17670 and 18650 in series

[Pacecar]

On a Spiderfire L2 Cree LED flashlight that has a bore too small for a 18650, can I use a 17670 in series with a flashlight extension with a 18650? Is there a technical reason why I shouldn't run these two cells (both are li-ion rechargeables) in series? Thank you.

 

[Black Rose]

The 17670 and 18650 cells have different capacities and would have different discharge curves.

Chances are pretty good that the 17670 cell would become overdischarged or it's protection circuit would trip (if it's protected).

 

[KiwiMark]

can I use a 17670 in series with a flashlight extension with a 18650? Is there a technical reason why I shouldn't run these two cells (both are li-ion rechargeables) in series? Thank you.

It would work

BUT

You run risks of a major problem with the lower capacity cell if you let it go too flat or even worse get pushed into reverse polarity. It would be far safer to run 2 x 17670 cells that are closely matched to each other.

 

[Sgt. LED]

Throw away your money by killing cells and/or risk burning down your house while you sleep if something goes wrong.

OR

Spend a little bit more $$.

 

[kramer5150]

Very dangerous. the charge capacities are very different between the two. There is a risk of over-discharging the 17650 and it will explode/vent.

Lithium gas that vents from these cells is highly toxic, flamable and a violent reaction. See the lithium cell safety sticky.

Never ever mix cells of differing capacity. Its MUCH safer to drive two 17670 cells in that light+extension.

 

[redsfairlane]

I think it just won't work.

The cell in the back has to reach forward into the body of the light to make contact with the first battery, Remember that the body extends past the battery so that there is room for the switch to go on, so the 18650 battery is too big to do this.

It is also a bit risky as mentioned.

 

[alpg88]

why not just use 2 17670??

 

[AnAppleSnail]

why not just use 2 17670??

It's possible that idea is that the 17670 would go flat and the 18650 would run at reduced voltage. I've never tried lithiums this way.

 

[yellow]

when one cell goes flat, the protection kicks in and there will be no light at all,
no good idea


(btw: it is not good to discharge Li-Ions down till the protection acts. That is a totally "last ditch cell survival device", but not intended for regular use)

 

[redsfairlane]

If the cells are not protected, then this is an excelent example of the possible danger.
When the 17670 goes flat, the 18650 will or at least may begin reverse charging it, this is what can lead to venting with flame

Lithiums should not be run this way. Protected cells maybee if you need light and have no choice, but unprotected run like this is simply dangerouus.

It's possible that idea is that the 17670 would go flat and the 18650 would run at reduced voltage. I've never tried lithiums this way.

 

[Pacecar]

Thank you for the excellent advice! --- Pacecar