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Float Charging Lithium-Ion Batteries at Lower Voltages
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mhz:
I understand from various sources that float charging a Li-Ion battery at full voltage (usual 4.2V case) is bad, degrading the battery lifetime and potentially causing an unsafe situation. For example, https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries: "Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible."

I also have seen in some sources that leaving a Li-Ion battery to float charge at a lower voltage (say 4.1V) is safe, however I haven't seen this stated at any references I'd consider authoritative. Does anyone have any references for or against this practice that come from a seemingly authoritative source like "battery university"? Ultimately I'd like to know if I can leave a 4.1V cell in CV stage indefinitely without worrying about causing it damage or creating an unsafe event. (Actually, my application is 13s5p pack, made of Samsung 25R cells).

For that matter, any general references (books, etc) that can help bridge the divide between practical use and the underlying chemistry would be appreciated.




BravoV:
Although I am not sure if this is a real legit authoritative reference, from the video, at least to me, its very convincing to watch Prof. Jeff Dahn presentation.

Bottom line is, the "ideal" nitpicking spot is 4.03 volt (at 80% SOC), and anything above 4.1 will get worst, especially if we aim for cell life longevity.

This video is quite long > 1 hour, suggesting to watch from the beginning.

But don't worry, I marked the time to start the video at 1:07:05 at below link, watch and note carefully what he said.

https://youtu.be/pxP0Cu00sZs?t=1h7m5s

Hope this helps.
edpalmer42:
Look for info on solar powerplants that use Lithium batteries or something like a Tesla Powerwall system that's used during AC failures.  AFAIK, these systems float their batteries rather than occasionally doing a charge cycle.

I've heard that the rule of thumb is to float the cells a bit above the voltage that they drift to when they're idle.  That voltage puts the least amount of stress on the cells.

Sorry, but I don't have any more definitive info.

Ed
digsys:
I work with lithium EV packs for many years and the comments are pretty much spot on. They do NOT like "trickle" charge of any sort, especially above their
"resting" state ~3.7 - 3.8V. IF you know the the self-discharge rate, you can safely put that back, but that changes with time / temp etc, so not easy to find.
If the cells are in "reasonable" condition, they shouldn't need ANY trickle maintenance anyway. Button Li cells in RTCs for example hold charge for 10-20 yrs+
jmaja:
I was designing a device that would work on a very small solar panel + 50 mAh lipo cell. Since I could not find a charging IC that would have Iq below 1 uA I planned to use a LDO setup for 4-4.1 V. I did find some references that supported constant voltage charging 0.1-0.2 V below max.

I decided to dump the solar panel and use nonchargable lithium coin. Can't remember where I found the reference for trickle charging. I think there were tests how cell life was affected at different trickle voltages.
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