EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: Peabody on December 17, 2022, 02:56:03 am
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He says it's ok NOT to terminate charging, that it won't hurt the battery if the charger continues to apply 4.2V to a fully-charged battery. That goes against everything I've learned about lithium batteries. Also, if he's right, why do all lithium chargers I've ever seen include a termination feature? If continuing to trickle charge is ok, why bother to terminate? On the other hand, he has almost 1 million subscribers, and I don't.
https://www.youtube.com/watch?v=f2yMs-JAyQM (https://www.youtube.com/watch?v=f2yMs-JAyQM)
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He says it's ok NOT to terminate charging, that it won't hurt the battery if the charger continues to apply 4.2V to a fully-charged battery. That goes against everything I've learned about lithium batteries. Also, if he's right, why do all lithium chargers I've ever seen include a termination feature? If continuing to trickle charge is ok, why bother to terminate? On the other hand, he has almost 1 million subscribers, and I don't.
He doesn't say that, though. He says the charger "appears to keep charging, but it's not doing that really". He says at 7 minutes in that the module charges the battery up and it does terminate. He goes on to say that the load then discharges the battery a bit, until the voltage drops, and then the charger switches back on, charges it up again, and terminates again. So this is an endless cycle of charge/discharge/charge again.
The only variable in this scenario is how long the charger should wait before reactivating. I have a lithium charger that refuses to start charging even with a substantially depleted battery. It is really annoying. I have to drain the battery a lot before the charger will recharge it.
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That's when the load is less than the termination current. You need to go back to 6:00 when the load exceeds the termination current.
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It won't be good for battery life to keep cells at full charge, but it won't be hazardous either.
Once the cell reaches 4.2V, the charger won't supply any more current so it stops charging.
The problem is with pushing the voltage past 4.2V, which won't happen since the charger is a limited current + limited voltage source.
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That's when the load is less than the termination current. You need to go back to 6:00 when the load exceeds the termination current.
That's OK too. When the load exceeds the termination current the battery will reach a steady state where the battery voltage is constant and therefore the battery current is zero. If the battery current is zero it is not being charged, so that is also fine. Charging is effectively "terminated", even if the termination light does not illuminate.
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He says it's ok NOT to terminate charging, that it won't hurt the battery if the charger continues to apply 4.2V to a fully-charged battery. That goes against everything I've learned about lithium batteries. Also, if he's right, why do all lithium chargers I've ever seen include a termination feature? If continuing to trickle charge is ok, why bother to terminate?
Lithium batteries, definitely no - they can't be charged.
But let's talk about lithium ion batteries. He is kinda-sorta right. Li-ion battery is close to an ideal battery, which is a voltage source with some very small series resistance, and very large parallel resistance (self-discharge). The voltage of said "voltage source", also called open-circuit voltage (OCV), is a function of the State-of-Charge (SoC); for example, 100% is 4.20V and 0% is 3.40V and 50% is 3.65V. Not a straight line, but kinda close.
Now if you apply 4.20V to a cell which has open-circuit voltage of 4.20V, by simple Kirchoff laws, no current flows, except the tiny current equaling the self-discharge current.
There is tiny nitpick - manufacturers do not define 100% SoC as 4.20V OCV, but instead like "100% is when C/20 flows at 4.20V terminal voltage". Equivalent OCV is a bit smaller, like 4.17V. By floating indefinitely at 4.20V, you are thus keeping the cell at something like 101% or 102%. Hardly a problem, but do the tolerence analysis. If your voltage supply is inaccurate, better derate the voltage more.
Now why is it shunned? #1 reason, by far, is the shitload of pseudo-"information" by people who have absolutely no clue what they are talking about, and fake information sites like Battery University. But to be fair, they have a point. Cell manufacturers also instruct to terminate charging, not only with a stopping current like C/20, but also a timeout.
This is because manufacturers have thoroughly tested their products in "typical" use case, and that typical use case is charging a cell, then disconnecting the charger anyway, and starting to use the thing, until it needs to be recharged. In such use case, it makes no sense to float the cell indefinitely. Li-ion cells do not self-discharge much anyway, so no need to "keep it topped". It keeps topped on its own.
Terminating the charge with a combination of C/20-like stopping condition plus CV phase timeout, offers one simple check against a specific cell failure mode: namely skyrocketed leakage current due to internal failure in cell. If the cell consumes more than C/20 on it's own, by just being, CV phase never finishes, timeout triggers, and amount of charge put into the cell is limited. I'm very skeptical of this being of much use, but it's something. By floating the cell indefinitely, you bypass this.
But an internal short inside the cell finally causes a thermal problem, and some 2-hour timeout can't help with that. Temperature sensor is much better at that. And if the internal short is not enough to cause significant heating but trigger timeout, then the user sees some blinking red light, continues to use the cell anyway, and recharges it again, bypassing the result of this safety check. Unless you store "this cell is faulty" information in non-volatile memory, but neither the manufacturers nor the Battery University scientists require you to do that!
In real world, li-ion cells are used in float charge configuration (always connected to a CC-CV supply), despite all the Battery University Internet Scientists saying otherwise. It's just this use case is not widely documented and not supported by the cell datasheets. You need to discuss this with the cell manufacturers to be sure. And the voltage won't be 4.20V, more like 4.10V.
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In #919 Dave says you shouldn't leave the battery charging at 4.2V for an extended period. Is he wrong about that?
https://youtu.be/jNmlxBXEqW0?t=679 (https://youtu.be/jNmlxBXEqW0?t=679)
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. And if it's ok to float charge, why don't we have at least some chargers that don't provide any termination at all? Such chargers would prevent the cycling that occurs when the load current is less than the standard termination current.
Well I guess I need to look for the results of studies of this issue. I assume at some point the cell manufacturers have actually done such trials to see what happens. I just don't want to be wrong about anything involving lithium batteries.
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No, Dave is not wrong about that. Manufacturer recommendation is to cease charging when appropriate termination conditions are met, such as when the charging current in CV phase drops below a threshold like 100 mA.
But I have tried to explain above that in the Big Clive example the battery is not being float charged. 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.
In short, maintaining the CV charging phase at a battery voltage of 4.20 V is not recommended. But in this case that is not happening. The battery voltage is less than 4.20 V, and no charging is going on.
And if you say that even that is unacceptable, what would you like to do about it? Do you want the device to turn off the output while the charging cable is plugged in? Don't you think people would complain about that? Or do you want the device to refuse to accept charge while it is powering something? Don't you think people would complain about that too? And if it is my cell, and my charger, and my application, isn't it up to me if I wish to (potentially) shorten the life of the cell a bit?
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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. And if it's ok to float charge, why don't we have at least some chargers that don't provide any termination at all? Such chargers would prevent the cycling that occurs when the load current is less than the standard termination current.
I think you are having trouble with the idea that in the real world, things are not binary, black or white, true or false. There are shades in between. There are recommendations, and best practices, and sometimes you can deviate from recommendations, and maybe this will be a problem, and maybe it won't.
Also, lithium ion cells are not bombs that will detonate at the slightest touch. They do contain a lot of stored energy, and they can deflagrate with heat and flame if seriously damaged or abused, but slight variations in charging protocol do not constitute serious abuse.
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No, Dave is not wrong about that. Manufacturer recommendation is to cease charging when appropriate termination conditions are met, such as when the charging current in CV phase drops below a threshold like 100 mA.
But I have tried to explain above that in the Big Clive example the battery is not being float charged. 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.
In short, maintaining the CV charging phase at a battery voltage of 4.20 V is not recommended. But in this case that is not happening. The battery voltage is less than 4.20 V, and no charging is going on.
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.
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?
And if you say that even that is unacceptable, what would you like to do about it? Do you want the device to turn off the output while the charging cable is plugged in? Don't you think people would complain about that? Or do you want the device to refuse to accept charge while it is powering something? Don't you think people would complain about that too? And if it is my cell, and my charger, and my application, isn't it up to me if I wish to (potentially) shorten the life of the cell a bit?
What I've always thought I had to do about it if the load current prevented charge termination is to use a load sharing circuit. With such a circuit, load current is never provided by the charger, so load current can't prevent termination. I've used circuits like that, and even designed one for use with solar panels that actually works. Another option is to periodically reduce the load current below the termination current, through sleep or whatever, so the TP4056 will terminate properly. But now you and Clive are telling me that I don't need to bother with such measures, at least not to protect the battery.
And as for turning off charge current when the charging cable is still plugged in, that's exactly what the TP4056 does if the load current is smaller than the termination current. And people do complain, but that was thought to be necessary to protect the battery. A load sharing circuit eliminates that problem, but you would think that there would be a TP405x charger that has no termination, so cycling wouldn't occur in the first place. If there is such a chip, I've never seen it.
I'm still looking for an explanation of what can happen to a cell if 4.2V is applied continuously. I've always accepted the industry dogma not to do that, but never saw any reasons for it. If Clive is right, I can forget about it, and buy fewer mosfets and schottky diodes.
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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.
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.
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?
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 (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.
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I fail to see what myth exactly is busted in this video, or even what exactly its purpose is.
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I fail to see what myth exactly is busted in this video, or even what exactly its purpose is.
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.
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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.
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".
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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.
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.
If Dave is right, why is he right?
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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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.
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.
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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.
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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.
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.
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.
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.
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.
Saft also talks about charging rate:
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.
Their emphasis, not mine.
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.
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.
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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?
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.
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So you can't really use Clive's method in UPS mode.
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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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.
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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.
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.
Is such a low current limit for a discharged battery really needed? I don't know.
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.
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I take care not to let the cells ever overdischarge, and consider overdischarged cells a failure, either in cell or cutoff circuit.
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?
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I take care not to let the cells ever overdischarge, and consider overdischarged cells a failure, either in cell or cutoff circuit.
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?
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.
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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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There are two competeing considerations at play, and I think this is a large part of why people disagree on this, some people have a focus on longevity of the cell (loss of capacity over time), and some people have a focus on safety of the cell (not setting your house on fire).
"Float charging" if you like to call it that, having a 4.2v supply connected to a cell which is sitting at 4.2v, is perfectly safe, but it could conceivably cause an accellerated reduction in capacity (aging) over time (at least anecdotally, I don't know the actual research).
It gets worse when the "longevity" people think because it's not as good for longevity that it must be bad for safety. And the "safety" people think that because it's perfectly safe that it must be fine for longevity too. These two don't necessarily correlate!
Personally, I go for the "it's safe" position, I don't care about capacity loss.
But in any case, a TP4056 is hard pressed to maintain an amp at the best of times anyway. A P-Fet bypass to provide the load directly from a connected supply is pretty trivial. It's kinda weird that none of the chinese TP4056 boards include one, and yet for powerbanks almost universally power the load directly from a connected supply.
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It's kinda weird that none of the chinese TP4056 boards include one, and yet for powerbanks almost universally power the load directly from a connected supply.
I think power banks have changed a bit since the early days? I thought the early ones with a cylindrical form factor just had one single USB A port? I suppose the newer ones with multiple USB ports can be charged through one port while loaded through the other?
My favorite USB power bank is going to be this one:
https://www.ryobitools.com/products/details/33287177264 (https://www.ryobitools.com/products/details/33287177264)
It doesn't have all the latest USB standards, but it is powered by a massive tool battery and it is 85% efficient, so a 4 Ah tool battery can provide about 12 000 mAh at the 5 V USB output (4000 x 18 / 5 x 0.85).
If the battery runs out, you can just swap it for another one, no recharging necessary. It is not, of course, suitable for air travel.
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"Float charging" if you like to call it that, having a 4.2v supply connected to a cell which is sitting at 4.2v, is perfectly safe, but it could conceivably cause an accellerated reduction in capacity (aging) over time (at least anecdotally, I don't know the actual research).
Yeah, but if the alternative is microcycling the cell between 4.20V and 4.00V, this is (probably, I'm not sure!) going to be even more detrimental to the lifetime.
According to my own tests plus others I have read, you really have to go low enough storage voltage, say 3.90V and below, to get any significant decrease in calendar fading.
In any case, you are absolutely right, and if one wants to design a long-life "float" application, you can choose a tad larger cell and float it at 3.8-3.9V or so.
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A P-Fet bypass to provide the load directly from a connected supply is pretty trivial. It's kinda weird that none of the chinese TP4056 boards include one, and yet for powerbanks almost universally power the load directly from a connected supply.
Yes I've always thought it was strange that the module makers of the Far East don't seem to know about power sharing. You even have combination charger and boost converter modules, but they don't have power sharing either. There are some charger ICs with that feature built into the chip, but they don't come in hobbyist-friendly packages, and few if any modules use them (maybe Adafruit is an exception, but that would not be low cost).
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A P-Fet bypass to provide the load directly from a connected supply is pretty trivial. It's kinda weird that none of the chinese TP4056 boards include one, and yet for powerbanks almost universally power the load directly from a connected supply.
Yes I've always thought it was strange that the module makers of the Far East don't seem to know about power sharing. You even have combination charger and boost converter modules, but they don't have power sharing either. There are some charger ICs with that feature built into the chip, but they don't come in hobbyist-friendly packages, and few if any modules use them (maybe Adafruit is an exception, but that would not be low cost).
Because they know it's not necessary and just increases cost?
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Or maybe define "module makers". Or whatever.
Just because there are zillions of modules, mainly targeted at hobbyists, made around the TP4056 (which itself comes in various clones, with slightly different characteristics, fun stuff), doesn't mean that this is the only asian chip available or that it is even relevant. Yes, there is a life beyond the TP4056! Believe it or not. ;D
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A P-Fet bypass to provide the load directly from a connected supply is pretty trivial. It's kinda weird that none of the chinese TP4056 boards include one, and yet for powerbanks almost universally power the load directly from a connected supply.
Yes I've always thought it was strange that the module makers of the Far East don't seem to know about power sharing. You even have combination charger and boost converter modules, but they don't have power sharing either. There are some charger ICs with that feature built into the chip, but they don't come in hobbyist-friendly packages, and few if any modules use them (maybe Adafruit is an exception, but that would not be low cost).
Because they know it's not necessary and just increases cost?
Well, even if power sharing isn't necessary, it would prevent the cycling between 4.2V and 4.1V that occurs when the load current is less than the termination current. And it would eliminate any potential startup issues if the battery is discharged to below 3V and the load current is greater than the initial float current. Some might want those benefits.