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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:

https://www.ebay.com/itm/5-10-20Pcs-TP4056-5V-1A-Micro-USB-Lithium-Battery-Charging-Module-Charger-Board/172856541571?hash=item283f0d8183:m:m1YzA8TerE_cywHj69klcVA

https://www.ebay.com/itm/10x-3A-Protection-Board-for-1S-3-7V-18650-Li-ion-lithium-Battery-W-Solder-Belt/282474147244?hash=item41c4c56dac:g:hu4AAOSwY3BZFV3A

Am I missing something? Would it be safe to connect these to a bare lithium polymer cell?

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  • \$\begingroup\$ How do these compare to the quality US-engineered units you have looked at from reputable vendors? \$\endgroup\$ – Harper Nov 7 '17 at 21:27
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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:

  1. When talking about voltage of a battery, it is important to distinguish quiescent voltage and voltage under load.
  2. 2.5V to 3.0V is a reasonable interval for shutoff voltage under load. 3.5 V is certainly too high.

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.

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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.

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    \$\begingroup\$ You're repeating the question as an answer. Cool. \$\endgroup\$ – Harry Svensson Nov 7 '17 at 20:03
  • \$\begingroup\$ @HarrySvensson What I meant is these cheap eBay ones aren't really good for the batteries as they claim. A higher quality protection circuit will definitely cutoff at 3.0V or 2.75V max. \$\endgroup\$ – The_Vintage_Collector Nov 7 '17 at 20:15
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    \$\begingroup\$ @HarrySvensson I did not get it as that. Rahul added a lot of info. Often a question is "I'm guessing this, that, etc. Is that true?" An answer of "yup. This for this reason, that for that reason, etc." is not a repeat of the question. \$\endgroup\$ – Harper Nov 7 '17 at 21:25

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