Charging 18650 Li-ion w/Lab Power Supply

[spectral532]

Would it be possible to charge a 18650 Li-ion with a Lab (bench-top) power supply? The power supply I have access to is voltage regulated with an adjustable current limit. I was thinking of setting the voltage at 4.2v and current limit at 0.5C (~1A). That way the current limit would hold the charge current at 1A until the current draw decreases. Then the power supply would hold steady at 4.2v for the remainder of the charge. I could then manually cutoff power when charge current drops below 0.1C.

I'm still rather new to Li-ion batteries though, and I'm not sure if this would be a safe way to charge the batteries. Any advice would be appreciated.

[Mr Happy]

This works fine as long as you watch the charging progress and make sure to stop it at the right time.

[T24]

when the power supply would hold steady at 4.2V --> safe

if not --> maybe

[65535]

Just make sure you have an expensive power supply, a lot of them have poor meters and less than perfect regulation. If it will hold .01V you should be fine. Keep an external VM on the cell the first time or two as a double check.

Otherwise the idea is completely sound, and ideal.

[TranceAddict]

i was using lab CC/CV power supply as li-ion charger as well before i built this http://www.candlepowerforums.com/vb/...d.php?t=233932 which worked pretty fine and its algorithm far surpass ultrafire charger and even Pila IBC.


as long as you set voltage to 4.2v it will be fine even if you leave them on forever, when supply is 4.2v and cell is also 4.2 there will be no potential as so you will get 0amp if you measure it. this solution has only one drawback, which is when cell voltage raise to around 4v, it seem to take infinite time to reach 4.2v due to too small current flowing pushed by the 0.2v potential difference. same rules apply when you connect many cells in parallel to the power supply, they will balance each others and charge perfectly to 4.2v.

[45/70]

Just make sure you have an expensive power supply, a lot of them have poor meters and less than perfect regulation. If it will hold .01V you should be fine. Keep an external VM on the cell the first time or two as a double check.

I've used my Metex MS-9140's PS for charging Li-Ion cells. It is very important to watch the current.

The DMM in the 9140 has always checked out to be better than +/- 1% @ 3.3, 5, and 12 Volts, but the meter on the PS is more like +/- 3%. What you can do, if you don't want to monitor the voltage/current with a separate DMM, is set your PS voltage to 4.10 Volts. Provided the readings on your PS aren't totally off, this will charge the cell(s) to 90% within a reasonable safety margin.

as long as you set voltage to 4.2v it will be fine even if you leave them on forever, when supply is 4.2v and cell is also 4.2 there will be no potential as so you will get 0amp if you measure it.

As I understand it, this is not a good idea. Edit: From B.U.

"No trickle charge is applied because lithium-ion is unable to absorb overcharge. A continuous trickle charge above 4.05V/cell would causes plating of metallic lithium that could lead to instabilities and compromise safety."

The CV stage should cutoff at a current no lower than 0.03C, or you run the risk of metallic lithium plating out onto the anode. A lot of hobby chargers cut the CV stage at 0.05-0.1C. This would add an acceptable safety margin without sacrificing much capacity, as well as probably prolonging cell life.

In reality, it's better to just use a good charger. As has been pointed out, using a bench supply takes a long time and you have to watch the current closely. The only time I ever use the Metex, is when I'm determining cell condition, and want an actual 4.20 Volts "off the charger". As I have chargers for most of my LiCo cells that do charge to 4.20 Volts, I rarely use the Metex anymore.

Dave

[rmteo]

I have charged Lithium cells using a bench power supply but no longer do it. My PSU is a 1200W (0-30V, 0-40A) switching Xantrex unit. It is just too easy to accidentally nudge the controls (or to make an incorrect setting setting due to loss of concentration) to make the process potentially dangerous to your health.

With 6-cell, 5A balancing hobby chargers starting at under $50, there really is no need to look at alternative charging methods. Also, these hobby chargers have several safety mechanisms built in such as cell count determination, low voltage, auto cut-off, etc. which you do not have with a bench supply.

[SilverFox]

Hello Spectral532,

Welcome to CPF.

As others have mentioned this is a great way to charge Li-Ion cells.

However there is one "issue." Li-Ion chargers will terminate the charge when the charge current drops below 50 - 100 mA. The reason they do this is because Li-Ion chemistry does not handle "trickle charging" very well.

In this case, we are defining "trickle charging" as the charge that occurs when the cell is at 4.2 volts and the current is in the range of 50 - 0 mA.

If you let your power supply go, it will continue to charge until the current drops to 0 mA. The "trickle charging" phase tends to oxidize the electrolyte inside the cell, and is detrimental to long term life. It also takes a long time to complete this final stage, so charging time is extended. This is especially true when the cell has developed some internal resistance and is no longer holding 4.2 volts rested. With a cell that has been in service for awhile and is at around 90% of its original capacity, the amount of time spent "trickle charging" can be equal to or exceed the total charge time if the charge was terminated when the current dropped to around 100 mA. As the cell ages further, you get into the situation where the "trickle charge" never ends.

There are a couple of ways to work around this.

One way is to be very vigilant and stop the charge when the current falls below a certain amount. This is a very good way to go, but sometimes we get distracted and let things go a little longer than we had planned.

A better way is to simply reduce the voltage you are charging to.

Cycle life is related to the maximum voltage you charge the cell to. If you charge to 4.2 volts, you should get somewhere around 500 charge/discharge cycles. However, if you limit the charge to 4.1 volts, the cycle life extends to somewhere around 1500 charge/discharge cycles.

With this information in hand, you can now adjust the settings to avoid any problems with "trickle charging." You simply set the voltage to 4.15 or 4.18 volts. With cells in good condition I usually use 4.18 volts. As the cells age and develop some internal resistance, I drop that to 4.15 volts. Many people simply use 4.15 volts all the time.

With this method you still have to watch the charge process, but you don't have to be glued to it. Since our definition of "trickle charge" is very specific, at a lower voltage we are not "trickle charging." Charging using a power supply is safest using this method.

There is one other method that can be used. Normal charge rates are in the 0.5C - 1.0C range. An optimum rate is about 0.7C. The idea is to balance the CC phase and the CV phase of the charge. The faster you charge, the more time you spend in the CV phase, and the greater the possibility of "trickle charging." The ideal rate (from the cells perspective) is 0.7C, but everyone wants faster charging and I am not sure there is much of a difference in cell life between charging at 0.7C or 1.0C.

At any rate, if you charge at a low charge rate, it seems that you can eliminate the CV stage completely. I have not tried this, but have read that if your charge current is about 0.18C, or lower, you can simply terminate the charge when the voltage reaches 4.2 volts.

Once again you have to watch the end of the charge closely, but it is another option. The general rule that the termination current is usually about 10% of the initial charge current does not apply in this case, but you simply terminate upon the voltage reaching 4.2 volts.

One final caution... from experience.

If you have curious cats, watch them carefully while charging. I had a cat become interested in the changing displays and was pawing at the control nobs...

Tom

[BVH]

Tom, I've read where a Schotkey diode should be installed in the + lead of the PS to prevent backflo of power. I did not have one in my 1st Mastec 30V-50A and I believe I had some backflo when I happen to carelessly make a final connection to energize an HID ballast. With a bare wire, I ended up making and breaking hot contact probably 3 or four times in a fraction of a second. I smoked the supply. I now have a Schotkey in-line and have had no issues, although I am much more careful about my connections and have a toggle switch in-line to make the final connection. Your thoughts?

[45/70]

As others have mentioned this is a great way to charge Li-Ion cells.

One way is to be very vigilant and stop the charge when the current falls below a certain amount. This is a very good way to go, but sometimes we get distracted and let things go a little longer than we had planned.......

With this method you still have to watch the charge process, but you don't have to be glued to it.......

Once again you have to watch the end of the charge closely, but it is another option.......

One final caution... from experience.

If you have curious cats, watch them carefully while charging. I had a cat become interested in the changing displays and was pawing at the control nobs...

What's really neat, is charging single NiMH or NiCd cells with a bench PS. Your eyes are so "glued" to the display, watching for that 0.005 Volt -dV that the cat isn't a problem!

Before anyone asks, no, I haven't done this.

Dave

[Mr Happy]

I've read where a Schotkey diode should be installed in the + lead of the PS to prevent backflo of power. I did not have one in my 1st Mastec 30V-50A and I believe I had some backflo when I happen to carelessly make a final connection to energize an HID ballast. With a bare wire, I ended up making and breaking hot contact probably 3 or four times in a fraction of a second. I smoked the supply. I now have a Schotkey in-line and have had no issues, although I am much more careful about my connections and have a toggle switch in-line to make the final connection.

I know this was addressed to Tom, but how were you using the power supply?

Here is what I think is a typical safe way to use a lab power supply:

1. Set the power supply to the minimum voltage
2. Set the current limit just above the highest expected load value
3. Discharge the power supply terminals with a small load (bulb or medium value resistor) and remove
4. Discharge any capacitors in the equipment to be tested with a resistor
5. Connect the equipment to be tested to the power supply
6. Slowly increase the power supply voltage to the required value while watching for any unexpected events in the equipment

This should save you from untoward happenings, protecting both the power supply and the equipment being tested.

[SilverFox]

Hello BVH,

I have also heard that using a diode is a good thing to do, but I have never used one. While I frequently charge Li-Ion batteries with my power supply, that is not its main use. If I ever get to the point of dedicating a power supply to charging, I will look further into this.

Tom

[HKJ]

Hello BVH,

I have also heard that using a diode is a good thing to do, but I have never used one. While I frequently charge Li-Ion batteries with my power supply, that is not its main use. If I ever get to the point of dedicating a power supply to charging, I will look further into this.

Tom

There are both positive and negative aspects to using a diode for charging on a power supply.

The plus side is that the batteries will be not discharged if the power supply is turned off, while connected to the batteries.
A badly designed power supply might also be damaged, if turned off while the batteries are connected.

The minus side to diodes, it the voltage drop, the diode will drop some voltage (between 0.5 to 1 volt depending on current), making it a bit difficult to adjust the power supply to exactly the required voltage.

[SilverFox]

Hello Mr Happy,

I know this was addressed to Tom, but how were you using the power supply?

Here is what I think is a typical safe way to use a lab power supply:

1. Set the power supply to the minimum voltage
2. Set the current limit just above the highest expected load value
3. Discharge the power supply terminals with a small load (bulb or medium value resistor) and remove
4. Discharge any capacitors in the equipment to be tested with a resistor
5. Connect the equipment to be tested to the power supply
6. Slowly increase the power supply voltage to the required value while watching for any unexpected events in the equipment

This should save you from untoward happenings, protecting both the power supply and the equipment being tested.

I set things up a little differently for Li-Ion battery charging...

I turn the current down to near 0 mA, then adjust the voltage to 4.15 volts. I then hook up my battery and adjust the charging current. When the charge is complete, I zero the charging current and disconnect.

Tom

[Mr Happy]

I set things up a little differently for Li-Ion battery charging...

I turn the current down to near 0 mA, then adjust the voltage to 4.15 volts. I then hook up my battery and adjust the charging current. When the charge is complete, I zero the charging current and disconnect.

I agree. The procedure is a little different as you describe when connecting to an energized device like a battery (which is a slightly unusual use for a lab power supply).

I believe BVH was connecting to an HID ballast which is more of a passive load.

In every case, it is always a good plan to short the power supply terminals with a dummy load after setting the voltage and before connecting the real device. This will discharge any output capacitors in the power supply and prevent any unexpectedly high voltage spikes being fed into delicate equipment.

When working with electronic circuits one should always be careful of residual charge on capacitors. In some cases they can kill.

[45/70]

I turn the current down to near 0 mA, then adjust the voltage to 4.15 volts. I then hook up my battery and adjust the charging current. When the charge is complete, I zero the charging current and disconnect.

That's the way I've always done it for Li-Ion cells, except I adjust the current to zero, initially. When I use the PS for forming NiMH or NiCd packs, I turn the voltage up to 30V, and then once the pack is connected, adjust the current up (from zero) to 0.1C.

The 0-30V section of my PS is only 3A max, but I guess that really wouldn't make any difference, in regards to the intensity of any spikes, or would it?

Also, I added a quote and link from Battery University to post #6

Dave

[spectral532]

Thanks for the replies everyone. I couldn't have asked for better answers.

In response to the concerns of a few members, I am using a digital power supply with a built in volt/ammeter. It has an accuracy of about ±4mV, so it should be fine for this purpose. I plan on manually monitoring the current, and will stop the charge when current drops to ~.1C.

@Silverfox

Why is it that "trickle charging" at 4.15V is fine, while doing so at 4.2V isn't? I dont think I completely understand your explanation. Additionally, why is 0.7C the optimal charging current? Would it be any better for the batteries if I charged them at less than 0.7C? Say, if I decided to charge the batteries at 0.1C all the way, would that in any way effect the cycle life?

Again, thanks for the help.

[SilverFox]

Hello Spectral532,

If you review the definition, you will remember that "trickle charging" occurs when the cell is at 4.2 volts and the current drops very low. Slow charging at 4.15 volts is simply slow charging and not "trickle charging."

I have not had the opportunity to explore, study, test, or experience plating first hand, so I am just providing information from others who have run into this. It seems that the conditions for plating are most favorable when the cell is at full charge, or is overcharged. This is one of the main reasons for recycling cells when they fall below 80% of their initial capacity.

The 0.7C charge rate comes from the battery manufacturers. They feel that this is the optimum rate to balance cell life and charge time.

Tom

[45/70]

My interpretation of "trickle charging" with regard to Li-Ion cells, is any additional charge beyond the predetermined current cutoff point (usually 0.03-0.1C).

If your charging your cells to 4.15 Volts, as SilverFox suggests (and I normally do, as that's about where my "el cheapo" hobby charger stops (actually, 4.16V) when set to "LiPo" anyway), that's OK, provided the cell doesn't continue to receive current past the predetermined cutoff charge current level, of the CV stage. If the cell does continue to charge past this point, then that constitutes a trickle charge.

According to multiple sources, "trickle charging" can happen at any point from 4.05 Volts, or above. Below this voltage, I guess it's just considerd a really slow charge.

Dave

[SilverFox]

Hello Dave,

The early Li-Ion cells, and many of the current Saft cells, require limiting the charge to 4.1 volts. With these cells, plating can begin at a lower voltage. I don't think the current mix of chemicals that allow for 4.2 volt charging plate out at that low a voltage. Unless the cell has deteriorated to below 80% of its initial capacity.

The point is that Li-Ion cells can not tolerate overcharging. When the cell reaches full charge, additional charging is harmful. If you are charging to 4.2 volts, and the cell settled down to 4.16 volts, you would be better off using a higher cut off current. On the other hand, if you have a cell that is designed to be charged to 4.2 volts, and you limit the charge to 4.15 volts, you never achieve full charge, and the risk of plating is greatly reduced.

Tom

[45/70]

The point is that Li-Ion cells can not tolerate overcharging. When the cell reaches full charge, additional charging is harmful. If you are charging to 4.2 volts, and the cell settled down to 4.16 volts, you would be better off using a higher cut off current. On the other hand, if you have a cell that is designed to be charged to 4.2 volts, and you limit the charge to 4.15 volts, you never achieve full charge, and the risk of plating is greatly reduced.

My hobby charger indicates it is charging at 4.20 Volts during the CV stage (set at "LiPo"). When "good" LiCo Li-Ion cells complete on this charger, the voltage reading is 4.16 Volts when checked with an accurate meter.

When these same cells are charged with a YOHO-122 (or with the Metex PS), they come off at 4.20 Volts, and after 24 hrs read 4.18-4.19 Volts, and stay there, about forever. I consider these cells to be "good", and not suffering from significant degradation. This also leads me to believe that my $35 (delivered) charger (Dynam Supermate DC6), #1 has an inaccurate voltage reading, and #2 only charges during the CV stage at 4.16 Volts.

It's also noteworthy, that when the charger is set to "LiIo", it indicates 4.10 Volts during the CV stage, but the cells come off at 4.06 Volts. There definitely seems to be a 0.04 Volt error in the charger, somewhere. I think that's just what you get with a $35 hobby charger. I have to say, after redoing the heat sinking of the power transistors, I'm quite pleased with it though.

As for the 4.05 Volt trickle charging, I'm still not convinced. I will have to look into this some more. I know I read about this in other places besides B.U., but you may be correct, that formulations have changed since this information was published, and the parameters may have changed.

EDIT to add that the Supermate does in fact have a 0.1C current cutoff termination, so that may very well be why it under charges the cells.

Dave

[Justin Case]

With this information in hand, you can now adjust the settings to avoid any problems with "trickle charging." You simply set the voltage to 4.15 or 4.18 volts. With cells in good condition I usually use 4.18 volts. As the cells age and develop some internal resistance, I drop that to 4.15 volts. Many people simply use 4.15 volts all the time.

With this method you still have to watch the charge process, but you don't have to be glued to it. Since our definition of "trickle charge" is very specific, at a lower voltage we are not "trickle charging." Charging using a power supply is safest using this method.

There is one other method that can be used. Normal charge rates are in the 0.5C - 1.0C range. An optimum rate is about 0.7C. The idea is to balance the CC phase and the CV phase of the charge. The faster you charge, the more time you spend in the CV phase, and the greater the possibility of "trickle charging." The ideal rate (from the cells perspective) is 0.7C, but everyone wants faster charging and I am not sure there is much of a difference in cell life between charging at 0.7C or 1.0C.

At any rate, if you charge at a low charge rate, it seems that you can eliminate the CV stage completely. I have not tried this, but have read that if your charge current is about 0.18C, or lower, you can simply terminate the charge when the voltage reaches 4.2 volts.

Once again you have to watch the end of the charge closely, but it is another option. The general rule that the termination current is usually about 10% of the initial charge current does not apply in this case, but you simply terminate upon the voltage reaching 4.2 volts.

One final caution... from experience.

If you have curious cats, watch them carefully while charging. I had a cat become interested in the changing displays and was pawing at the control nobs...

Tom

So with method #1 with my bench supply, I will be operating in constant current mode. If I let the Li-Co cell continue to charge up to say 4.15V and just leave it alone, what happens to the charge current? Let's say I start out at 0.5C, just to pick a charging level. Does it drop because the potential isn't high enough to push the current anymore, even though the power supply is in CC mode? Presumably, it is still wise to terminate the charge when voltage hits 4.15V rather than letting the power supply continue to feed the cell?

Method #2 seems good in that the cell spends less time at 4.20V, but again presumably you ought to terminate the charging when voltage hits 4.20V rather than letting the charging continue. I suppose that with the low charging rate, you do have some leeway and don't necessarily have to terminate the charging right on the dot.

Has anyone used a programmable bench supply as a charger?

[SilverFox]

Hello Justin Case,

Your power supply should follow the CC/CV profile. When you set your maximum voltage to 4.15 volts, the current will begin to taper off when the voltage reaches that maximum amount. With a maximum voltage set to less than 4.2 volts you can just let the current taper down to 0 mA.

Tom

[45/70]

So with method #1 with my bench supply, I will be operating in constant current mode. If I let the Li-Co cell continue to charge up to say 4.15V and just leave it alone, what happens to the charge current? Let's say I start out at 0.5C, just to pick a charging level. Does it drop because the potential isn't high enough to push the current anymore, even though the power supply is in CC mode? Presumably, it is still wise to terminate the charge when voltage hits 4.15V rather than letting the power supply continue to feed the cell?

Your power supply should follow the CC/CV profile. When you set your maximum voltage to 4.15 volts, the current will begin to taper off when the voltage reaches that maximum amount. With a maximum voltage set to less than 4.2 volts you can just let the current taper down to 0 mA.

I think what Tom is saying here, is that once the voltage reaches the maximum voltage you set the PS at, it is no longer in the CC stage, but in the CV stage.

What I still can't figure out, is why current levels below 0.03C (where "C"=the charging current), aren't considered a "trickle charge", regardless of the voltage. I said I would research this further, but haven't to any serious degree. I guess it could be, that with LiCo cells, there is a low enough voltage point, below the "magic" 4.20 Volts, where trickle charging just isn't trickle charging. I'm pretty sure, as I said before, that I read elsewhere, that trickle charging is not recommended above 4.05 Volts, when charging LiCo cells. Maybe this was in reference to the older 4.10 Volt LiCo's?

Dave

[hiuintahs]

Edit: I didn't realize this topic had been brought up until my thread was merged. I pretty much have all my answers after reading through this.

I wanted to ask you guys what you thought of charging a lithium ion with a power supply where you can set the voltage and current limit. I am just getting into rechargeable lithium-ions and have recently purchased two Redilast 3100mAh 18650's.

I have yet to buy a charger and if I do, it will be a hobby charger as I'm one of those guys that has to have more control over the parameters of charging a cell.

So for the fun of it, I set the voltage on my Agilent (HP) E3631 power supply to 4.15v and the current limit to 0.500 amps and connected my battery. The voltage at first was 3.62 volts (that's what it was when shipped to me). Then I watched the voltage climb while the amperage stayed at 0.500 amps (actually 0.499) until it got to 4.15 volts and then the current started to ramp down. I was going to shut it off when the current got to 0.05 amps but noticed that when it got to 0.048 amps that it suddenly went to 0 amps and held at 4.15 volts with no further current charging. I then shut it off.

Anyone want to chime in as to whether there are any pitfalls to doing it this way. The only problem I can think of is that I have to watch it but if I'm in my work room anyhow, I can just keep an eye on it.

Another question. What is the best voltage to charge these batteries to since I can program any value within 0.01 volts? I heard if you stay off of 4.20 volts that the battery might last longer. And what constant charging current? Also what current should the battery charging process be stopped at? I was thinking around 50 to 100 mA, but noticed it suddenly dropped to 0 mA right shortly after passing the 48 mA value.
Thanks.



Thread merged - Norm

[HKJ]

I wanted to ask you guys what you thought of charging a lithium ion with a power supply where you can set the voltage and current limit. I am just getting into rechargeable lithium-ions and have recently purchased two Redilast 3100mAh 18650's.

It works perfectly fine, but you must manually shut it off, when the charge current drops below some limit. I uses this method for my LiIon battery test.
With most batteries the best voltage is 4.2 volt.

[Mr Happy]

That's a nice power supply.

What you did should work fine. As HKJ said, 4.2 V is correct for most ordinary lithium ion cells. The current limit will typically be not more than 0.5C, so perhaps 1.5 A for a 3100 mAh cell. Using a lower current like 0.5 A or 1.0 A will do no harm, it will just take a little longer to complete the charge. I am not sure why the current appeared to go to zero after passing 48 mA. Could be a limitation of the power supply or maybe below the display resolution?

[hiuintahs]

Thanks for the comments.

Well, I found the battery today was only at 3.64volts. I'm not sure I measured it off of the charger yesterday as I was in a hurry and had to go. And I was wondering why it didn't take very long for it to charge up. Well if it had been charged up it would be sitting closer to the 4.15 volts but was sitting just a little higher than the 3.62v that it came as. I kind of suspected something wasn't right because it didn't take very long to charge and was acting kind of weird at some point. I think I had a poor connection resulting in high resistance between battery and PS.

So today I moved the battery over to the other channel which has 6 amp capability and set the power supply this time to 4.17 volts with a 0.8amp current limit. Now it looks just like textbook charging. And the time was a lot longer than yesterday. I terminated it at 50mA. Now the battery is sitting at 4.153v. The voltage at termination (50mA) at the power supply was 4.166v. Everything looks good now. I'm going to have to come up with a better contraption for connecting up the battery like some clamps with screws on them that I've seen on CPF.

[acolin]

Thanks for the comments.

Well, I found the battery today was only at 3.64volts. I'm not sure I measured it off of the charger yesterday as I was in a hurry and had to go. And I was wondering why it didn't take very long for it to charge up. Well if it had been charged up it would be sitting closer to the 4.15 volts but was sitting just a little higher than the 3.62v that it came as. I kind of suspected something wasn't right because it didn't take very long to charge and was acting kind of weird at some point. I think I had a poor connection resulting in high resistance between battery and PS.

I got nailed on exactly this, too. My thin wires resulted in a drop of ~0.6v between PS ground and Vbatt negative lead. Hence, when the PS built-in meter reported 4.2v, the battery leads were only at 3.6v. That was confusing, and is now resolved; but could someone please help diagnose this other issue:

The battery is a 4325mAh, 3.7v. With PS settings Ilimit=0.5C=2A and V=4.2v, the CC-stage seemed to work, Vbatt steadily rose to 4.0v. But then it stopped. I expected it to continue rising until 4.2v and the PS to switch into V-const.

I have the following weird observation: the voltage on the battery leads depends on the charging current. That is, if I set Ilimit=1A, the PS goes into const-I mode and I see Vbatt=3.0v and I=1A; but if I set Ilimit=1.5A, I see Vbatt=4.0v and I=1.5A. With PS off, Vbatt is ~3.8v. I expect voltage on battery leads to be independent of charge current.

(1) Is the above normal?
(2) If Vbatt no longer increases, but I-const keeps flowing, am I overcharging the battery?

Hypothesis: I thought, maybe my battery is not a 4.1v, nor a 4.2v, threshold kind, but lowering the V just makes Vbatt stagnate at a lower value as described above, so lowering V down to the hypothesized threshold Vt < 4.1v doesn't help me reach the const-voltage stage.

Thank you in advance!!