18650 hot while charging in XTAR VP1

I finally got around to charging a Sanyo 2600 mAh cell I got from a laptop battery pack a while ago ( http://www.candlepowerforums.com/vb/showthread.php?344741-Test-Review-of-Sanyo-18650-2600mAh-(Red) ). It was in the left slot of my XTAR VP1 charger (the other slot was empty) and it was charging at 1A. Everything was going fine, till I noticed that the cell was getting warm after it had been at 4.20V for about 80 minutes. About 25 minutes later, it was even hotter, and while touching it, the voltage dropped to 4.17V. At that point I removed the cell from the charger, and it was too hot to really hold tightly (touching wasn't too bad). About 5 minutes later, I measured a temperature of 52 degrees Celsius, so I'm guessing it has been around 60 degrees or maybe even higher.

When I measured the voltage with a multimeter, it was showing 4.04V, and stabilized at 3.98V about an hour later. So I'm guessing this cell is no good anymore, and it probably has high internal resistance, as mentioned in http://www.candlepowerforums.com/vb/showthread.php?259404-18650s-hot-while-charging . I don't understand why it gets hot though, or why the voltage dropped. I know NiMH cells drop voltage when they are full and start heating up, but I've never heard that for Li-ion cells. Does the cell voltage drop while the charger is supplying a constant voltage, which results in higher charging current being converted into heat because the battery is already full? Would it be safe to just remove the cell as soon as soon as it hits 4.20V (which would mean it wouldn't be fully charged of course). Would there be any risk in using the charged cell? Could there be a problem with the charger?

I'm just asking because I want to understand how this stuff works. I'll probably just dispose of the cell properly and try the other ones from the pack.

Shouldnt have been in the charger 80 minutes after it hit 4.2v. It should have beem OUT of the charger as soon as it hit 4.2v or your charger should have terminated the charge cycle.

You're asking aboit a recycled laptop battery salvaged from a pack.

I wouldnt touch them with a ten foot pole. Putting a 1amp charge rate on a cell like that of questionable stability/origin?

Sounds like a in the making to me.

Sent from my SCH-S720C using Tapatalk 2

lightcycle1 said:

Shouldnt have been in the charger 80 minutes after it hit 4.2v. It should have beem OUT of the charger as soon as it hit 4.2v or your charger should have terminated the charge cycle.

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HKJs review of the XTAR VP1 shows that the constant voltage phase takes about 70 minutes at the 1A setting: http://www.candlepowerforums.com/vb/showthread.php?355379-Review-of-Measurement-on-Charger-Xtar-VP1
Why do you think they should be removed as soon as the charger shows 4.2v?

lightcycle1 said:

Putting a 1amp charge rate on a cell like that of questionable stability/origin?

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The Sanyo cells are some of the best available, to quote HKJ's from his review: "it is a very good cell". The origin of the pack is known to me, it was in good working condition, and only being recycled because the laptop it belonged to was retired. I removed the cells from the pack myself, and they were over 4 volt when I got them out. So I don't see why I should question the state or origin of the cell.

FWIW: I had a VP1 failure which just happened to be in the left channel. The right channel continued to operate correctly. The failure initally manifested itself by charging erratically in the left channel and then stopping the charge for a longer and longer time, finally not charging at all. The voltage was still being measured. I returned that VP1 for an exchange. The replacement VP1 operates satisfactorily on the 0.5A charge rate for dozens of 18650 cells, mostly AW 2900 and Eagletac 3400 cells. I have had the replacement VP1 for about a month having charged a couple dozen cells with it from different charge states. I have never had an 18650 cell become too 'warm', much less 'hot' while under charge.

Which is a seque to comment that I generally charge 18650 cells at the 0.5A rate, a practice I have followed for at least a couple years with several different chargers. I understand that the prevailing practice seems to be to charge 18650 cells at a 1.0A or even higher rate. I believe that the lower charge rate is easier on the cells. "Easier" meaning that the reconversion of chemicals is more even thruout the cell with fewer hotspots of high current density.

For example, I still have my original AW cells with no obvious deterioration in capacity compared with new AW cells. Acknowledged that AW cells are very good. With a proper CC/CV charging algorithm, the only real advantage to a charge rate higher than 0.5A is the shorter charge time. I have had to recycle a couple dozen 14500 and 10440 cells as being worn out.

moldyoldy said:

FWIW: I had a VP1 failure which just happened to be in the left channel. The right channel continued to operate correctly. The failure initally manifested itself by charging erratically in the left channel and then stopping the charge for a longer and longer time, finally not charging at all. The voltage was still being measured. I returned that VP1 for an exchange.

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Thanks for the heads up, I'll keep an even closer eye on it. I haven't charged a lot of cells with it yet.

moldyoldy said:

Which is a seque to comment that I generally charge 18650 cells at the 0.5A rate, a practice I have followed for at least a couple years with several different chargers. I understand that the prevailing practice seems to be to charge 18650 cells at a 1.0A or even higher rate. I believe that the lower charge rate is easier on the cells. "Easier" meaning that the reconversion of chemicals is more even thruout the cell with fewer hotspots of high current density.

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From what I know, charging Li-ion cells with a lower current shouldn't have any disadvantages for the cells or charger. NiMH's have the problem that delta V termination detection gets worse with lower currents. From what I read, 0.5C to 1C charging current should be fine for Li-ions, and 1.0A is even lower than that for these 18650's with 2600 mAh. It's mainly a question of time, coupled with the fact that I want to monitor the charging closely, and I do that in a location where an explosion or something wouldn't do a lot of damage. That does mean having to walk over to check the charger and cell temperatures though (till maybe I get a thermometer and webcam so it can be done remotely).

moldyoldy said:

For example, I still have my original AW cells with no obvious deterioration in capacity compared with new AW cells. Acknowledged that AW cells are very good. With a proper CC/CV charging algorithm, the only real advantage to a charge rate higher than 0.5A is the shorter charge time. I have had to recycle a couple dozen 14500 and 10440 cells as being worn out.

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14500's and 10440 cells should be charged with a lower current of course. I charged my 14500 (salvaged from a pack with probably 600 mAh capacity per cell when new) with 250 mA, which is a bit under 0.5C. That low current was one of the things I liked about the VP1. It would be interesting to see any results for differences in Li-ion cell life time for different charging currents.

Hello Serenity,

With a resting voltage below 4.0 volts after charging and the fact that it got hot, you should recycle that cell and not use it.

Keep an eye on how the charger behaves with other cells and if this pattern repeats you should recycle the charger.

On the positive side of things it is wonderful that you had the awareness to closely monitor the charge process and catch this issue before things got out of hand.

Tom

SilverFox said:

With a resting voltage below 4.0 volts after charging and the fact that it got hot, you should recycle that cell and not use it.

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Well, I've been using it for a bit in a flashlight, but checking the voltage and temperature often (and removing it when not in use). It does work fine, no idea about the capacity though. But I probably won't charge it again after it's empty. I might try to calculate the internal resistance (from voltage drop under a known load). But the cell is probably just worn out.

SilverFox said:

Keep an eye on how the charger behaves with other cells and if this pattern repeats you should recycle the charger.

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The charger is new and still under warranty, so I would be returning it to the vendor if it had a problem. I've charged another cell from the same battery pack, and that didn't get warm at all. The voltage drop at the end is probably just normal (for old cells), because I didn't notice that the charging was finished (HKJ did put in his review that it wasn't very clearly indicated). So the charger is probably fine. This cell is 4.12V now, about 24 hours after charging, so that doesn't seem too bad.

Thanks for your reply.