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Is Big Clive right about lithium battery charging?

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tszaboo:
All this generic talk about the exact operation of Li-Ion batteries is pretty useless because it's an umbrella term, covering batteries with a dozen chemistries. The Saft article has quite good generalization, but they are only operating in the high-end battery space, and what they write might not be applicable to some low end unbranded polimer case lithium battery. I mean they sell lithium batteries which is only charged to 3.65V, not 4.2 but it works at extremely low temperatures.

Generally, you should terminate a charging cycle when the current reaches C/10 in the CV part. And that's probably a good rule of thumb for most chemistries. If you are interested in how the batteries go bad, look up Coulombic Efficiency. As far as I understand, the losses in the Coulombic Efficiency is because those electrons are actively damaging the battery cell, so every electron that is pumped into the battery, and not stored, is bad for the battery. Hence terminating a charge is useful. At least this was the working theory a decade ago.

Siwastaja:

--- Quote from: Peabody on December 18, 2022, 06:40:47 pm ---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.
--- End quote ---

You are totally right. In my opinion, instead of this stupid micro-cycling, it is better to just float the cell at, say, 4.15V indefinitely using a CC-CV supply. But if your manager reads from the battery datasheet that the CV phase must be terminated when current drops below C/20, or after three hour timeout, then how do you convince them to do otherwise? You are left with nothing but micro-cycling.


--- Quote ---Is such a low current limit for a discharged battery really needed?  I don't know.
--- End quote ---

Usually, cell manufacturers specify the normal charging current, which is applicable to empty (0% SoC) cell as well, no reduction needed. By default, charging a cell below certain voltage threshold should not be done at all. (Is it 2.5V? 2.0V? 1.5V? I don't remember exactly, I use something like 2.0V myself. The problem is with copper dissolution and related dendrite risk. But who knows for sure if there are other reasons not to charge overdischarged cells? Possibly the limit could be something else than that of copper dissolution.)

Sometimes manufacturers additionally specify some kind of recovery process to allow restoring slightly overdischarged, possibly slightly damaged cells. Surprisingly many li-ion charger ICs implement something like this, and usually using higher current than recommended.

I think the whole "low current recovery" thing is a mess. I personally don't design my products to implement it at all. I take care not to let the cells ever overdischarge, and consider overdischarged cells a failure, either in cell or cutoff circuit. But your mileage may vary, maybe if you manufacture a million devices you will get a few angry customers and such automatic recovery thing is not that dangerous after all. I truly don't know for sure.

If I do the recovery thing, I do it manually with a lab supply, with C/50 (not C/5!), and keep an eye at the cell. If not seriously damaged, voltage will rise to something allowing normal charging to commence within an hour or two, and if the cell has increased leakage enough to cause danger, it will prevent the voltage from rising.

IanB:

--- Quote from: Siwastaja on December 18, 2022, 07:37:00 pm ---I take care not to let the cells ever overdischarge, and consider overdischarged cells a failure, either in cell or cutoff circuit.
--- End quote ---

But presumably if the battery pack has a protection circuit, this will have some residual current drain, and if left unattended for long enough the battery could get over discharged by this? Or can the parasitic drain be made low enough that it is not important?

Siwastaja:

--- Quote from: IanB on December 18, 2022, 07:42:50 pm ---
--- Quote from: Siwastaja on December 18, 2022, 07:37:00 pm ---I take care not to let the cells ever overdischarge, and consider overdischarged cells a failure, either in cell or cutoff circuit.
--- End quote ---

But presumably if the battery pack has a protection circuit, this will have some residual current drain, and if left unattended for long enough the battery could get over discharged by this? Or can the parasitic drain be made low enough that it is not important?

--- End quote ---

This is basics of the basics for the battery system designer. If they fail to analyze and do this properly, I consider them failed as professionals. And yes, it does happen.

The math is easy, you have to choose how much unused capacity to leave on the bottom, how long you can let the customer keep the device fully discharged on their shelf. If you choose wrong, you will have unhappy customers. For example, I don't consider it good design if the product dies when the customer fully discharges the battery, forgets to recharge and then puts the thing on their shelf for two months after which they rediscover the thing and try to start using it again, but now it's dead. But two years on shelf, maybe that is acceptable. Then it needs to be printed on the manual, with a big red font: CHARGE TO 50% BEFORE LONG-TERM STORAGE.

Nowadays, the problem is widely recognized and most BMS chips have very low quiescent current draw, especially in some kind of sleep mode they enter at low cell voltage.

james_s:
Float charging a lithium ion battery is not really a thing because unlike other battery types, a lithium battery does not self limit the charge voltage, if you continue to pass current through the cell, the voltage will keep rising. If the charger is in CV mode though when the battery voltage equals the charger voltage no current will be drawn and charging stops. If a load connected to the battery is drawing current then the charger will supply current to the load, the battery will not be charging.

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