This is the split-off discussion about Li-Ion technology from this thread:
Ah, so it was Bulk or whomever, that makes sense. Your name is on the post, so I assumed it was you making the comments over there.Originally Posted by Geologist
Okay. Now, question, do you know what actually causes the cell to shut down? It is the cell's protection mechanism. This is specifically designed to protect the cell if all else fails. You don't need to explain to me what a regulated light is, I do understand much better than many folks, how a switcher works, and how in this design the uC is sampling the current thru a sense resistor, to alter the switchers operating point at the FB node.Originally Posted by Geologist
As the cell voltage drops, when you have a regulated output, the input current has to go up, in so you have enough power (think watts) comming in.
So, as the cell voltage drops, the input current goes up. This causes more current to flow thru the Li-Ion cells PTC (which is one of the last ditch protection mechanisms- short the vents that are designed to make the cell spew stuff at a slower rate than a totally violent explosion- and those don't always work either) (assuming you don't have a sealed cavity, like a water tight flashlight with o-rings). The higher current causes the PTC (Positive Temperature Coefficient- resistance goes up with temp) to start heating up.
These PTC protection devices have a knee in them, so that when you exceed a certain threshold, their resistance rapidly goes up, which causes the current to go down (the cell's temperature also adds to the heating of the PTC). This is the mechanism that is shutting down the cells.
That PTC shutdown mechanism is the last ditch effort to protect the cell before it vents. It is certainly **not** designed for routine protection. Anyone that is relying on this as the protection mechanism is certainly playing with danger.
This PTC mechanism is installed right under the + button of the cell.
There is another type I've seen which is actually a polyfuse. When these blow, they actually disconnect. They must cool down before "resetting/healing" to allow any current to flow. Most battery vendors get around this, and make them work like PTC resistors (since they are cheaper), by squeezing the polyfuse under pressure in the cell. This causes them to act more like PTC resistors, but causes a rather considerable variation from cell to cell. The amount of pressure put on the polyfuse causes more current to be able to flow, before the polyfuse kicks off. This pressure is not something that is easily controlled, nor will necessarily stay constant over time.
The other protection mechanism you mentioned is in the switcher chip itself. This protection mechanism kicks off way up there at 125C, which is certainly a highly dangerous area for a Li-Ion cell to be at.
Also note, he mentioned right off the LTC3441/3443 datasheet, that hitting this 125C temperature isn't something you want to do, as the chip could blow.
Now, did you realize that 130C is considered the actual thermal runaway point for Li-Ion cells? As the switcher chip, which is the flashlight's "thermal protection" is not thermally connected directly to the cell, plus as Georges80 mentioned, the chip is thermally relieved by vias into a broad ground plane on the backside, it would be fool hardy to rely upon this for thermal protection of the cells, since it isn't monitoring the cell's temperature.
Most manufacturers will recommend the cell stays below 60C. This is a sheet for the Molicel:
"Temperature: Discharge ˆ20 to +60° C Charge 0 to +45° C Storage ˆ20 to +25° C (occasional short excursions to +60° C)"
From another datasheet:
Operating Specifications- Temperature Range
-Discharge -20C to +60C
3.8 Operating Temperature Charge : 0 to 45C
Discharge : -20 to 60C
Please, keep in mind that LG and Molicel are the top Lithium Ion rechargable cell makers on the market, and most cells don't come close to their standards.
Once a cell hits thermal runaway, even removing the current draw will not stop the thermal runaway.
Now what happens when the chip blows? It is very likely it will short out. This puts a short on the Li-Ion cell. You [b]**really**[b] don't want to be shorting out Li-Ion cells...
Neither of these protection mechanisms would be something I'd even consider a true protection mechanism. Both are designed to function in disaster situtations, and each have their issues. This is nothing like the ARC or HDS design where things are actually monitored and controlled, so things don't reach this dangerous situation in the first place.
Protected cells are one step better. They put additional electronics in/on the cell, that protect against overvoltage, undervoltage, overcurrent, overtemperature.
With higher end Li-Ion cells, like the LG Li-Ion, they add a porous plastic "screen/mesh" in the cell. This is there in case the Lithium starts to kick off. What it does is to melt, hopefully cutting off the anode and cathode current path. There still have been quite a number of Li-Ion rechargable cells that have gone off with external protection circuitry, PTC, and the melting mesh. Due to this, Molicell has added an additional high temp mesh, on top of the lower temp melt mesh, that melts at yet a higher temperature, in an attempt to yet further protect against explosions.
These Li-Ion rechargeable cells really are not something one should be trying to push for all they are worth!