I have a Fenix ALB-18-3500U cell. It was stored at 3.7v over the summer and more recently started being used again, I decided to fully discharge it and then recharge it with a VC2 to see how much capacity it’s retained. To discharge it I left it in my PD32v2 on overnight and in the morning it had powered off but would still turn back on and then just drop back down to the lowest mode. It stayed on for several seconds at this point until I turned it off.
I’m confused by this as it can only have been powered off by either the light or the cell protection, is the fact that it now will turn back on simply an artefact of voltage rebound?
I didn’t want to risk over-discharging so I just put it in the charger which read 3v, but am curious to know if repeatedly turning the light back on would be safe and whether the cell hasn’t actually been fully discharged. If the latter is true then the capacity report on the charger won’t be accurate.
It has been discharged to the low-voltage limit setting of the flashlight controller, which may be different than the discharge voltage limit setting of the VC2. One is used to protect the cell from damage, the other to determine the maximum capacity as determined by the VC2 algorithm. A healthy cell should exhibit some rebound, if it doesn’t then it is damaged or defective.
Nothing good is going to come from trying to fully discharge a lithium ion battery. Not all cells or flashlights have the same low voltage limit. If you don’t know what that batteries low voltage limit is per the manufacturer don’t try to go lower. If you don’t have a digital voltmeter you are really in the dark trying to measure anything. Battery capacity is measured while discharging a battery not while charging. The rate of discharge will affect the measured capacity. There are some more expensive chargers that can do a discharge capacity test.
Since it’s a protected battery, you shouldn’t be able to discharge it too low. Even if you put it into a flashlight without LVP, the battery should shut down below a certain voltage. Not sure how the LVP works on the Fenix PD32, but you probably have double protection for the battery. And since it’s dropping down to the lowest mode, it appears the flashlight LVP is kicking in vs the battery. If it was the battery, the flashlight would not turn on or just turn off.
It seems normal for the flashlight to turn on then quickly shut down when the LVP kicks in, even on a depleted battery. My ZebraLights do the same.
The Japanese like to test down to 2.5v, the South (good) Koreans like to cease discharge at 2.75v.
Doing this everyday, won’t benefit li-ions, but once in a while, it’s not going to kill them, either.
The real and dangerous problem was in leaving a lithium-cobalt cell in a really low voltage…say <2.5v for weeks and months and then charging them up.
The copper separator could form copper dendrites in that discharged state and cause a short. If you charged them quickly back up, you’re left with just damaging the capacity of the cell and not much more.
Since the last Li-Co/ICR cell made by the Big 5 was the NCR 2900 by Panasonic, which was years ago, we don’t have to worry about venting with flame.
Now stuff made by generic ChiCom companies…all bets are off.
The separator is a porous plastic membrane about 25 microns (0.001”) thick that allows the positively charged Lithium ions to easily pass between the anode and cathode in the electrolyte. Copper foil is the negative current collector that is coated with a Lithium-Carbon(graphite) paste that is the negative terminal (anode). Aluminum foil is the current collector for the positive terminal coated with Lithium-Cobalt Oxide, or -Manganese Oxide, or whatever is the latest gee whiz cathode.
During discharge Lithium ions leave the graphite matrix and transport thru the separator and embed into the cathode matrix, and visa versa for charging.
If a cell is discharged too low, then all the Li+ ions get pulled out of the anode matrix and then the chemical reaction will start pulling copper ions from the collector. This form sharp copper dendrites which will puncture the separator and create a short circuit directly to the Aluminum collector.
During a subsequent charging session this short circuit current path causes the interior of the cell to heat up (I^2*R) and increase the interior pressure causing swelling. The copper dendrite reaction is not reversible which is why folks warn not to try to use a cell that has been over-discharged, and why such a cell will tend to self-discharge and not hold their voltage. The rate of over-discharge likely affects the dendrite formation and damage threshold. During a fast discharge the cathode matrix can fill up and cause heating also as ions are fighting each other to find an empty seat at the bar.
There is information on the websticle about how NASA selects lithium cells for space flight applications, in which they totally discharge a cell to zero volts, then check to see if it rebounds. Then charge it back up to full and do a capacity test down to some normal level. Charge it up to some level and put it in storage and check the voltage in a year. Cells that survive the testing are allowed to be used. Other cells of the same design get accelerated charge-discharge cycling to determine lifetime and qualify the design.
