What actually happens when lithium batteries are over-charged or deep discharged?

/ / Battery Technology

Charging Process:

Lithium-batteries are charged with constant current until a voltage of 4.2 V is reached at the cells. Next, the voltage is kept constant, and charging continues for a certain time. The charger then switches off further charging either after a preset time or when a minimum current is reached. In the rare event that the charger does not switch off, for example continues to allow the charging current to flow, there is an electronic protection circuit and possibly a passive safety element located on the cell. These protective mechanisms ensure the safety of the user. But what dangers can arise when batteries are over-charged or deep discharged?

In order to operate lithium-batteries safely and optimize their life span, they should not be over-charged or deep discharged.

What happens when a battery is over-charged?

If neither the charger nor the protection circuit stops the charging process, then more and more energy enters the cell. As a result, the voltage in the cell rises – this is known as over-charging. On the one hand, this is harmful to the battery and bad for its life span. On the other hand, it can pose a safety risk for the user. The excess energy leads to heat generation. “In the worst case, this can lead to a so-called ‘thermal runaway’. This means that the temperature in the cell continues to rise and chemical reactions start from it, which proceed exponentially. These reactions can no longer be stopped,” explains Dr. Jürgen Heydecke. In the case of lithium-batteries, this can lead to the cell opening and possibly burning down. “With lithium-polymer batteries, it should also be noted that gas formation can occur in the cell, which leads to the severe swelling of the cell.” The next step would also be thermal runaway and, thus, burnout.”

And what about deep discharge?

This is referred to when the cell voltage drops significantly below the cut-off voltage. In itself, this phenomenon does not lead to any safety problems. However, it slowly destroys the cell’s performance, which can also cause the battery to swell under certain circumstances.

“If a battery does become deeply discharged, special care must be taken during the subsequent recharge. With the aid of very low current, an attempt must be made to rebuild the basic voltage so that charging can then resume normally from 3 V,” says Heydecke.

Key take-away:

Users must therefore ensure that suitable chargers are used and to avoid damaging the devices and batteries. This ensures that the protective mechanisms can take effect in an emergency and prevents the dangers that can arise especially when lithium-batteries are overcharged.

More from our “Ask the Doctor of Chemistry” series:

What special features are there to consider when designing-in a lithium polymer cell?

What is the difference between a Protection Circuit Module (PCM) and a Battery Management System (BMS)?

15 Comments to “ What actually happens when lithium batteries are over-charged or deep discharged?”

  1. Muntaha Amber says :

    wow, great post! …

  2. ida says :

    best and effective

  3. Martie says :

    my lithion ion battery on solar went completely down red light on. can i damage the battery

  4. Selina Ruof says :

    Hi Martie,

    thank you for your question. We recommend to directly contact the manufacturer.

    Best regards
    Selina

  5. Phil Denton says :

    Do Battery Management Systems help to mitigate these situations? I’ve also heard that some lithium-ion batteries can be discharged to such a low state of charge that they can’t be recharged again (even if they’re relatively new). Is that true?

  6. Selina Ruof says :

    Hello Phil,
    thank you for your questions.
    A BMS definitely helps to mitigate these situations, because overcharging or deep discharging is avoided as far as possible. However, if lithium batteries are not charged and left for a long time, they can still be pulled into deep discharge because the BMS also has a quiescent current. We recommend to always keep applications charged to avoid deep discharge.
    Hope this helps 🙂
    Kind regards
    Selina

  7. John says :

    Hey, this blog gives me a basic idea of Li batteries like overcharging or deep discharging, it was simple and easy to understand. Thank you

  8. Selina Ruof says :

    Hello John,

    thank you very much for this feedback. We’re happy that the article helped you.

    Best regards
    Selina

  9. Kyle says :

    Hi, keep up the great work!

    I have some brand new batteries that seems to have been deep discharged. How would you recommend trying to bring them back up to a chargeable voltage?

    They are part of a wireless BT headset and are
    really tiny and really difficult to replace.

  10. Selina Ruof says :

    Hi Kyle,

    thank you for your feedback and question.
    I spoke to one of our experts and the recommendation is that you try to charge the application with supplied charger. It is very important that it is the supplied charger, because it may be that it recognizes the low voltage and uses the small charging currents that are necessary to reach the basic voltage. It could take a few hours until you see something. However, if the LED is not lit after a few hours the battery is too damaged and must be replaced.
    Hope this helps.

    Best regards
    Selina

  11. Noel McDowell says :

    Hello, I have a new 45ah CMAX bank that I use for competitive car audio and electronics. Max charge voltage recommended is 16.8v. Unfortunately I left the bank on my bench and forgot that I was charging it and by the time I got back to it the voltage was 17.6v. I hooked up a couple computer fans to the bank to slowly draw the voltage down overnight and this morning the bank sits at 16.3v. I plan to continue the draw until it’s at a safe operating voltage for my car at 15.8v. The bank is showing no swelling of the cells or signs of overheating. My question is, did I make the right decision to slowly draw on the bank or should I have rapidly drawn the voltage down with an AC/DC converter and a juice thirsty electronic device?

    Thank you in advance.

  12. Selina Ruof says :

    Hello Noel,

    As this is not one of our power banks, we cannot provide any information on this. We recommend contacting the manufacturer or looking in the manual for recommended actions.

    Kind regards
    Selina

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