General > General Technical Chat
Is Big Clive right about lithium battery charging?
IanB:
--- Quote from: Peabody on December 17, 2022, 10:43:29 pm ---I guess we have a difference of opinion about what's going on in the video. According to the datasheet, the charger is in the constant voltage phase, and that voltage is 4.2V, not some smaller value. If it is capable of supplying nearly 1A at that voltage, I doubt there will be any sag when the current drops to 170mA. And of course the "4.2V" is subject to individual part variation (1.5% per the datastheet), so a particular TP4056 could easily be supplying 4.2V to a fully charged cell, which puts you in the same situation as in Dave's video. So as far as I can tell, Dave is telling me one thing, and Clive is telling me the opposite.
--- End quote ---
Perhaps I may be wrong about whether the voltage is exactly 4.2 V, and whether the current is zero or not. Perhaps you are right.
--- Quote ---The basic question is still whether it is ok to continue to apply 4.2V (not some higher voltage) to a fully charged cell - even if there is no voltage differential with the cell. I understand the logic that says if there's no voltage differential, no current will flow, so it's no problem. It's just that as far as I can tell Dave and the industry say otherwise - for reasons that are not clear. If Dave is right, why is he right?
--- End quote ---
And honestly, I think the answer will be "it depends". No one is right, and no one is wrong.
I found the following article from a battery manufacturer that might shed some light on the matter:
https://www.saftbatteries.com/energizing-iot/charging-your-lithium-ion-batteries-5-expert-tips-longer-lifespan
Note tip 2, where they say that float charging is a possible scenario with solar applications.
They also suggest that reducing the CV voltage below 4.2 V (perhaps to 4.1 V or 4.15 V) is another strategy that could help to extend the life of batteries, especially if they might be float charged and held at that voltage for a long time.
Apparently the possible mode of damage to the cell is stress on the electrodes, especially the graphite electrode that has to absorb the lithium ions during charging. The lithium ions take up volume and thus stretch the electrode structure. Keeping the electrode excessively full of ions can cause mechanical damage to it over the long term.
SiliconWizard:
I fail to see what myth exactly is busted in this video, or even what exactly its purpose is.
IanB:
--- Quote from: SiliconWizard on December 18, 2022, 12:13:26 am ---I fail to see what myth exactly is busted in this video, or even what exactly its purpose is.
--- End quote ---
The myth is that people believed that the TP4056 module would keep charging the battery forever, and that would be bad for it. But it doesn't do that, and the battery voltage does not go above 4.2 V.
Siwastaja:
--- Quote from: Peabody on December 17, 2022, 04:02:18 pm ---I'm just having trouble with the idea that all the manufacturers of cells and chargers specify termination when they don't need to do that.
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You don't know what they actually specify. All we have are some leaked partial datasheets of cells that were written for some customer, for some specific application.
If you were considering a floating application that would be sold in millions, you would be discussing this with Samsung or whoever directly, and they would probably say "it's ok, given these and these conditions". And you would get a different datasheet.
One point I forgot to list in my previous post: forcing to terminate also forces the start of charge. And start of charge, as recommended, includes initial voltage qualification. Again, increased self-discharge not only would prevent the CV phase from finishing, triggering the timeout, but it could also bring the cell below the safe starting voltage (say, 2.5V) during the time the cell is not charged.
But this is all still iffy. You can follow the usual recommendation of having a cell permanently connected to a charger, which microcycles the cell with a hysteresis between some 100% and 80% and never qualifies why the cell was discharged to 80% - was it increased leakage, or a load?
And, the same manufacturers who recommend the CV timeouts, also recommend very low current "conditioning" process for overdischarged and damaged cells. It's pretty weird they do that.
I think I know this shit pretty well but I have never heard a truly good reason not to float charge a li-ion cell. There are many weak and iffy explanations that have some validity, though, and it's not a surprise that manufacturers recommend the "known good practice".
Siwastaja:
--- Quote from: IanB on December 17, 2022, 06:09:11 pm ---The voltage is less than 4.20 V due to the load, and the charging current at the battery terminals is zero, therefore the battery is not being charged.
--- End quote ---
You are right and wrong. If you connect a charger IC plus a load to a cell, as long as the load current is smaller than the maximum CC current the charger IC can supply, then the voltage sure will be 4.20V at the cell terminals. Kirchoff laws! Whether the load is there or not does not matter the slightest.
You are right that the current does not flow, but that has nothing to do with the load or charger; it's just how li-ion cells work. Once charged to 4.20 OCV, applying 4.20V to terminals does not cause current to flow beyond the tiny self-discharge current.
I'm fine with your definition "battery is not being charged because current does not flow", but that is actually the core of the discussion. Many consider that having a li-ion cell hooked up to a current-limited voltage source means it is "being charged" or "float charged" or "trickle charged" (whatever term you want to use), even with no current flowing, and they want you to "terminate", i.e., disconnect the voltage source after some time.
--- Quote from: Peabody on December 17, 2022, 10:43:29 pm ---If Dave is right, why is he right?
--- End quote ---
If Dave is right, he's right because most cell manufacturers, at least in publicly available datasheets, recommend some kind of charge termination system.
Even if the manufacturers were not exactly right in that, it's generally a good idea to follow their advice unless you are 110% sure about what you are doing.
Me, I of course float charge li-ion cells against this advice because I am confident enough. But not at 4.20V, but something like 4.10V instead.
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