I was looking for a battery charge- discharge circuit from ebay, and most if not all, state to have overdischarge protection voltage at ~2,5V. Isn't this way too low? By my knowledge Lithium batteries (LiPos in this case) can only be brought to around 3.5V before riskin damage to the cell? Certainly no lower than 3.0V. Here are some examples of the boards I found:
Am I missing something? Would it be safe to connect these to a bare lithium polymer cell?
This may not be a definitive answer, but a couple of points to note when choosing the threshold voltage.
The output voltage of a cell while discharging is lower than its quiescent voltage and the difference is higher for higher current. If you disconnect the cell after reaching the low voltage limit, its voltage will recover a bit. That's why discharge controllers should have hysteresis or memory to prevent them from turning on again without a charge cycle.
Another thing is if you look at a discharge curve of a Li-Ion battery, you will see that the voltage drops very quickly at the end of the discharge cycle. There is just little difference in terms of remaining state of charge between 2.5V and 3.0V. So the actual implication of choosing 2.5V vs 3.0V threshold voltage may not be as dramatic as it looks.
UPDATED. This document includes a discharge diagram: https://engineering.tamu.edu/media/4247819/ds-battery-panasonic-18650ncr.pdf.
At low current (0.2C), the difference in charge between 2.5V and 3V is ~3%, the cell is practically empty. At higher currents (2C) the difference is higher, however, this is partially due to the discharging battery being "unable" to yield all stored energy quickly. In this case, voltage recovery will likely be more significant.
UPD2. Summarizing, to make my points a bit more clear:
UPD3. By the way, all diagrams on the datasheet above are specified under V_cutoff = 2.5V, including the particularly interesting "Cycle Life Characteristics" one.
"[Overdischarge] protection is normally not used, i.e. it is allowed to […] [remain] outside the [usage] range for best lifetime. […] Do not use over discharge protection as signal for when to charge batteries, it might wear the batteries down faster." (source)
Maybe this answers your question already: discharging until over-discharge protection kicks in will wear a battery faster and reduce its lifetime. It will not outright damage it as you say, but reduce its lifetime, and should not happen during regular use. The device itself should also measure the discharge voltage and shut off earlier than the over-discharge protecting circuit, namely at 2.5–3.0 V measured under load, depending on battery chemistry (source).
This is called "minimum voltage for terminating discharge".
2.5 V as a cut-off limit for an over-discharge protection circuit is not "way too low", for the following reason:
The voltage measured by this circuit is measured during discharge (that is, under load), while the lowest voltage for safe recharging that you have in mind is measured as open circuit voltage (that is, under no load). For a lithium-ion cell, the two ways of measuring voltage will differ by up to 1.2 V due to the rebound effect. It depends on the load applied (higher load leads to higher rebound afterwards) and on the chemistry, age, temperature and manufacturer of the cell.
An example of the rebound effect can be seen in the following diagram (sourced from here), where a cell was discharged to 2.5 V at a load of 2.0 A, and then discharging was stopped:
The correct voltage at which over-discharge protection should kick in at latest depends on battery chemistry, manufacturer and load, but is typically 2.3-2.5 V for low-current applications (0.5-1C) and 2.0 V for high-current applications (assuming suitable cells, of course). (source)
You mention "Lithium batteries […] can only be brought to [certainly no lower than 3.0V] before risking damage to the cell". We already established that 3.0 V is the voltage under load at which discharging should stop to maintain the maximum lifetime of the cell. But discharging further will not immediately damage the cell, or make it unsafe to use.
So for completeness, let's see what voltage will actually damage the cell.
After cut-off by the over-discharge protection circuit and waiting for about one hour, we can then measure the open-circuit voltage of the cell. This voltage is relevant for deciding if it is still safe to recharge the cell. It is hardly ever stated explicitly that this has to be measured as open-circuit voltage, but (1) this is just how you measure cells in storage, (2) if it would not be an open-circuit voltage then the load to measure at would have to be given as well. Plus, (3) the following source implies that it is open circuit voltage by mentioning "dwelling" at that voltage, which does not happen if it were a voltage under load:
"Do not boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer." (source)
Typically, a lithium-ion cell will not become permanently damaged or unsafe to use until its open-circuit voltage falls below 2.0 V:
"Do not charge batteries measuring below 2 volt, except if you have data [sheets] saying it is safe." (source)
So the 1.5 V limit mentioned above is not applicable always, but it (and sometimes even lower limits down to 1.0 V) can be applicable for some batteries of some manufacturers. Below that voltage, permanent damage to the cell occurs by chemistry degradation and copper dendrite growth. The dendrite can cause internal short-circuiting and battery fires during a future recharge and are the reason why a cell that has been discharged too low must be discarded.
Also note that any over-discharged lithium-ion cell must not be charged at full current but rather must be pre-charged at low currents with a special charger ("boost mode") until it reaches 3.0 V open-circuit voltage or more precisely, until an appropriate over-discharge protection circuit disengages. Most chargers of consumer devices cannot do this and will consider the battery "defective", but the cells are not actually damaged and can still be recharged safely when applying a pre-charging step as described.
Yes, you are correct. Li-ion batteries can be discharged to a minimum of 2.5V, but it is recommended that the lowest voltage that a Li-ion or LiPo battery be discharged be 3.0V only. This will increase the longevity of the battery. Draining it to 2.5V causes changes to the chemistry like increased internal resistance etc. I've accidentally discharged below 2.5V and then the battery goes to short internally (I've had experience).
If you take a look at some ESCs from RC electric cars they usually shut-off at 3.0V. So, I guess it depends on the brand and quality of the protection circuit used.
The usual protection IC on most batteries I have seen which incorporate protection is the DW01A which cuts of at 2.4 - 2.5V.
If you want to cut off at 3.0V then you need a different protection IC. The Ablic IC for 3.0V is the S8261ABTMD G3TT2.
https://www.ablic.com/en/doc/datasheet/battery_protection/S8261_E.pdf
I'm also having issues finding suitable overdischarge bms modules.. The battery manufacturer is very firm on cutting off at no lower than 2.7v
I have a programmable battery discharger that lets me set the discharge current and termination voltage, and it reports the full milliamp hour rating for the battery at reasonably high currents despite the fact the voltage will rebound a bit when the load is removed.
The voltage cutoff being lower (2.4v is what most modules have now, a DW01 protection IC) would leave the battery depleted such that any small parasitic load from the protection circuit itself can damage the battery if not immediately recharged. Also, smaller loads won't cause as much voltage drop as bigger loads and with a small load and a too low cutout voltage the battery will be drained too much, there will be very little rebound of the voltage with a small load.
I trust the battery manufacturer's specs, they're right on target for rated loads, rated low cutoff voltage, and rated watt-hours for the cells.
I'll not mention my opinion of why 3rd parties would supply only boards that will shorten the life of batteries for no added value to the consumer.
