Battery Safety: What does deep discharge and overdischarge mean and why is it dangerous?

Lithium ion batteries are regularly in the news because of individual batteries catching fire. But most people don't know, what mechanisms are actually causing a safety threat.
In this video, you will learn what deep discharging of Lithium ion cells means, what overdischarge is, and in which cases it poses a safety risk and why.
Throughout the video, I am referring to the following papers and resources:
Connor Fear - Elucidating Copper Dissolution Phenomenon in Li-Ion Cells under
Overdischarge Extremes
doi.org/10.1149/2.0671809jes
Hui-Fang Li - Effect of Overdischarge on Swelling and Recharge
Performance of Lithium Ion Cells
doi.org/10.1002/cjoc.200890286
Rui Guo - Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries
doi.org/10.1038/srep30248
Thomas Langer - Studies on the deposition of copper in lithium‑ion batteries during the deep discharge process
doi.org/10.1038/s41598-021-85...
Jan Philipp Schmidt - Beneficial Use of a Virtual Reference Electrode for the Determination of SOC Dependent Half Cell Potentials
doi.org/10.1149/1.3684800
Electrochemical potential series
leverageedu.com/blog/electroc...
André Karwath - Digital Multimeter
commons.wikimedia.org/wiki/Fi...
Credits to the title music:
Your Intro von Audionautix unterliegt der Lizenz Creative-Commons-Lizenz "Namensnennung 4.0". creativecommons.org/licenses/...
Künstler: audionautix.com/
Short Guitar Clip von Audionautix unterliegt der Lizenz Creative-Commons-Lizenz "Namensnennung 4.0". creativecommons.org/licenses/...
Künstler: audionautix.com/

[@mrlillebroder]

Just found your channel. Very interesting! Looks forward to see more from you!

[@meandthebattery9628]

Thanks for watching and your feedback. Very much appreciated!

[@rf1133]

Such an inspiring video! May I please ask a question about the figure at 1 min: why there is a sudden voltage drop when the current becomes -3.4 from 0 (start of safe discharge)? Thanks for your time!

[@oleksii.zagorskyi]

Thanks for your efforts to make these videos!

[@meandthebattery9628]

Thanks for watching and your feedback. Very much appreciated!

[@danielreyesmartinez6361]

Excelent explanation! It was very clear, will look for the SEI video

[@meandthebattery9628]

Thanks for watching and your feedback. Very much appreciated! I hope I'll soon find the time to continue with the series....

[@premkarthikmanoharan6395]

Great Explanation!! But we had a scenario of a 48V 15Ah NMC pack which went in to deep discharge by keeping it Idle for several days and OCV is around 9V. Then we tried to charge at 1A , then slightly increased to 5A when it reaches 16V . But thermal runaway happened at 28V . What could be the major reason in this?? Any clues

[@FJL4215]

Wow, great video with interesting information.

[@meandthebattery9628]

Thanks for watching and your feedback. Very much appreciated!

[@djr3485]

Interesting videos, thanks for the in depth explanation. Do you see any harm in "bumping" a cell which has fallen slightly below threshold in order to allow it to accept a charge again from the charger? I understand there may be a degradation in performance from the chemistry deterioration, but if the cell is only slightly below this threshold, say >2.0V I can't see why it would be so problematic.

[@meandthebattery9628]

There might be cases where that does not lead to a problem. But very often there is a reason that led to the deep discharge (even if it is still >2.0V). So unless you know exactly what history the cell has seen and you understand the cell chemistry well enough to judge what electrode potentials were reached when the cell has this voltage, my clear advice is don't do it.

[@djr3485]

@@meandthebattery9628 Fair enough. I'm not entirely convinced it is as problematic as made out to be. There are scientific papers showing that the copper formations risking an internal short do not pose a significant problem until a state of -12% discharge. This would suggest that in the vast majority of cases, if the battery is 0V or above, the cell can safely be restored to service without any appreciable loss in capacity.

[@przemekostrowski5836]

Hello.
We are making a battery powered (Li-Ion) device at work. The original design assumes that the device goes to sleep when it is not used, however it can never be completely turned off and it always drains the battery. How big of a problem is that in your opinion?

[@meandthebattery9628]

Hi, thanks for watching and your question. If your device can discharge the battery below the limit specified by the cell manufacturer, it is a potential safety hazard. I would propose a two step safety approach to avoid deep discharge.
1. send the device to sleep mode whenever possible and prevent wake up if one of the cell voltages drops below e.g. 3V.
2. switch off the device completely if the cell voltage drops below e.g. 2.5V. The values for the limits depend on your specific cell.
In any case, you should prevent recharge of the battery in case it was deep discharged. Hope that helps!

[@pufferyreviews3273]

Ur videos are great , keep it up

[@meandthebattery9628]

Thanks for watching and your encouragement!

[@yazhengli4310]

The graphit on the copper current collector will have very poor "arrtion"?

[@meandthebattery9628]

Adhesion. It will not stick to the copper any more since the copper surface dissolves

[@sayantanjana7571]

So what about cell balancing in electric vehicle? Should we deep discharge (soc 10% or lower) and the. Recharge to 100% for better cell balancing and battery health....or is this practice bad

[@meandthebattery9628]

Thanks for watching and your question. As long as your BMS works correctly, it should at least not harm your battery (preventing deep discharge) . If it is beneficial or not depends on the balancing strategy implemented. In general, a good system should not require a full discharge for proper operation.

[@themartinandersson]

If I fully discharge a battery and I let it just sit somewhere forever in a fully discharged state. Is this dangerous? Or is it, the RECHARGE after full discharge that is dangerous? I can only pick up hints from this video and other sources online that is actually the activity of recharging which is the dangerous part, yet y'all keep saying "deep discharging is dangerous". Is this just sloppiness on your end or is it really the act of depleting all energy from a battery that is dangerous (in my uneducated, yet logical brain, this doesn't make much sense lol)?

[@meandthebattery9628]

Hi, you are right, it is the recharge that causes the problem. Deep discharge is only dangerous if you don't detect it and prevent recharge.