Danger in using dissimilar sized Li-ion Protected cells?

[txgp17]

Guys,
My seachfu must be weak, cause I couldn't find anything on it using the search or "Google Search" feature.

I'm thinking of boring a Surefire M3 out to hold 18500's. Presently, I have a A19 extender on it with 17670's.

Boring of the A19 isn't presently offered.

If I bore the M3, but not the A19, is there a danger in running one 18650 and one 17670? Both would be AW protected cells, and I'm running the lowest output lamp Lumens Factory offers.

Thanx in advance.....

 

[Benson]

Yes, there's somewhat increased danger. Ordinarily, the smaller cell's PCB would cut out on undervoltage, preventing you from seriously overdischarging it. But you're now single point-of-failure, whereas with two cells of the same capacity, if either protection circuit works, they'll shut off before either cell overdischarges much.

More to the point than the exact level of danger, though, why would you bother? You gain nothing from this, since the capacity of the 17670 is the limiting factor, you'd get just as good runtime from 2x17670. When there's no benefit to doing something weird and somewhat more dangerous than normal, it should be an easy decision...

 

[Kestrel]

Benson is right on. Both cells will deplete at the same rate, and when the 17650 is 'empty', it will trigger it's protection circuit, cutting off all current flow to the light, while there will still be some juice left in the 18650, nothing gained. There is nothing wrong with 2x17670 unless you bore both for 2x18650.

 

[txgp17]

Thanx for the input guys. Here was my line of thinking....

The 18650 out to provide a larger portion of the amps to the bulb than the 17670. Under equal amp loads, the 18650 would retain a higher voltage, causing it's electrons to have more pressure (voltage), and thus it would tend to provide slightly more amps than the 17670 until the point where their voltages (under load) were equal.

Does such a scenario only apply to batteries in parallel?

 

[Mr Happy]

With two (or any number of) batteries in series, all batteries pass exactly the same current. The same applies in general to all series circuits with any kind of device. It is one of the fundamental laws of electric circuits.