http://www.mpoweruk.com/lithiumS.htm says 2V too -- I wonder where they are getting figure from.
http://en.wikipedia.org/wiki/Lithium_ion_battery says
Li-ion batteries should never be depleted to below their minimum voltage, 2.4 V to 3.0 V per cell.
http://batteryuniversity.com/parttwo-34.htm says
Some lithium-ion batteries fail due to excessive low discharge. If discharged below 2.5 volts per cell, the internal safety circuit opens and the battery appears dead. A charge with the original charger is no longer possible. Some battery analyzers (Cadex) feature a boost function that reactivates the protection circuit of a failed battery and enables a recharge. However, if the cell voltage has fallen below 1.5V/cell and has remained in that state for a few months, a recharge should be avoided because of safety concerns. To prevent failure, never store the battery fully discharged. Apply some charge before storage, and then charge fully before use.
http://www.buchmann.ca/article28-page1.asp says
To prevent the battery from over-discharging, the control circuit cuts off the current path at about 2.50V/cell.
But http://www.mpoweruk.com/life.htm says
By restricting the possible DOD [depth of discharge]
in the application, the designer can dramatically improve the cycle life of the product. Similarly the user can get a much longer life out of the battery by using cells with a capacity slightly more than required or by topping the battery up before it becomes completely discharged.
This last doesn't seem to refer specifically to lithium, but
http://www.mpoweruk.com/lithium_failures.htm also says a 2 V minimum.
Under-voltage / Over-discharge
Rechargeable Lithium cells suffer from under-voltage as well as over-voltage. Allowing the cell voltage to fall below about 2 Volts by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.
Anodes
First the anode copper current collector is dissolved into the electrolyte. This increases the self discharge rate of the cell and can ultimately cause a short circuit between the electrodes.
Cathodes
Keeping the cells for prolonged periods at voltages below 2 Volts results in the gradual breakdown of the cathode over many cycles with the release of Oxygen by the Lithium Cobalt Oxide and Lithium Manganese Oxide cathodes and a consequent permanent capacity loss. With Lithium Iron Phosphate cells this can happen over a few cycles .
Where does the 3.6V figure originate? Is that an older or newer cell (meaning how long ago the cells associated with that number was made), or does it refer to older cell (meaning used more or aged more), and is the for the same chemistry as cells referred to above? WHICH li-ion chemistry? And is 2V a critical minimum while 3.6 is minimum for best practice and cell life?
But there seems to be something wrong with that 3.6 volts figure is
http://www.ibt-power.com/Battery_packs/Li_Ion/Lithium_ion_tech.html is correct in that
Lithium Ion Cell Specification Nominal Voltage 3.7V
With the voltages in the graph:
Typical Discharge Graph
CC/CV Charge at 4.2V,1C, +25ºC. CC Discharge at 0.2C to 2.75V.
Did you recall that figure incorrectly, or is there another criteria here? It does look like with most of the discharge curves letting it go below 3.6V would lead to the ight soon going dim, and perhaps with a regulated light sucking too much juice from the battery.
But then
http://www.batteryjunction.com/protected-14500-lithium.html
UltraFire Battery LC14500 3.6V
Capacity: 900mAh
Rechargeable for 600-800 times
Peak Voltage: 4.2V
Nominal/Working Voltage: 3.6V
Temparature: -40C~60C
Dimension: 14x52mm
Protected against short circuit, over discharge, over-current discharge.
It looks to me like your 3.6V figure is mistaken, as far as damaging the cell.
