How far can LiIon be discharged?

How far can LiIon be discharged?

I do often see a question about how far LiIon can be discharged and if it is safe to charge a discharged LiIon. These questions can be fairly difficult to answer precisely, LiIon manufactures do not publish much about it and LiIon is not just LiIon, there are many different chemistries with similar but not equal specifications.

How to measure the discharge cut-off voltage?

Some people do not wish to discharge below 3.6 volt (or something), but why stop there when the specifications say 3.0 or lower?

There is some sort of reason for this and it has to do with how you measure voltage: Usual voltage is measured with the battery out of the equipment, i.e. the battery is unloaded. Here is an example:

Here I discharge a cell to 2.5 volt with 2A, when I stop the voltage jumps to 3.3 volt and it jumps fairly fast.

Zooming in on the stop, the curve shows 3 minutes. It takes 12 seconds for the voltage to raise from 2.5 volt to 3.1 volt. This means that if you take the battery out and measures it you will never see the 2.5 volt.

Lets try with another cell.

This is also a 3 minute view, the voltage raises from 2.5 volt to 3.1 volt in 3 seconds and in 7 seconds it is about 3.2 volt. Again it is not possible to measure the voltage where the equipment turns off.

How far the voltage jumps depends on the discharge current, a lower current will give a smaller jump and on the battery chemistry, different chemistries gives different voltage jumps. This means that measuring voltage with the battery out of the equipment will not give a correct indication of the minimum voltage the equipment discharged the battery to.

Due to the above jump in voltage it is rather difficult to know what the actual battery voltage is inside the flashlight when it is on and the safe way is to assume it is empty when the unloaded voltage is below some value. The 3.6 volt is fine for some batteries, but it is possible to go lower with many modern cells as seen above. The table at the end of the remaning capacity article can also be used.

How far to discharge batteries during normal use?

The first answer to this is easy, check the datasheet:

Here are some values from 4 different datasheets. The most common values are 2.75V and 2.5V today.

These values are the lowest voltage that is allowed on the battery if the full lifetime must be reached*. This means that for any normal usage this is the lowest value to discharge the battery to. This voltage do not include any current, i.e. the limit is not one that can only be discharged to at high currents, it is allowed to discharge with a very low current to this specification.

It is not within specifications to discharge below this voltage, just because the battery will recover to above this voltage when the load is removed.

*Usual, the 2.0 volt specification is not used for lifetime tests on that cell, there the lower limit is 2.5 volt.

But my over-discharge protection do first trip at 2.3 volt, is it faulty?

Usual no, protection is normally not used, i.e. it is allowed to go outside the range for best lifetime.

Here is some data from Samsung, the over-discharge protection is supposed to be outside the normal usage range.

And if I include a bit more of the table it can be seen that it varies with application. At high current the protection can be at a lower voltage.

From this table it looks like it is acceptable to discharge LCO/LMO (Real ICR/IMR) cells below minimum value, when it is done at high current, but with the more modern cells it is best not discharging too much below rated voltage.

How low voltage is safe to recharge?

Here are some illustrations from LiIon handbooks:

In both illustrations there is:

Operating range, that is the voltage range from the datasheet.

Protection/safety range, that covers a wider voltage range, but may add extra wear to the battery.

Keep out/unusable range, that is very low or very high voltage and the battery is dangerous.

The Samsung guidelines also list a minimum voltage.

These 3 items gives the minimum voltage as 1.5 volt, somewhere below 2.3 volt and 1 volt. This covers different brands and different chemistries.

Another importance detail is pre-charging, when the voltage is low the battery must not be charged at full current. A reduced current must be used.

When a battery is discharged to this low voltage it may lose some capacity and will lose some lifetime.

The reason to not charge very discharged batteries is because the chemistry breaks down at low voltages, depending on how much the chemistry has degraded the battery may explode when charged!

Conclusion

With the above as basis I will recommend the following:

1. Do not use over discharge protection as signal for when to charge batteries, it might wear the batteries down faster.
2. Do not charge batteries measuring below 2 volt, except if you have data saying it is safe.
3. When cells are down to 3.3 to 3.6 volt unloaded they are empty according to my tests, when the load is a few ampere.
4. Just because the cell recovers voltage, do not make it advisable to discharge below the limits while current is on.

And remember that if a battery gets hotter than usual during charger and/or will not charge fully (i.e. charger will not terminate), it is dead and maybe dangerous.

Notes

Lithium Ion Rechargeable Batteries Technical Handbook (Sony)

Lithium Ion Batteries Technical Handbook (Panasonic)

Pack Design Guideline (Samsung datasheet, I have done some cut and paste in the table)

More voltage jumps with and without load
4 Thanks

Thanks.

Thanks! very useful info.

Hey mate correct me if im wrong but if the battery says 2.8 volt cut of for example you need to reach that voltage to get the advertised mah?

Yes, but depending on chemistry and current you might get very close at higher voltage. The rest of the way down to 2 volt will only be few mAh, but will increase the wear of the battery.

10x m8,

I just watched a Discovery documental movie about telecommunication satellites, they stated that some of those use LiIon batteries( they didnt mentioned what type) : they never charge the LiIon pack over 65% of its potential, and nevel let it go down under 30%: thus the pack coud be recharged over 80k times! I was quite impressed realy

1 Thank

This thread should be a sticky. Maybe a title change, so it doesnâ€™t look like a question.

Zero

Thank you for summarizing and providing references for us!

The discrepancy between battery voltage under load (with the flashlight turned on) and the rebound voltage measured after pulling the battery out had always been a nagging issue in the back of my mind. Your explanation helped clear that up.

And caused me to re-think my charging/re-charging practices. In the past, I targeted 3.0V (resting) as being the minimum Iâ€™d like to see my battery levels when recharging. However, if the rebound voltage is 3.0V, then the voltage under load might have been somewhere under 2.5V - not good! So, that means Iâ€™ll need to adjust my target level to something more like 3.3V or 3.4V.

I like my titles to match for all places where I publish. This is not the first article with a question in the title, there is also this one: My charger do not charge LiIon to 4.2V, is it faulty? about low charge voltage, it was from there I got the idea for this article.

Anyway people looking for information will probably open a thread with a question title, to see if it has a good answer (At least I hope so).

Thanks HKJ. Again advice we should all heed.

Great info! Thanks

I would suggest â€śHow far can LiIon be discharged? (detailed answer)â€ť

Thanks for this interesting data HKJ.

I've recently been giving some advice on FastTech's forums to certain users concerned about the safety of using unprotected batteries on single-cell (1SxP) buck driven led flashlights. Since the absolute minimum voltage a typical (Cree) led would drop for any current to flow is 2'5V (checked XM-L/XM-L2 datasheets), my conclusion is that it is pretty safe. It must be understood that, as we approach that voltage, current flow diminishes exponentially: it'd take â€śagesâ€ť to reach there (and it would be undeniable to notify on the light's output.

It is a bit surprising to me to see such a jump between load and â€śno-loadâ€ť (voltmeterâ€¦) voltages in your measurements for that current flow, because that would mean there's close to 0'4Î© of resistance among:

• The battery's internal resistante (supposedly lower than 0'4Î©).
• Whatever stuff between the battery terminals and your readout locationsâ€¦ ?
• Am I missing something?

Hope the expounded above helps.

Regards,

This question is asked a lot on the forum so will ask here.
If the batteries voltage is measured less than the recommended minimum does it make any difference to the chemistry and safety of the battery if its discharged below the limit very quickly or slowly, for eg in a flashlight that has very low parasitic drain?
Many are tempted and it is often mentioned that it is safe to recharge a battery if it has been drained below the recommended voltage slowly.
Hope that all makes sense.

Draining slowly mean the battery spends more time at the lower voltage, i.e. more damage.
But as I say above the 2.5V or other end voltage is not the danger limit.

Well, Iâ€™ve barely read a thing about those Nyquist plots, yet anyway, that explains â€śeverythingâ€ť. Thank thee. In such a case, that just confirms (my) the predicted safety of using single (1SxP) buck driven li-ions led flashlights because of their minimum (led) Vf, which coupled with the intrisically higher internal impedance of the battery when approaching that low Vfâ€¦ means it is even more difficult for the battery to reach â€śthat lowâ€ť. Hope this is understood my dears. :laughing:

Cheers

What would your advice then be for discharging phone batteries to ensure a proper calibration of them?

For instance, I am using an iPhone 6 Plus. I read that the battery should be completely drained once per month. However, I read online that it is unwise to completely discharge the battery (run the iPhone on max brightness and play a video with max volume till it shuts off).

I, for now, donâ€™t let my battery drop below 10. I just wonder how I can fully calibrate it by letting the capacity of the battery drain fully so I can charge it from 0-100 and have the highest accuracy of calibration for the said battery.

You only need to calibrate it if your believe the remaining time is far off.
A full discharge/charge will allow the fuel gauge to recalibrate itself, these chips are counting mAh into and out of the battery and this will go out of calibration over time. Some of the newer chips can also correlate with battery voltage and do not need much calibration.

Iâ€™ve not done a â€śbattery calibrationâ€ť for months on my XT1032, and still it shows me accurate enough charge estimationsâ€¦ :sunglasses:
I minimize battery wear by not charging it above 60%, generally. See here:
BU-808: How to Prolong Lithium-based Batteries
Additional information: in the article it is reported that, for 4â€™2V maximum voltage li-ion chemistries, 3â€™92V or below no-load voltages are reported to do away with all voltage related stresses. 3â€™92 / 4â€™2 equals 14/15 so Iâ€™d use this fraction to determine/estimate this equivalent voltage for the now commonly used higher maximum voltage ICR chemistries (commonly used in modern mobile phones).

Stupid myths and [censored] seem to abound everywhere. :person_facepalming:

Your deviceâ€™s battery is completely firewalled through its power management circuitry. That means in no [censored] way will it allow the battery to be over-discharged.
I bet the lowest no-load voltage your phoneâ€™s battery has ever seen is fairly above 3V. Iâ€™ve never seen below 3â€™1V (load voltage, which is always higher than no-load) through my battery monitoring software before a forced low battery system shutdown.
What is more: what you call a â€śchargerâ€ť is not really a charger but, usually, just some kind of more or less capable constant voltage supply with @#\$% gimmicks.
The aforementioned power management circuitry inside your device decides what/how and when whatever crap is done: â€śnegotiatesâ€ť power input with the PSU/charger for up to the required amount, feeds the deviceâ€™s internals and then uses the remaining power to charge the battery.

How could they, the manufacturers, gladly offer the kind of device warranties you seem to enjoy in these iCraps, Crapdroids, Wincraps, etc. in the hands of uneducated [censored] morons without this kind of sophisticated power management? :person_facepalming:

Well, hope this is properly understood. Maybe I should write a sticky about this somewhere, for the love of @#\$%â€¦

Regards,