0
\$\begingroup\$

I know that the maximum charging voltage for lithium ion batteries is 4.2v. I also know that charging a lithium ion battery involves a constant current and constant voltage phase.

But what will happen if I continuously charge a lithium ion battery with a maximum of 4.0v at 100mA? Will it destroy the battery?

\$\endgroup\$
  • \$\begingroup\$ I think that you will have many problems, unbalanced charge, rather small charge delivered to the battery, etc. Why not use a real charger? What is the capacity of your battery? \$\endgroup\$ – Claudio Avi Chami May 8 '18 at 5:05
  • \$\begingroup\$ I have a 2600mAh battery and I have asked this question as a thought experiment because all the resources say charge at a maximum of 4.2v but no one have said what will happen if you charge at less than 4.2v \$\endgroup\$ – jasper samson May 8 '18 at 5:29
  • \$\begingroup\$ You mean you have two limits? 4.0V or 100 mA, whichever one triggers first? Because you don't get to set voltage AND current simultaneously. If you set the voltage, then the battery decides how much current to accept (or supply). And if you set the current, then the battery gets to decide the voltage. \$\endgroup\$ – mkeith May 8 '18 at 6:02
  • \$\begingroup\$ I see no issues with it. \$\endgroup\$ – winny May 8 '18 at 6:15
  • \$\begingroup\$ Just to clear things up, what I'm trying to say is, if you continuously charge a lithium ion battery at 4.2v, it kills the battery. So what will happen if you continuously charge a lithium ion battery at say 4.0v \$\endgroup\$ – jasper samson May 8 '18 at 6:26
2
\$\begingroup\$

Yes, charging a Li-Ion cell at constant voltage without ever terminating the charge will likely destroy the cell.

What will happen is that your battery will get (maybe slowly) to 4.0 V, and, if the voltage stays, the charging current will gradually decrease, and will decrease to zero. This will put the cell into overcharged state, even if the voltage was not at maximum for the cell's capacity.

Most common rationalization of overcharging process is that “the lithium builds up faster than it can dissipate. The result is that metallic lithium plates up on the anode. At the same time, the cathode becomes an oxidizing agent and loses stability”.

In other words, if the cell is subjected to CV and the current stops over time as it normally goes, ions of Lithium started to build up without the current forcing them to move. That's why the charging process must be stopped at some point. So it is not the voltage level (although overvoltage causes other bad effects), but the fact that current eventially stops, and Lithium build-up starts.

As result of electrolyte decomposition some gassing might occur resulting in bulged/bloated/swollen cell, and the cell typically looses 50-70% nominal capacity due to some irreversible changes in microstucture of electrode materials.

Bottom line - don't charge Li-Ion cells continuously.

For more scientific explanation, see this article in "Frontiers in Energy Research"

\$\endgroup\$
  • \$\begingroup\$ Thank you! You have greatly satisfied my curiosity. I was almost convinced of trying it out myself! \$\endgroup\$ – jasper samson May 8 '18 at 6:34
1
\$\begingroup\$

I also know that charging a lithium ion battery involves a constant current and constant voltage phase.

It usually does, but it's not necessary. That's the way commercial chargers work, to get the fastest charge while staying within the no-damage parameters.

One common regime that chargers use is to charge to 4.2v, then turn off until the voltage has fallen to 4.1v, then recharge to 4.2v.

For my money, 'providing enough charge over time' to keep the cell at 4.1v would be kinder if that charge was delivered steadily and the cell never exceeded 4.1v, than if it was provided in bursts and the cell cycled between 4.1v and the higher more damaging 4.2v. The first option is of course a continuous trickle charge.

However, I'm not a battery manufacturer, and I've yet to find data from any of them that discusses longevity under sub-maximum voltage trickle charge conditions. An immense amount of effort has been put into characterising rechargeable cells to get fastest charge rates and largest usable capacity (which, let's face it, is where the volume and the money is), and rather less into using them more gently.

To your specific charging conditions of 4v 100mA. If 100mA is less than the battery's max charge current, then your CI phase will be OK. Once the cell gets to 4v, the charging current will fall. I expect it would fall to essentially nothing.

I would risk keeping it on CV at 4v indefinitely, taking appropriate precautions to mitigate fire risk. You might be tempted to do the same. You will not find any reputable sources that will tell you this is OK. I will not be responsible for your cells if you do this, and find they degrade more quickly than you hoped.

It would be interesting for someone, perhaps you, to test a few cells with differing lower voltage trickle charge regimes and report the results, perhaps quarterly over a five or 10 year period. I've considered it, but am unlikely to get around to it.

\$\endgroup\$
  • \$\begingroup\$ Yes. Certain things seem to be true. Cycling between 4.1 and 3.5 is surely better than cycling between 4.2 and 3.5. And if 4.1 is a good voltage to trigger a top-off, then surely floating at 4.1V should be reasonably safe? But I don't know for sure. Seems nobody is ever willing to come out and say this. \$\endgroup\$ – mkeith May 8 '18 at 6:09
  • \$\begingroup\$ Thanks a lot to your opinion! I have never thought that the current will drop as the voltage reaches 4.0v. I will try trickle charging a Panasonic 18650 battery (with fire safety measures in place) at 4.0v and see if there would be adverse effects and hopefully share my results. \$\endgroup\$ – jasper samson May 8 '18 at 6:32
  • \$\begingroup\$ @jaspersamson that's how the charger knows to end the CV stage, when the current has fallen to <5% of C, or whatever the figure is. Obviously a charger that terminates at 0.1C will result in a lower capacity and a longer life than one that quits at 0.03C. At a lower terminal voltage, I'd expect the current to fall to much less than that. \$\endgroup\$ – Neil_UK May 8 '18 at 8:37
  • \$\begingroup\$ The current will eventually drop off to some low level at 4.1 or 4.0 (or any voltage). I am pretty sure a good battery will always drop below 0.1C at 4V. If it accepts 0.1C at 4V without the voltage rising, the battery is probably permanently damaged. \$\endgroup\$ – mkeith May 16 '18 at 22:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.