anymore!

 

[hamster]

The LEDs are constant current devices, not voltage driven. In practice it means that the driver circuit is what determines the input voltage. Many of the LEDs have built-in protection in the emitter itself.

 

[Standish]

boy was it bright accompanied by a smell of burning as the circuit popped Classic!

 

[legrandefromage]

Its absurdly bright. The battery has been sent to purgatory so I dont accidentally plug it in to the replacement.

 

[JamesM]

I'd guess its probably a linear regulator driving 3 LEDs wired in parallel. That would explain why these packs are 4.2v instead of 8.4v as they don't want a big voltage difference between the supply voltage (input to the reg.) and the forward voltage of the LED (output of the reg.). Linear regulators lose the difference between their input and output as heat, so 8.4v would generate a lot of heat and could cook the regulator.

 

[hamster]

Most of them seem to have some kind of switched drive - I can see strobing effects on my LED lights even on continuous when I look at the tread of the front tyre whilst riding. Many have very cost-reduced circuit topologies which may limit their ability to deal with out of range voltages.