General > General Technical Chat
Is Big Clive right about lithium battery charging?
Siwastaja:
--- Quote from: IanB on December 18, 2022, 12:08:11 am ---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.
--- End quote ---
I tested a large bunch of cells years ago, and also looked at research by others, at Uni. My takeaway is, be very careful with such generalizations! Of course it's safe to reduce CV voltage from 4.20V to say 4.10V but if you expect significant lifetime improvement, it's 50-50% you either succeed, or are disappointed.
I remember seeing a paper where a Panasonic (IIRC) cell did not show any lifetime advantages until going significantly below 4V. IIRC, 4.00V was even slighly worse than 4.20V or something like that. I found out that charging current is a MASSIVE factor, but only at high SoC, but well before the CV phase. And small differences can take the cell over the edge. Don't remember exact numbers off-hand, but charging current like 1.2C caused the cell to fade in just tens of cycles, and reducing it to 1.0C or so made it last hundreds. And on the other hand, you could charge it at 1.5C until 3.9V, then drop to 1.0C, and yet it lasted for hundreds of cycles.
I also investigated the effect of charging temperatures at high charging currents, and low temperature was catastrophically bad, to the point that cells performed with the best lifetime when exceeding the datasheet maximum charging temperature. This was a 45degC rated maximum charging temperature, and a certain customer requested us to do testing at 50degC. To our surprise, it performed better in all regards, including lifetime.
Again, if you take the general advice of "keeping cells cool" and combine that with high charging rates, you are doing more harm than good.
And all this data is irrelevant with some other cells than what I tested.
Pretty much the only generic hints I can give for good lifetime are, store below 50% SoC and charge as slowly as you can.
Siwastaja:
I realized this was not mentioned yet:
One source of confusion is, some other chemistries like NiCd or lead acid can be charged with unlimited-voltage constant current, as long as current is small, because cell starts to consume excess charge and convert it into heat, without causing (serious) damage to the cell. For example, lead acid uses the extra charge to produce hydrogen and oxygen (+heat), which are converted back into water in SLA battery, or with flooded battery, water is added manually to compensate.
This is not possible with li-ion, so people who say "don't float charge" "don't trickle charge" etc. originally meant "do not apply unlimited voltage". Nowadays it is blatantly obvious to almost anyone that a li-ion cell needs a CC-CV charging circuit, but the original advice is still parroted, with the original meaning lost.
IanB:
--- Quote from: Siwastaja on December 18, 2022, 01:01:56 pm ---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.
--- End quote ---
Yes, I was wrong about the voltage. If the CV voltage is set to 4.20 V, then the charger will maintain that voltage in the face of a small load current that is less than the CC current.
--- Quote from: Siwastaja on December 18, 2022, 01:13:38 pm ---I tested a large bunch of cells years ago, and also looked at research by others, at Uni. My takeaway is, be very careful with such generalizations! Of course it's safe to reduce CV voltage from 4.20V to say 4.10V but if you expect significant lifetime improvement, it's 50-50% you either succeed, or are disappointed.
--- End quote ---
Indeed, however the article I linked was by a particular battery manufacturer (Saft), and they do refer to the batteries they manufacture and know about as specific examples. They mention that just because their batteries may have certain characteristics, it doesn't mean that all batteries from other manufacturers will have the same characteristics.
--- Quote ---I remember seeing a paper where a Panasonic (IIRC) cell did not show any lifetime advantages until going significantly below 4V. IIRC, 4.00V was even slighly worse than 4.20V or something like that. I found out that charging current is a MASSIVE factor, but only at high SoC, but well before the CV phase. And small differences can take the cell over the edge. Don't remember exact numbers off-hand, but charging current like 1.2C caused the cell to fade in just tens of cycles, and reducing it to 1.0C or so made it last hundreds. And on the other hand, you could charge it at 1.5C until 3.9V, then drop to 1.0C, and yet it lasted for hundreds of cycles.
--- End quote ---
Saft also talks about charging rate:
--- Quote ---At low charging speed (C/2, C/5 or even less), the lithium ions are intercalating themselves smoothly in the graphite sheets, without damaging the electrodes. When the charge rate increases, this intercalation gets harder and harder. If the rate is too strong, Lithium ions have no time to penetrate the electrode properly and just deposit on its surface, which causes the battery to age prematurely.
--- End quote ---
Their emphasis, not mine.
--- Quote ---I also investigated the effect of charging temperatures at high charging currents, and low temperature was catastrophically bad, to the point that cells performed with the best lifetime when exceeding the datasheet maximum charging temperature. This was a 45degC rated maximum charging temperature, and a certain customer requested us to do testing at 50degC. To our surprise, it performed better in all regards, including lifetime.
Again, if you take the general advice of "keeping cells cool" and combine that with high charging rates, you are doing more harm than good.
And all this data is irrelevant with some other cells than what I tested.
Pretty much the only generic hints I can give for good lifetime are, store below 50% SoC and charge as slowly as you can.
--- End quote ---
That whole Saft article is a good read, and it pretty much echoes the advice you give above. They also indicate that you should match the cell to the application, since cells differ in their characteristics.
Peabody:
--- Quote from: Siwastaja on December 18, 2022, 12:54:16 pm ---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?
--- End quote ---
I don't understand. If the load current is less than the termination current, the TP4056 will terminate charging normally, at which point the battery supplies the load current. But the TP4056 will automatically resume charging when battery voltage falls below 4.1V. So for small loads, this process results in continuous battery cycling between 4.1V and 4.2V. But the very low voltage qualification (<3V) isn't involved at all.
But even at below three volts, a small load current won't prevent the battery from eventually charging because battery voltage will eventually rise to 3V. However, that's not true with Clive's 170mA load. If the battery is ever fully discharged, charging current will be limited to 100mA, so the battery will probably never recover. So you can't really use Clive's method in UPS mode. You need a load sharing circuit, which solves this problem. Is such a low current limit for a discharged battery really needed? I don't know.
IanB:
--- Quote from: Peabody on December 18, 2022, 06:40:47 pm ---So you can't really use Clive's method in UPS mode.
--- End quote ---
But it's not "Clive's method". He did not invent it or promote it. He is simply explaining the behavior of the cheap and ubiquitous TP4056 chip that is commonly available and useful for charging reclaimed lithium ion cells in non-critical applications. You are really not going to use that chip in anything big or important.
The criticism was that a TP4056 module could "overcharge" a cell, where overcharge should be understood to be charging the cell above the recommended 4.2 V. Clive simply demonstrates that the cell voltage does not go above 4.2 V, and the cell is thus not "overcharged" in that sense.
Whether you are happy to accept the cell being "float charged" at a constant 4.2 V without termination is up to you. Siwastaja apparently would probably accept it, you may not wish to accept it. It's really your choice. Since you are likely using a TP4056 module on a cheap or free reclaimed cell, I don't think it really matters if you happen to shorten its lifetime this way. I certainly wouldn't be very concerned about it.
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