Something I don't understand about 'protected' cells.

Firstly, please forgive any ignorance that may become obvious in this post - I'm pretty new to directly handling lithium ion cells.

One of the concerns I have with the 'protected' 18650 cells I've recently started using is that they have a conductive metal strip running down the side of the cell which is only insulated by weak film coverings. This strikes me as particularly dangerous with a relatively high likelihood of leading to a short. After searching I found this issue has already been mentioned several times on these forums.

I may be being ignorant, but it seems unnecessary to me. I know that the protection circuitry requires power and hence connection to both terminals of the cell, however I don't see why it needs to take the form of a thick, high current capable copper strip with such poor protection. Surely the actual control circuitry only requires a tiny amount of power and as such could be serviced by a VERY thin wire in place of the usual thick strip. This would have several advantages; firstly, it would be incapable of carrying sufficient current to represent a serious safty hazard in the envent of a short it would simply burn out like a fuse. Secondly, it's smaller size could allow it to be better insulated and protected (even that hair thin magnet wire one can buy has a durable non-conductive coating). I don't see any reason why that should cost any more to implement than the normal copper strip arrangement.

Last point, most cells I've encountered are coated in very fragile and easily damaged film material, leading to exposure of conductive spots along the sides of the cells, clearly not an ideal safety situation. Much stronger polymer films are available and the per cell cost difference would be minimal, it seems odd to me that even 'premium' cells have such poor coatings.

I may well have missed or failed to understand something, if so please let me know. If not, does anyone have any idea why the cells are made as they are when there appear to be cheap and easy ways to improve safety?

Thanks!

if there were an abrasion and the strip were exposed, what would it touch? the inside wall of your flashlight's body...the same that is connected to the negative of the battery...where the strip is ultimately connected as well. no short.

positive and negative only meet in the driver/led circuitry.

now, if you had a flashlight where the positive of the battery made contact with the body (or put the battery in backwards), then you'd have an issue.

ProstheticHead said:

Firstly, please forgive any ignorance that may become obvious in this post - I'm pretty new to directly handling lithium ion cells.

One of the concerns I have with the 'protected' 18650 cells I've recently started using is that they have a conductive metal strip running down the side of the cell which is only insulated by weak film coverings.

Click to expand...

however, it is thick enough to be a barrier to the low voltages used in these systems. Generally, they use kapton tape or similar.

ProstheticHead said:

This strikes me as particularly dangerous with a relatively high likelihood of leading to a short. After searching I found this issue has already been mentioned several times on these forums.

I may be being ignorant, but it seems unnecessary to me. I know that the protection circuitry requires power and hence connection to both terminals of the cell, however I don't see why it needs to take the form of a thick, high current capable copper strip with such poor protection. Surely the actual control circuitry only requires a tiny amount of power and as such could be serviced by a VERY thin wire in place of the usual thick strip.

Click to expand...

the flatness is so that it still fits in ~18mm applications. The relativly large cross section is probably necessary so that the resistance of the copper strip does not cause a voltage drop that would confuse the protection circuit. Plus, it's easier to deal with than a bunch of tiny strands of wire.

ProstheticHead said:

This would have several advantages; firstly, it would be incapable of carrying sufficient current to represent a serious safty hazard in the envent of a short it would simply burn out like a fuse.

Click to expand...

now, that would be neat. Safety's all about compounding safety measures, and while the circuit is designed to cut off in the event of a short, it would also be nice to have a secondary circuit breaker.

ProstheticHead said:

Secondly, it's smaller size could allow it to be better insulated and protected (even that hair thin magnet wire one can buy has a durable non-conductive coating). I don't see any reason why that should cost any more to implement than the normal copper strip arrangement.

Last point, most cells I've encountered are coated in very fragile and easily damaged film material, leading to exposure of conductive spots along the sides of the cells, clearly not an ideal safety situation. Much stronger polymer films are available and the per cell cost difference would be minimal, it seems odd to me that even 'premium' cells have such poor coatings.

Click to expand...

I haven't dealt much with aftermarket cells, I only have/use bare cells. As such, I can say that the sleeves used on those are fragile as well, but that these cells were meant to be handled once or twice during the manufacture of power packs, then never to be seen or replaced again, and because of that there really is no need to make the sleeves more sturdy. For people like AW and Redilast who rewrap OEM cells with protection circuits, perhaps it would be something worth looking into. However, again we get into the diameter constraints.

ProstheticHead said:

I may well have missed or failed to understand something, if so please let me know. If not, does anyone have any idea why the cells are made as they are when there appear to be cheap and easy ways to improve safety?
Thanks!

Click to expand...

Always a balance between safety, performance, convenience, and price. If you do find some specifics, you can send the details along to AW or the redilast vendor. They will certainly appreciate helpful suggestions or useful links.