| General > General Technical Chat |
| LiPo Battery on Holding/Float Charge |
| << < (4/6) > >> |
| Siwastaja:
--- Quote from: magic on July 02, 2021, 08:35:05 am ---Perhaps it could also be that self discharge at 4.2V exceeds the CC/10 threshold if CC is set too low. --- End quote --- No, only going to happen with a completely dead/destroyed cell and then it could happen at any voltage really. Self discharge at 4.2V on a good cell is approx 1-10%/year so put in C rate, approx. C/1000000 to C/100000. This may be one of the reasons for the recommended "charging timeout" feature, though, although I think it's insufficient if you don't flag the cell as dead and refuse using it again (and then false positives would be disastrous). One otherwise-broken-by-design-as-well TI/Benchmarq IC I used in error had this great idea of having gazillion of pins to configure things yet they shared the setting of CV ending current and "overdischarged cell revival current". You couldn't even set the revival current low enough to be safe, without setting the CV cutoff to some abysmally strange value like C/100. So with something sane like C/10 stopping, overdischarged cell precharge became also C/10, basically way beyond what any cell manufacturer recommends. And you couldn't disable the damn feature. This chip was full of self-destruction features and traps and actually using it is the only case ever where I managed to overcharge cells unintentionally, charging a few packs, two of which already at customers, to 4.5V per cell. No fires, no problems, by the way. The cells worked fine and provided nice extra capacity for the few customers before the recall :phew:. Having never seriously failed a from-scratch li-ion management design, this experience is clear cut proof to me that I'm never going to put any BQ anything chip in any of my designs again, period. (Many of them are OK, I guess.) |
| SiliconWizard:
--- Quote from: magic on July 02, 2021, 08:35:05 am --- --- Quote from: SiliconWizard on June 29, 2021, 08:25:03 pm ---The reason EOC can be hard to detect for low charging currents is simple. Most charger ICs roughly use the following algorithm: 1 - If Vcell < Vmin => source a predefined constant current < charging current (to "prepare" the cell); 2 - If Vcell < Vmax (usually 4.2V for LiPo) => source constant current = charging current; 3 - When Vcell reaches Vmax => switch to constant voltage mode @Vmax, and monitor current to the cell until it goes below (charging current)/10; 4 - Stop charging => charger output in Hi-Z. The condition in step 3 is one reason EOC detection can fail for very low charging currents. Since the threshold is usually (charging current)/10, it might be below the usable range for the internal current monitoring circuitry if charging current is set too low. Something to keep in mind... --- End quote --- Perhaps it could also be that self discharge at 4.2V exceeds the CC/10 threshold if CC is set too low. --- End quote --- That can be a factor too, although it's pretty unlikely, unless your charging current is freaking extremely low, to a point of making no sense. But Li-ion battery chemistry is not that simple, and for low charging currents (for a given battery), you can start observing some weird non-linear behavior. That's why I recommend charging currents in a reasonable range. Of course refer to datasheets, but that's commonly between C/4 and C/2. Some batteries of course can widthstand > C, but that's still most often reducing battery life significantly. |
| tooki:
--- Quote from: Siwastaja on July 02, 2021, 02:25:04 pm ---One otherwise-broken-by-design-as-well TI/Benchmarq IC I used in error had this great idea of having gazillion of pins to configure things yet they shared the setting of CV ending current and "overdischarged cell revival current". You couldn't even set the revival current low enough to be safe, without setting the CV cutoff to some abysmally strange value like C/100. So with something sane like C/10 stopping, overdischarged cell precharge became also C/10, basically way beyond what any cell manufacturer recommends. --- End quote --- Are you sure about that? The cell datasheet I attached above lists a max precharge current of 150mA, with 1C=2750mA, which is just above C/20. And the same datasheet recommends no less than C/20 termination current.. So very, very similar values, and definitely not “way, way beyond” a typical recommendation. --- Quote from: Siwastaja on July 02, 2021, 02:25:04 pm ---And you couldn't disable the damn feature. This chip was full of self-destruction features and traps and actually using it is the only case ever where I managed to overcharge cells unintentionally, charging a few packs, two of which already at customers, to 4.5V per cell. No fires, no problems, by the way. The cells worked fine and provided nice extra capacity for the few customers before the recall :phew:. Having never seriously failed a from-scratch li-ion management design, this experience is clear cut proof to me that I'm never going to put any BQ anything chip in any of my designs again, period. (Many of them are OK, I guess.) --- End quote --- FWIW, the bq25895 I’m using is working great. It defaults to values that are conservative for an 18650, but offers a gazillion parameters to set via I2C — including separate precharge and termination currents. ;) |
| Siwastaja:
--- Quote from: tooki on July 02, 2021, 05:12:28 pm ---Are you sure about that? The cell datasheet I attached above lists a max precharge current of 150mA, with 1C=2750mA, which is just above C/20. --- End quote --- Yes I have read through that and I have decided I don't just trust Samsung on this. Read it again, carefully, and you'll see the text allows charging a cell at 0.0V or even -99.9V at 150mA forever with no timeout. This does not match the precharge as usually described. There almost always is some minimum voltage requirement which is more than zero, and also a timeout required in case the cell does not reach some "normal charge can begin" threshold like 3.0V here (which is a sensible value). Problem with high currents is localized heat generation at the site of the dendrite. And C/20 is surprisingly lot IMHO. After all, the whole point of the "precharge" feature is to see, carefully, with very low charging current, if copper dissolution has happened within the cell, and if it has caused internal shorts by copper dendrites piercing the separator; possibly even forcing these dendrites back to the copper current collector, at least to some extent. I.e.; see if the cell is damaged; try to recover; bail out if impossible. The only thing that makes this procedure safe is correct settings and thresholds. You do detect the presence of the dendrites by the fact that the cell voltage does not rise while charging (slowly) within some sensible time (like 30 minutes). Or maybe Samsung has carefully calculated/simulated/tested that their 29E cell, completely overdischarged to any voltage, including negative voltages, can take continuous 150mA forever without issues, just heating up some and dissipating that heat. But I wouldn't base a charger logic on this because others have stricter rules, and quite frankly, the fact you can do something does not mean you necessarily should, this includes not running at the maximum values. --- Quote ---FWIW, the bq25895 I’m using is working great. It defaults to values that are conservative for an 18650, but offers a gazillion parameters to set via I2C — including separate precharge and termination currents. ;) --- End quote --- Good to hear that. Personally, I don't just bother with "precharge", at least not an aggressive one. It's relevant only in case of BMS or cell failure, trying to "save" a possibly slightly damaged cell. Primarily, the BMS should be fixed so that overdischarging doesn't happen in the first place. I refuse to charge cells below say 2.5V, which is the "classic" way. Maybe I limit charging current below say 3.3V but I won't call that precharge, but just because cells show significantly higher DC ESR near 0% SoC it's a good idea to charge more slowly to generate less heating power. |
| tooki:
--- Quote from: Siwastaja on July 03, 2021, 09:46:32 am --- --- Quote from: tooki on July 02, 2021, 05:12:28 pm ---Are you sure about that? The cell datasheet I attached above lists a max precharge current of 150mA, with 1C=2750mA, which is just above C/20. --- End quote --- Yes I have read through that and I have decided I don't just trust Samsung on this. Read it again, carefully, and you'll see the text allows charging a cell at 0.0V or even -99.9V at 150mA forever with no timeout. --- End quote --- It took me a few readings of your claim to actually figure out what you meant: your wording makes it sound like it's allowing a charge current of 150mA at -99.9V, whereas you meant "the text allows charging a cell whose voltage is 0.0V or -99/9V with a charging current of 150mA…", correct? Anyhow, I think that's a rather contrived interpretation, since the datasheet expressly warns not to ever discharge below 2.5V. What is true is that they do not define what should to be done with a cell that nonetheless has fallen below 2.5V. But there is no ambiguity that <2.5V is NOT an allowed condition. Since -99.9V<2.5V, and 0V<2.5V, they clearly aren't "allowing" charging in that state. A sensible interpretation of the datasheet is this: precharge current of up to 150mA from 2.5V to 3V, then regular CC charging up to 4.20V, then constant voltage to C/10. --- Quote from: Siwastaja on July 03, 2021, 09:46:32 am ---Or maybe Samsung has carefully calculated/simulated/tested that their 29E cell, completely overdischarged to any voltage, including negative voltages, can take continuous 150mA forever without issues, just heating up some and dissipating that heat. But I wouldn't base a charger logic on this because others have stricter rules, and quite frankly, the fact you can do something does not mean you necessarily should, this includes not running at the maximum values. --- End quote --- Correct. But your assertions only exist by ignoring the maximum values given elsewhere in the datasheet. The datasheet says to use 3V as the 0% SOC, and 2.5V is given as the absolute maximum discharge voltage, not as recommended. 3V is clearly not zero or a negative voltage. --- Quote from: Siwastaja on July 03, 2021, 09:46:32 am --- --- Quote ---FWIW, the bq25895 I’m using is working great. It defaults to values that are conservative for an 18650, but offers a gazillion parameters to set via I2C — including separate precharge and termination currents. ;) --- End quote --- Good to hear that. Personally, I don't just bother with "precharge", at least not an aggressive one. It's relevant only in case of BMS or cell failure, trying to "save" a possibly slightly damaged cell. Primarily, the BMS should be fixed so that overdischarging doesn't happen in the first place. I refuse to charge cells below say 2.5V, which is the "classic" way. --- End quote --- Of course. (Again, the 3V specified 0% SOC is greater than 2.5V, so they're clearly telling the designer to design around 3V, not 2.5V.) The datasheet also specifies maximum discharge currents for <3V and <2.5V, which is presumably to limit the chances of a cell going below 2.5V to begin with. Of course, the (separate) protection IC should be selected to enforce the absolute maximums as a last resort. |
| Navigation |
| Message Index |
| Next page |
| Previous page |