Series Protected/Unprotected Li-ion

[divine]

I have one of Kai's 4x123 lights made for use with 3.6 volt cells and it came with 4 unprotected cells. I have protected cells, but 4 protected cells won't fit in the light. Two protected 880mAH and two unprotected 1000mAH cells will fit.

My reasoning tells me that since the unprotected cells are higher capacity than the protected, the protected will reach their low voltage state earlier than the higher capacity unprotected cells and the protection circuit should open the circuit protecting all of the cells. (As long as I make sure they are all charged at the same time.) Also, if there is a short circuit or circuitry problem, the protected circuit should open in that case, too, and protect the unprotected cells.

Someone school me if my reasoning is wrong here. Is two protected cells alternating with two unprotected cells enough protection? Maybe I should have the protected on the outside with the two protected on the inside, but if they are really in series, it shouldn't matter where I place them.

Is there any other situation that it would be a bad idea to use protected/unprotected in series like this?

 

[mdocod]

in a perfect world, your reasoning is very accurate and the protected cells would protect the unprotected cells from over-discharge, this is a far from perfect world where cheap cells are never consistent from cell to cell and routinely deliver 40-50% lower capacity than the label advertises. My point being that the "1000mAH" cells are probably closer to 400-600mAH like every other RCR123 out there, and so are the "880mAH" cells. Which cell actually has more capacity is unknown at the moment. The other problem is that I wouldn't be surprised if the unprotected cells would have slightly lower resistance in the circuit, which may or may not have some impact on how they discharge compared to the protected cells. You should never plan on relying on the low volt protection on a cell to terminate the discharge anyways. It should only happen by mistake. If you have discharged to a point that the protection trips, you have discharged the cell farther than ideal anyways.

 

[divine]

Okay, looks like I'm looking for new protected cells. :sick2:

Thanks.