Should my AA Eneloop XX batteries get this warm and take this long to charge?

I use my Nitecore i4 charger literally every day to charge Li Ion batteries for my electronic cigarette. I also use it for charging AA and other batteries. Today I took out my Fenix TK 41 for the first time in about a year, and it's been barely used at all in the first place. It sits in its box, with the 8 Eneloop XX batteries I bought for it in summer 2012. These batteries have NEVER been completely discharged, hardly used, and only topped off maybe 2-3 times total.

So it's been at least a year since they have been topped off, maybe more. I measured their voltage at 1.25v today, so decided to top them off 4 at a time. The first 4 took several hours to finish, and the next 4 took several hours, but still weren't quite finished when I pulled them off the charger just now. But they tested to 1.46v and we're very warm. If they were any warmer I'd call it hot.

Is it normal to take Eneloop xx batteries 5+ hours for 4 to charge on the i4 from 1.25v and end around 1.46v very warm?

I use the same batteries in my Fenix TK45. I use two Powerex 4-cell chargers. It takes about 2-3 hrs to fully charge them. They only get warm; never hot. I previously bought a Nitecore i4 Charger but I returned it when I saw how hot the batteries got.

Hi.

It looks like the charger has missed the end of charge voltage drop, on new or battery`s that have not been use for a while this can happen, if you think this might have happened, I just check the temperature with my finger, if it`s hot it`s done, I temperature when charging an nimh should not really go above 45c and higher and damage will occur.

Some really smart charger will detect and old or unused battery and charge it more slowly, I just do the finger temp test, if it`s hot it`s done.

The same with new or unused battery can signal full charge when they have only been charging a few minutes, I normally watch the battery charge for the first 15minutes and if they show as charged i re-install the battery and let it charge again, you can feel the temperature while doing this.

That is why i use my Accumanger 10 and 20 chargers, they put a lot of charge in the battery but they only get warm never hot.

John.

I think several factors come in here. One is that the batteries haven't been charged for a long time. While they keep most of their charge, their internal resistance still gradually increases, which makes them get more warm when recharging.

Another factor is that they only ever have been topped off and never were completely discharged. In my experience, over time this also leads to elevated internal resistance with the effects above. The same thing happened in my cordless phone when I still used it. Discharging the batteries completely and recharging them might bring their internal resistance back to a normal level, if it doesn't, they've sustained permanent damage. But at only 2-3 "toppings" I can't imagine that permanent damage to have taken place that early, so you should be able to bring them back to a reasonable level.

I also think your charger has missed termination in this case and that's why they've become so hot.

If you turn on the light on full power, does it bring its full light level or does it seem somehow weak?

TinderBox (UK) said:

That is why i use my Accumanger 10 and 20 chargers, they put a lot of charge in the battery but they only get warm never hot.

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Yes, those are good chargers. Not only do they do a good job charging (and detecting end-of-charge), but they will automatically charge old batteries more slowly, when their IR is too high to do a quick charge.

Kurt, apropos of what you said, for a given internal resistance, is the dV/dt termination easier to detect with greater or lesser charging currents? Or does it matter?

magellan said:

Kurt, apropos of what you said, for a given internal resistance, is the dV/dt termination easier to detect with greater or lesser charging currents? Or does it matter?

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-dv/dt requires some minimum current to work, when I get some time I will publish something about it with actual tests and not only hearsay.

Had this happen recently on a couple of old Sanyo white and orange 2500 mAh cells. They're probably five years old. They don't hold a charge very well, and someone explained to me that higher self discharge goes with higher internal impedance. This affects the charger too and could be why the charger missed termination as the batteries got pretty hot and the capacities showed in excess of 3100 mAh.

HKJ said:

-dv/dt requires some minimum current to work, when I get some time I will publish something about it with actual tests and not only hearsay.

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Thanks HKJ. I know there are things I don't understand about that and anything to explicate it in more detail would help.

For one thing there's the problem of taking the derivative based on a series of discrete readings. I suppose the line is fitted using the least squares method but I really don't know.

magellan said:

For one thing there's the problem of taking the derivative based on a series of discrete readings. I suppose the line is fitted using the least squares method but I really don't know.

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A charger usual do it a much simpler way, it simply records the maximum voltage and when the actual voltage drops a few millivolts below that it stops.
The trick is to do some averaging and have a initial lockout or the charger might terminate premature.

And setting an end voltage of say 1.55 volts, some old/faulty battery would never get there if you left them forever, all chargers have some sort of safety timer cutout at 15-18hrs , this limits the maximum capacity battery you can charge, unless you want to remove the battery and but it though a second time.

John.

TinderBox (UK) said:

And setting an end voltage of say 1.55 volts, some old/faulty battery would never get there if you left them forever, all chargers have some sort of safety timer cutout at 15-18hrs , this limits the maximum capacity battery you ca n charge, unless you want to remove the battery and but it though a second time.

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All chargers do not have a backup, but a good charger will have:
-dv/dt
Maximum voltage
Temperature raise speed, i.e. dT/dt
Maximum temperature
Time/capacity.
Some chargers is smarter than the above in their termination.

The maximum voltage is often low enough to override the -dv/dt as you can see in some of my review.

magellan said:

Kurt, apropos of what you said, for a given internal resistance, is the dV/dt termination easier to detect with greater or lesser charging currents? Or does it matter?

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Well, since you ask me specifically... ;-)

generally, the dV/dt termination is easier to detect with greater charging currents. Here's why...

At the end of the charging, when the battery is full, the excess energy starts to get converted to heat, so the battery heats up. First the voltages rises relatively sharply because not being able to take suplus charge results in a higher resistance to charging. However, because the battery heats up, the internal resistance decreases as a function of temperature (as it always does, not only while charging and no matter if the heat is generated by the battery itself or not). Since the internal resistance determines how much the voltage raises on top of the no-load voltage, that voltage raise and thus the overall voltage decreases. But since the cell takes some time to heat up, the voltage reaches a peak when the cell isn't at full temperature yet and then drops. Eventually, if charging isn't stopped, the cell will have reached full temperature, and the voltage will stay at a certain level.

Now if you apply a very low charging current, the cell won't heat up that much, thus the decrease of its internal resistance by heating up and with it the voltage drop from the peak won't be as pronounced, to the point where it's barely noticeable.

Some examples:
I've charged some LSD D cells with 8 Ah at a charging rate of 500 mA (which is the one I always use for those cells)... in this case the voltage peaks at 1.45 and then returns to 1.443 in the course of about an hour, and won't drop much more than that. (I only use a dumb charger for these cells... it's so dumb it doesn't know it's actually being used as a charger ;-)

Then I charged some older Uniross cells with 1500 mAh at a charging rate of 110 mA (which one of my dumb chargers applies). One cell peaked at 1.449 V and never got down from there. This happened again with two of those cells charging at a much lower charging rate. This is actually the trickle charge applied by my (fairly) smart charger after it calls the cells full. I left them in the charger because I suspected it ended the charge before the cells were actually full. One indication for this was that the cells after being shut down dropped to about 1.4 volts very quickly while still being trickle-charged (the actual end voltage was about 1.5 V). Well, I left them in the charger for several hours and tracked the voltage by hand. The voltage of both cells started out at about 1.38 V some minutes after the trickle charge began and then showed an actually fairly normal charging curve including an accelerated voltage rise near the end (about 10-12 hours later). However, eventually (at 1.45-1.46 volts) the voltage rise started to slow down again, and the voltage went towards a plateau. I actually left the cells in for a full day before taking them out, at which point they had reached 1.516 and 1.485 Volts, but in that curve, the voltage never dropped, and the cells were only a bit warm from the trickle charge. With that low charging current, of course it wouldn't have been possible to detect a dropping of the voltage.

I've also noticed that cells with a very high internal resistance (I have some with 1-2 Ohms each, though I have trouble finding an application they're still useable for) already heat up from the normal charging current before they're full... at least if you try to blast 1 amp into them. In this case, the voltage already drops at the start of the charge from the cells heating up, which might confuse the charger. Another thing that might happen is that the internal resistance is so high that the charging voltage exceeds the range of the charger's built-in meter or software and thus it's unable to detect if a voltage drop occurred (which is probably why the Maha charger displays "HI" beyond a certain voltage point). What adds to this is that some chargers will apply a lower charging current if the voltage is high, which will annihilate some part of the voltage curve as well.

So this is why it's harder to detect a voltage drop at a lower charging current. On the other hand it's probably hurting the cells more if they get charged, and especially overcharged, with a higher current. So maybe it would be better to use a different method of detecting when the cells are full which isn't dependent on a voltage drop, such as inflection, so that a lower charging current can safely be used while still correctly terminating the charge.

15-20 years ago was it nicd or nimh, timed charger, no smart -vD or anything, you charge battery`s that are virtually full already, the plastic cover starts to shrink and melt, and this was an eight hour charger, not the fancy 15min of today, the battery`s were so hot, they virtually glowed in the dark, I think it was an white uniross charger and battery`s it took 4xAA a single red led and a little white start button.

The charger never failed, and i most likely have one or two battery`s knocking around, I just learned to make sure the battery`s were empty before i charged them.

I still keep a couple of totally manual chargers for battery`s that are zero volts or slow low that smart chargers will not charge them, it does not even have an safety timer, but it`s only 150ma to 300ma charging rate so what could go wrong.

John.

Kurt, thanks for the long and detailed post and info. I've copied that into my permanent battery notes file.

Intuition told me that a higher charging current would make dV/dt easier, since a really low current would just run up against the average noise level, but ya never know, sometimes intuition is wrong.

Just a note here... my write-up above was according to a concept I figured based on various facts, not a scientific examination of how things work, so it might be wrong... but I'm pretty confident things work the way I described, and the examples I gave are from my own batteries, so at least those are for real, even if they are only anecdotal evidence.

magellan said:

Kurt, thanks for the long and detailed post and info. I've copied that into my permanent battery notes file.

Intuition told me that a higher charging current would make dV/dt easier, since a really low current would just run up against the average noise level, but ya never know, sometimes intuition is wrong.

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I only need a slight charge, and then pop them in a smart charger to finish the rest of the charge.

John.

magellan said:

<<for battery`s that are zero volts or slow low that smart chargers will not charge them, it does not even have an safety timer, but it`s only 150ma to 300ma charging rate so what could go wrong.>>

LOL. I like the way you think!

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Kurt_Woloch said:

At the end of the charging, when the battery is full, the excess energy starts to get converted to heat, so the battery heats up. First the voltages rises relatively sharply because not being able to take suplus charge results in a higher resistance to charging. However, because the battery heats up, the internal resistance decreases as a function of temperature (as it always does, not only while charging and no matter if the heat is generated by the battery itself or not). Since the internal resistance determines how much the voltage raises on top of the no-load voltage, that voltage raise and thus the overall voltage decreases. But since the cell takes some time to heat up, the voltage reaches a peak when the cell isn't at full temperature yet and then drops. Eventually, if charging isn't stopped, the cell will have reached full temperature, and the voltage will stay at a certain level.

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I agree with most of what you're saying, however I don't agree that the voltage drop is due to a reduction in the internal resistance of the cells. If the voltage drop was entirely due to a reduction in internal resistance, the voltage drop would only be visible whilst applying current. In my own testing, measuring the cell voltage during short rests between charging also shows a drop in voltage.

I agree that the voltage drop is due to the increase in cell temperature (and that's why you don't get a voltage drop when charging at low rates that don't give much of a temperature rise when the cell is full) but the actual mechanism (IMHO) is most likely due to changes in the electrochemical reactions at increased temperatures.
http://chemwiki.ucdavis.edu/Physica...ect_Of_Temperature_On_Equilibrium_Composition

This fits in with charging NiMH which is an exothermic reaction, but doesn't fit with charging NiCd which is endothermic - it could be a secondary (exothermic) reaction in NiCd which causes the voltage drop however...

Edit: I agree that Inflection termination is better because it works reliably at all charging rates (I've tested it successfully with charging an AA Eneloop at only 50mA) It's a pity that it isn't used in more chargers!

Power Me Up said:

I agree with most of what you're saying, however I don't agree that the voltage drop is due to a reduction in the internal resistance of the cells. If the voltage drop was entirely due to a reduction in internal resistance, the voltage drop would only be visible whilst applying current. In my own testing, measuring the cell voltage during short rests between charging also shows a drop in voltage.

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Well, that could also be because of the "surface charge" phenomenon as I call it, which causes some of the voltage dropping and rising action to be delayed because the chemicals continue to distribute evenly (or the charge does, whatever). An example here from my last measurings:

I recently tested how a set of AA NiMh batteries behave in my portable keyboard which takes a set of 6 AA's... in terms of how much runtime I get out of them and when the battery display drops the first bar. As part of that, I measured the voltage at various times...

After the first rehearsals, the working voltage was about 1.18 V per cell, which immediately went up to 1.23 V when I switched the keyboard off (it draws about 160 mA in this situation, and about 200 mA in average while playing it). Before the rehearsal on the next day, I measured again and the open circuit voltage had gone up to 1.255 V per cell. The end of that rehearsal saw a voltage of 1.143 V under load, 1.182 V immediately after turning the keyboard off, and 1.235 six days later before I started the next rehearsal (by then the batteries were nearly discharged and the keyboard dropped the first bar after about 15 minutes, so I didn't finish the rehearsal on battery... I suppose the keyboard drops the first bar at about 7-8% remaining charge, and this figure might even be lower with stronger batteries such as Eneloops).

I suppose that at a lighter load, the voltage immediately after switching the keyboard off would have been higher as well, at the same state of charge. I figure that the same thing applies to charging... some of the voltage change is immediate, but some of it is delayed, thus you still see a a voltage drop in the short rests between charging. But of course it may be that other factors come into play here as well.

Side note: These batteries are over three years old and already somewhat degraded, but still usable.