> down to ~3.0V (discharged).

That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...

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kees99 a day ago | parent | next [-]

Most cell OEMs will specify safe discharge (low threshold) voltage in a datasheet. 2.75V is quite common [1].

That being said, system designer might choose higher cut-off point, since:

1) charge/discharge curve is S-shaped. There is very little energy in that last few millivolts;

2) battery (protection) circuit, and/or battery itself probably have some small leakage current. However minuscule, over months/years on a shelf, even some nano-amps of leakage will add up. If you want device to survive that, you have to factor this in, so that rest cell voltage still stays above safety threshold even after storage.

Also, "Li-ion" is quite a wide category. Don't use arbitrary voltage as a fast rule. Look up datasheet, or characterize actual cell you use. For some[2], disconnecting at 3.6V would mean leaving 50% of capacity unused. For other[3], that would be a reasonable, if somewhat conservative threshold.

[1] https://docs.rs-online.com/080b/A700000007848112.pdf

[2] https://www.murata.com/-/media/webrenewal/products/batteries...

[3] https://ntrs.nasa.gov/api/citations/20140005830/downloads/20... (page 4)

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cenamus a day ago | parent | prev | next [-]

Of course you can go down there? That's literally the lower limit of design voltage?

Below 2,5V is usually when you don't wanna use them anymore

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dmitrygr a day ago | parent [-]

only if you want them to lose capacity fast and become spicy pillows in your lifetime

	▲	clumsysmurf a day ago \| parent [-]
		2.5V is the absolute minimum typically mentioned in spec sheets, 2.8V and above is perfectly fine.

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Kirby64 a day ago | parent | prev [-]

3.6V is considered the nominal voltage, certainly not the low end cut off.

3.0V is considered basically the highest voltage. Most chemistries suggest even lower, 2.8, or even 2.5 in some situations assuming you can control the cutoff carefully. Perfectly safe to do so. You only start to have issues when you’re south of 2.5 without a load.

Most advanced battery usages let the cells drop even below that during heavy load.

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shadowpho 20 hours ago | parent [-]

>3.6V is considered the nominal voltage, certainly not the low end cut off.

This is not right (3.6v certainly is and can be cut off depending on device and battery).

One thing you are not considering is discharge after the cut off. Fuel gauge, protection circuitry, the cut off circuitry and battery itself has some discharge.

So you don’t want to have the cut off being too low because then the battery is permanently dead after not using it for X period of time.

You want to leave some margin there.

Depending on product, battery chemistry and design I have seen cut-off at 3.0-3.6v.

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Kirby64 19 hours ago | parent [-]

Anyone setting cut-off at 3.6V either is using it in some insanely industrial, ludicrous application where you need to handle cases like multiple years in storage... or doesn't know how to properly design their protection circuitry.

The margin is already there at 3.0V. You can still recharge batteries discharged below 3.0V. It just becomes dicey below ~2.5V.

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shadowpho 17 hours ago | parent [-]

>Anyone setting cut-off at 3.6V either is using it in some insanely industrial, ludicrous application where you need to handle cases like multiple years in storage... or doesn't know how to properly design their protection circuitry.

It really depends on application, battery size and leakage. In consumer world of electronics for example there’s an often requirement to make sure device turns on after being on a shelf for 1/2 - 2 years.

Then when you do the math it ends up needing to set the limit to 3-3.6v.

>The margin is already there at 3.0V. You can still recharge batteries discharged below 3.0V. It just becomes dicey below ~2.5V.

The margin isn’t big enough for some products. Furthermore some of the more leading edge batteries (in terms of energy density) have higher leakage which requires having more margin.

	▲	dmitrygr 17 hours ago \| parent [-]
		^^ this