Voltage sag question, sorry. Simple answer probably!

This is basic and should not be a long lasting thread.

What are the differences in voltage sag between a 17650 and an 18650?

I have killed 1 bulb today and though I'd ask a question before killing another.

I have an old 2C incan that seems to run nicely on 1 17650 but when I stuck in an 18650 the bulb flashed .
So did it die because there was less voltage sag or did it just die because I have been running the bulb too hard on the 17650 to start with.
The bulb says: Philips HPX21 2.4V 0.8A Found the package, also says 1.92 Watt.

I guess this is an OK place to stick this thread? :thinking:

Sgt. LED said:

did it die because there was less voltage sag or did it just die because I have been running the bulb too hard on the 17650 to start with. :thinking:

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Both reasons are very possible, but it is true that the 18650 will have less voltage sag under the same load as the 17650 since it is a larger capacity cell. The 18650 may also be newer or in better condition and will have less internal resistance and therefore have a higher starting votage.

Sgt. LED said:

The bulb says: Philips HPX21 2.4V 0.8A

Click to expand...

It does seem like you would be over driving the bulb a bit hard? A charged Li-ion is going to run about 4 volts, and you are feeding a 2.4 volt bulb. I would not imagine the bulb being happy with more than about 3 volts or so, and even a discharged Li-ion will likely be exceeding that. I think you were most likely over stressing the bulb. (Since the internal resistance of Li-ion is low, I would not think the voltage would drop all that much at a 0.8 - 1 amp load.)

It might be voltage sag. It could also be amps pumped out by different batts. Larger ones can put out more amps than smaller ones, or maybe the 18650 is better quality and putting out more juice. Either way, pumping 4.2/3.7 volts through a 2.4v blub is the incan equivalent cruel and unusual punishment . Better would be a 3-cell blub, which should be rated at about 3.5v.

OK I just tried it with a charged 17650 and a new bulb and she instaflashed!

I knew better anyway! Too much voltage for the bulb, I guess the 17650 I had been using was partially drained before I cooked this up.

Time for more bulbs! This time I will use the 3 cell and quit being goofy.

What temperature was the light when you flashed the first lamp?

Looking at the data sheet... You get double the output at 3.6 volts, if it wasn't overdriven.

The lamp you were running is a 15 hour lamp, I'm surprised you could overdrive it at all!

Model Voltage Amps Lumens Hours
HPX21 2.4V 0.8A 21 15
HPX22 2.2V 0.61A 12 15
HPX23 2.4V 0.7A 15 50
HPX30 3.6V 0.82A 46 10
HPX31 3.6V 0.72A 34 36
HPX32 3.6V 0.91A 50 10

I don't know if I'd go for the 10 hour or the 36 hour lamp.

jerry i h said:

It might be voltage sag. It could also be amps pumped out by different batts. Larger ones can put out more amps than smaller ones, or maybe the 18650 is better quality and putting out more juice. Either way, pumping 4.2/3.7 volts through a 2.4v blub is the incan equivalent cruel and unusual punishment . Better would be a 3-cell blub, which should be rated at about 3.5v.

Click to expand...

You can't "pump" more amps across the same resistive load unless you have more volts, in the end, the only thing the "larger" cells are doing that results in higher current, is holding their voltage up higher due to lower internal cell resistance. The current flowing is always related to the voltage supplied. Small cells sag more, and less current flows naturally as a result. There is no magic pumping effect as a result of cell size going on, it's ALL about resistance.

Eric