trustfire 18650 2400mAh and RC charger

[A380]

Hi.
I have a little problem charging this cell on a hyperion eos0610inet. I mostly use it for A123 cell form my rc planes and never used li-ion before.
The thing is that cells are li-ion rated 3.7v but the charger sets li-ion chemistry with 3.6v, so I belive I'm not going to charge fully the batt.
Maybe I could set the charger to li-po that is rated to 3.7v? Or it could go ?

 

[SilverFox]

Hello A380,

Welcome to CPF.

The 3.6 volt setting will charge the cell to 4.1 volts. The 3.7 volt setting will charge it to 4.2 volts.

If capacity is important, charge to 4.2 volts. If cycle life is important charge to 4.1 volts.

A Li-Ion cell at 4.1 volts will have around 90% of its total capacity. A cell charged to 4.2 volts is expected to give you around 500 charge/discharge cycles. A cell charged to 4.1 volts is expected to give you around 1500 charge/discharge cycles.

Years ago, and in some cases even now, Li-Ion cells were only charged to 4.1 volts. Most common recent cells are now capable of safely being charged to 4.2 volts.

Tom

 

[A380]

thanks for the welcoming SilverFox.
I've just finish charging with the li.ion mode (4.1v) and the batt has taken nearly 2000mAh (it was 3.2v when it started).
I think I'm going to charge it up to 4.1v as i will use 2p configuration with a potentiometer, so going to have long runtime while going up with batteries life.

best regards

 

[wapkil]

If capacity is important, charge to 4.2 volts. If cycle life is important charge to 4.1 volts.

A Li-Ion cell at 4.1 volts will have around 90% of its total capacity. A cell charged to 4.2 volts is expected to give you around 500 charge/discharge cycles. A cell charged to 4.1 volts is expected to give you around 1500 charge/discharge cycles.

Do you know what the expected lifetime would be if only the constant current phase the to 4.2V is performed? I think the cell will drop the "excess voltage" immediately when disconnected so with the 1C charge current it should stay above 4.1V only for 5 or 10 minutes during the charge. I know that it will undercharge the battery (to ~80% capacity, I think) but will it be better or worst for the longevity than normal CC/CV charge to 4.1V?

 

[SilverFox]

Hello Wapkil,

I don't think that has been studied, but I would guess that the cycle life would be much better when charging to 4.0 volts rather than 4.1 volts. I believe there is an exponential increase in cycle life when the charge is limited to lower voltages.

Tom

 

[Black Rose]

A cell charged to 4.2 volts is expected to give you around 500 charge/discharge cycles.
A cell charged to 4.1 volts is expected to give you around 1500 charge/discharge cycles.

A difference of 0.1 volt can potentially add 1000 cycles to the life of a cell? That's amazing.

But don't Li-Ion cells degrade on their own, whether they are being used or not?

If you are a heavy Li-Ion user, it sounds like using a hobby charger and utilizing a 4.1v termination voltage would be beneficial to the cells and the wallet.
Maybe not so much for a more casual Li-Ion user.

Maybe I need to reconsider getting that hobby charger after all :thinking:

 

[wapkil]

Hello Wapkil,

I don't think that has been studied, but I would guess that the cycle life would be much better when charging to 4.0 volts rather than 4.1 volts. I believe there is an exponential increase in cycle life when the charge is limited to lower voltages.

Tom

Thank you for the answer Tom. I also believe that at the 4.0V the lifetime will be higher. This wasn't my question, though I'm not sure how to interpret what you wrote. Do you say that the charge regime I asked about wasn't studied and additionally inform me about the results when the cell is charged to 4.0V or is this just a misunderstanding?

I asked about the charge to 4.2V but interrupted immediately after the CC phase (so without the CV). I asked mainly out of curiosity but I noticed that for example some of the budget chargers illuminate their end-of-charge status LEDs when the CC phase is finished so this information may have some practical meaning for their owners.

 

[SilverFox]

Hello Wapkil,

No misunderstanding. You mentioned charging to around 80%. That works out to an ending voltage of 4.0 volts.

Tom

 

[wapkil]

Hello Wapkil,

No misunderstanding. You mentioned charging to around 80%. That works out to an ending voltage of 4.0 volts.

Tom

Ah, I see. You meant the resting voltage some time after charging and I was focused on the peak voltage during it, hence my misunderstanding.

Your conclusion that since the cell is charged to lower level, the lifetime should be higher is really interesting and somehow disturbing to me. I thought that both the charge level and the peak voltage (and the timespan the cell is under it) affect the cell longevity but I was under the impression that it is the peak voltage that is more important. I imagined that this is the reason why the standard charging method uses the CC/CV algorithm with 4.20V peak and not e.g. simply CC finished at 4.25V. I guess I need to read the description of what exactly happens inside during charging

 

[mdocod]

Hi wapkil,

You're on to something very intelligent here IMO:

A CC only charge to 4.20V followed by termination would result in the cell dropping off to a resting voltage lower than 4.20V, how far it drops down would depend entirely on the charge rate used.

Say for instance, you built a charger that was 50mA and charged an 18650 to 4.20V, you would actually end up with a off the charger fully charged at 4.20V (or very close, like 4.19V or whatever). If the charge rate were very fast, like 2000mA, the cell would likely fall to 4.0V or less after termination.

I would guesstimate based on my experience that a charge rate of ~0.25C to 4.20V would result in a cell falling to around 4.10V (+/- 0.04V) after termination.

As you may know, many poorly designed li-ion chargers do not use a CV stage at all, and have a CC only charge rate that runs up well over 4.20V to bring the cell to a state of charge that results in a resting voltage of around 4.20V after a supposed termination.

The life of a li-ion cell has a lot to do with how much of it's life it spends at the extremes of the voltage range. Optimal life is achieved at ~3.9V or around 50% state of charge. It has been "assuminated" (new word here watch out) that those cheap CC only chargers that ramp voltage above 4.20V during charge are likely having a negative impact on cell life, however, the amount of negative impact is likely pretty low since the cell is only spending a short time (minutes) in voltage situation above ideal. I think the same could be translated down to a charger designed to run a CC mode to 4.20V with intent to result in cells at ~4.10V. The short time the cell would spend above 4.10V during charging would likely not have a very dramatic impact on the cycle life compared to a 4.10V CC/CV charge.

A CC to 4.20V followed by termination would have the advantage of a simpler charger design, and reasonably fast charging (no waiting around for a CV phase to finish up, which can add a lot of time on to a charge).

-Eric

 

[wapkil]

Hi wapkil,

You're on to something very intelligent here IMO:

A CC only charge to 4.20V followed by termination would result in the cell dropping off to a resting voltage lower than 4.20V, how far it drops down would depend entirely on the charge rate used.

Say for instance, you built a charger that was 50mA and charged an 18650 to 4.20V, you would actually end up with a off the charger fully charged at 4.20V (or very close, like 4.19V or whatever). If the charge rate were very fast, like 2000mA, the cell would likely fall to 4.0V or less after termination.

I would guesstimate based on my experience that a charge rate of ~0.25C to 4.20V would result in a cell falling to around 4.10V (+/- 0.04V) after termination.

As you may know, many poorly designed li-ion chargers do not use a CV stage at all, and have a CC only charge rate that runs up well over 4.20V to bring the cell to a state of charge that results in a resting voltage of around 4.20V after a supposed termination.

The life of a li-ion cell has a lot to do with how much of it's life it spends at the extremes of the voltage range. Optimal life is achieved at ~3.9V or around 50% state of charge. It has been "assuminated" (new word here watch out) that those cheap CC only chargers that ramp voltage above 4.20V during charge are likely having a negative impact on cell life, however, the amount of negative impact is likely pretty low since the cell is only spending a short time (minutes) in voltage situation above ideal. I think the same could be translated down to a charger designed to run a CC mode to 4.20V with intent to result in cells at ~4.10V. The short time the cell would spend above 4.10V during charging would likely not have a very dramatic impact on the cycle life compared to a 4.10V CC/CV charge.

A CC to 4.20V followed by termination would have the advantage of a simpler charger design, and reasonably fast charging (no waiting around for a CV phase to finish up, which can add a lot of time on to a charge).

-Eric

Thank you or the follow-up Eric. I don't know how I could have missed the relation between the charge rate and the resulting charge level after the CC phase. I even tested for it myself before...

After reading your explanation I think that the main problem with the "CC-phase only" design is that to work well it would require user to set the current for every charged cell. If the current was pre-set to one value the charger would obviously work differently for different capacity cells. It would also, I think, work differently for the same cell as it ages. Aging cell loses its capacity. This results in each consecutive charge having higher impact on the cell life, because with lower capacity the pre-set charge current charges it closer to 100%. This rising impact would in turn accelerate the cell aging. Kind of a positive feedback loop. I think it is not such a good idea after all.

What I like about the CC/CV algorithm is that, as I believe, it self-balances. For different charging currents and cell capacities it makes the CC and CV phases have different lengths but ultimately the same charge result. I wonder why there seems to be no charger that would let user select between the cell life (defined as the number of charge cycles) and the charge level. It is not complicated to build. It could be really easy to operate, even for users without any knowledge related to Li-Ion batteries. It could have, say, four levels marked from "50% charge / 100% lifetime" to "100% charge / 10% lifetime" (just an example, the values are probably incorrect). Maybe it is that most of the people that may think about buying it already have hobby chargers for the task...

 

[wapkil]

But don't Li-Ion cells degrade on their own, whether they are being used or not?

If you are a heavy Li-Ion user, it sounds like using a hobby charger and utilizing a 4.1v termination voltage would be beneficial to the cells and the wallet. Maybe not so much for a more casual Li-Ion user.

I think that's exactly how it is. I read though that Li-Ion cells produced now seem to be able to stay vibrant much longer than previous generations so with them the balance between the charge level and cycle-life could be important for more users.

 

[mdocod]

Hi wapkil,

To makup for the variation in cell-to-cell capacities and internal resistance. A CC-only charger using the same (or similar) charge method as a WF-139 (some variants) but with a target voltage of 4.10V would probably work pretty well.

The WF-139 literally cycles the charge on and off every few seconds. The off cycle is only for a moment, but long enough for it to take a cell voltage reading without the charge applied. It continues charging at the CC rate until the cell reads 4.20V in one of those "off" portions of the routine. This is a problem in the current design, especially on smaller cells, because it will actually wind up taking charging voltages to 4.3+V.

If the target voltage were reduced to 4.10V, and 2 charge rates were available, (a "slower" charge rate on the "sideways RCR123 bays" and a "faster" charge rate on the full length bays), then I think a CC only charge to 4.10V (resting) could be applied to a wide range of cell sizes. The maximum voltage reached during charging would vary depending on the resistance of the cell in question, but most cells would probably ramp up in the 4.17-4.22 charging voltage range before termination, which would be fine.

-Eric