# Lithium battery charging-undervoltage

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?

• 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? May 8, 2018 at 5:05
• 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 May 8, 2018 at 5:29
• 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. May 8, 2018 at 6:02
• I see no issues with it. May 8, 2018 at 6:15
• 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 May 8, 2018 at 6:26

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.

• 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. May 8, 2018 at 6:09
• 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. May 8, 2018 at 6:32
• @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. May 8, 2018 at 8:37
• 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. May 16, 2018 at 22:00

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"

• Thank you! You have greatly satisfied my curiosity. I was almost convinced of trying it out myself! May 8, 2018 at 6:34
• While CC and CV modes have their quirks and one need to be careful, the OP asked for charging in CC+CV mode, meaning, with a both CurrentLimit and a VoltageLimit. This means that such a charger would reduce voltage or reduce current as the other value approaches the limit. This means, if voltage is measured properly at the cell, there is no way the cell can reach overvoltage condition of >4.2V,even if trickle-charge is allowed near the end. Also see batteryuniversity.com/learn/article/… & "Overcharging Lithium-ion" & "Charging Non-cobalt-blended Li-ion" Dec 23, 2020 at 15:54
• @quetzalcoatl, your advice of "measuring voltage properly" is nonsense. Are you aware that every battery cell has certain "internal resistance" (which varies depending on SOC and cell age), and the measured voltage will vary in considerable limits depending on load conditions. And please don't confuse "overvoltage" with "overcharging". Dec 25, 2020 at 6:13
• @Ale..chenski by "measuring voltage properly" I meant being aware of wiring/connector resistance. Measuring voltage near the 'output' of a charger vs measuring voltage "at the cell" (= as near the battery/cell as possible) can make a large difference when the charging current is high. However, indeed, while what I wrote is true, it's also a nonsense in this context, as charging current + wires' resistance will cause voltage drop, not rise, so unaccounted wires' resistance may only cause temporary undervoltage on the battery, and won't ever cause overvoltage, so no danger of overcharging. Dec 26, 2020 at 14:20

While CC and CV modes have their specific negative effects *) and one need to be careful, the OP asked specifically for charging in CC+CV mode, meaning, with a both CurrentLimit and a VoltageLimit. This is a different thing. It means watching both limits at the same time. This means that such a charger would reduce voltage or reduce current as the other value approaches the limit. This means, never exceeding the current limit, and never exceeding the voltage limit.

If voltage is measured properly at the cell **), there is no way the cell can reach overvoltage condition of >4.2V, even if trickle-charge is allowed near the end for a prolonged time.

If the battery is designed to be charged up to 4.2V, then trickle-charging it with diminishing current up to 4.0V means that the battery will be kept in an under-charged state.

According to https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries charging up to 4.0V instead of 4.2V means storing 70-80% of the maximum charge a li-ion could hold if charged to its maximum.

Further trickle (i.e. 0.05C) charging (with cut off condition of 4.0V) would not hurt the battery, if voltage is not allowed to exceed 4.0V, because if it would hurt the battery, than it would mean that, by design, the battery is either not allowed to be charged above 4.0V, or is not allowed to be charged with charging current lower than some value, or both, and we precisely know both those sentences are false: max V is 4.2 and those batteries only have max-charging-current and no min-charging-current.

Be sure to also read "Overcharging Lithium-ion" & "Charging Non-cobalt-blended Li-ion" paragraphs from the above-mentioned article. Especially "non-cobalt" batteries mentioning li-ions with lower target voltage rating.

You may also find Charging li-ion cell using constant-voltage only helpful.

**) All traces, wires, connectors, etc have resitance. If you measure the voltage not directly at the cell, your measurement may read "4.19V" while it's "4.22V" few wires further at the battery. Be sure what exactly are you measuring and with what uncertainties.

*)

• Using only CV, especially with a low-resistance charger output/cables/etc, may cause an excessive current to flow when battery's own voltage is much lower than the CV limit. This in turn may exceed battery's advised/safe charging current, may cause the battery to heat up, and cause all sorts of futher problems.
• Using only CC, once the battery reaches its max-voltage, CC won't stop, and will continue charging. Obviously this will eventually cause the battery to overcharge and its voltage to rise over the max-voltage. This may cause the battery to heat up, and cause all sorts of futher problems.

However, keep in mind that in both cases, "may" means it doesn't need to happen. Every voltage source has some finite max output current capability, every current source has some finite max voltage capability. If your CV voltage source has low max current output, trying to draw more than it can supply will simply overload the voltage source in some way and causing it to drop out of the CV mode and provide voltage lower than set. And similar for CC current source.

Of course if those sources are not designed for working in such conditions, it may damage them to some extent, but not the battery.
Of course a damaged voltage/current source may start misbehaving and may in turn damage the battery.
Of course you're doing it at your own risk. I did not advise you to do this.
I had to write this warnings in case some evil lawyers are reading this :)

• There is no such thing as "4.2V by design". The battery charge/discharge/capacity specifications are dictated by "usability" of cells, by commonly accepted number of charge-discharge cycles. Many Li-ion batteries are specified at 4.2V not because they are "designed to", but because they CAN DO this (~500-1000 cycles) for a particular chemistry, electrode structure, separator construction, additives/stabilizers, etc. Setting unterminated "trickle charging" at 4.2V is known as "bad thing", due to known "dendrite/metal plating" effect. Setting the same "trickle" at 4.0 V is no different. Dec 25, 2020 at 6:06
• @Ale..chenski that's all true in general, but I'll disagree regarding wording. "By design" means the choice of the chemistry inside. If there's cobalt/whathaveyou, then sensitivity to voltage levels is different. If (..) then it's 500-1000 at 4.2, if not (..) then it's 2500-3000 at 4.2V, etc. If I use a diode, spec'ced to continuous working conditions of 1W 600V, outside of its limits, and it happens to work fine at 1.1W / 700V, cool. Sure I can expect shorter life span. But that does not mean that I cannot say it was 1W/600V by design. Dec 26, 2020 at 14:28
• @Ale..chenski regarding 4.0V or 4.2V, please do note that while some "plating" effect may occur both at 4.2 and 4.0, it will be much weaker at 4.0 than at 4.2. Dec 26, 2020 at 14:31
• @Ale..chenski finally, I think you've missed one point re trickle-charging that I wanted to express properly (and apparently I have failed). The CC+CV mode I was writing about, doesn't trickle-charge indefinitely. When the battery reaches the voltage set, be it 4.2V or 4.0V, the charging ceases. It may trickle-charge at 4.195V, depending on exact design of the charging circuit, but setting a CV of 4.0V/etc means, stopping the current and not letting the battery voltage going any higher. If there any trickle-charge left, battery voltage would rise a bit causing charging to stop due to CV limit. Dec 26, 2020 at 14:34
• @Ale..chenski and yeah, bettery may have some leakage/etc, so a minimum-current limit may be a good idea as well, just like adding a minimum-voltage limit is a good idea as well, to make it safer against shorts/supercaps or just dead batteries, but that's another story IMHO. Dec 26, 2020 at 14:37