AA fast chargers - Delta V?

[MrAl]

Hi there,


A few quick points:

First, that article Tom linked is very interesting and informative. I did find
that their terminology is a little strange though when it came to what they
were calling "Inflexion Point", which is really the calculus "Inflection Point".
If you ignore the rather strange spelling, they are both the same.
And just to clear things up a bit, the inflection point detection when it
comes to charging cells like NiMH is very much the same as zero voltage
change (or zero slope) detection. It's the point where the slope either
changes sign or is just about to change sign, or viewed another way,
it's the point where the voltage has been rising and is now about to start
decreasing. The difference between true zero voltage and inflection point
detection methods is that the true zero voltage detection does not require
the voltage to actually start to fall, whereas the inflection point might, or
might pick up the voltage starting to rise again, which could also trigger
an inflection point detection. It largely depends on how the manufacture
pre-filters the signal first though. There are many ways to do this and this
changes the overall response to some degree too. It also depends greatly
on the manufacturers algorithm, which can be better or worse.

Second, the difference in pre-filtering also affects the minus delta
detection too, and doing this wrong could lead to overcharging also,
which is then blamed on the minus delta technique itself. The minus
delta technique isnt as bad as it's made out to be on some web sites,
and the proof is in the monitoring of the voltage *and* temperature
of several cells of different manufacturers and of different cycle ages.
What happens is that as the voltage rises, the temperature does too,
and since it's the temperature (times the time) that does the most damage
the shorter the time the cell is subjected to a higher than normal
temperature the longer the cell will last, but for short temperature
vs time increases not that much extra damage will be done to the cell.
This means that a small time addition where the temperature is higher,
although we want to avoid this, wont do that much extra damage to a cell.
I does matter however how the relatively noisy signal is filtered, and
that makes a big difference, and every voltage measurement has to be
pre-filtered even if that filtering is indirect (part of the algorithm).

Another thing (#3) to think about is that when comparing zero voltage or
inflection point detection to minus delta V detection one has to also take
into consideration the effect on cell capacity after a typical charge cycle.
Many cells capacities are defined by using the minus delta V technique,
so avoiding that method voids the cell capacity rating. Of course it's up
to the individual here to make the choice between highest cell capacity
and shorter charge time. For example, with my Li-ion cells i almost
always undercharge them just a little (10 to 20 percent) so i get longer
cell life.

[Mr Happy]

Hi MrAl,

Thanks for your comments.

Actually "inflexion" is an alternative spelling of "inflection" and both are the same word. In the past "inflexion" was the preferred spelling (and language purists would argue it still is), however in modern times "inflection" has become the norm.

If you examine my chart titled 'End of Charge Detail', you can see the inflection point where the purple arrow is, and the zero voltage slope where the red arrow is. The inflection point comes before the zero slope, and also before the temperature has started on its rapid rise.

An interesting point for me is that the particular cell I tested does not seem able to hold a charge any greater than 1830 mAh, so the inflection point essentially marks the point of 100% full charge. Continuing to the zero slope point does not add any more charge to the cell, and in fact is just overcharging it while heating it up. One might as well stop at the inflection point and consider the charge complete.

[wptski]

If the inflexion/inflection point is a 100% charge, it contradicts everything else that many of us ever read. I agree that this point occurs before the zero voltage and even before dT/dt.

This document was dated in 1994, so that means that they are refering to early Ni-MH cells. There were a few chargers that used dT/dt termination but they never worked well, so it would seem like using temperature input would be tricky.

[Mr Happy]

If the inflexion/inflection point is a 100% charge, it contradicts everything else that many of us ever read.

I'm only saying that it appears to be so for the Eneloop cell on test for this experiment. Eneloops are a new technology and they seem to perform better than previous generations of NiMH cell.

Here is the evidence for my statement:

When the C9000 terminated automatically at 1.47 V the charge input to the cell was 1754 mAh and a discharge test at 500 mA gave 1763 mAh, so the charge acceptance up to this point was 100%. In the second test when I terminated charging manually, the charge input to the cell at the −ΔV signal was about 1970 mAh and a discharge test at 500 mA gave 1834 mAh. This is an average charge acceptance over the whole period of 93%. If we assume the charge acceptance had again been 100% up to the 1700 mAh point, then the charge acceptance over the last part from 1700 mAh to 1970 mAh was 50% at best.

My observation is that the 100% cell capacity is about 1830 mAh, and the inflection point occurred at 1810 mAh. Making the presumption that the charge acceptance remained high up to this point, the cell was essentially fully charged at that time and further high rate charging was of little value.

I believe that if you want your cells to be fully 100% charged to the max, then the best approach is to stop the high rate charging before the rapid temperature rise begins and move to a lower top-off rate to complete the charge. The inflection point does look like a good predictor of the rapid temperature rise since it captured this point for a cheap Chinese cell of unknown manufacture and also for a modern high performing Eneloop.

[eluminator]

I guess the C. Crane NiMH and NiCad charger uses "inflection". The Saitek manual says it uses Negative Delta V and Delta V Square over Delta t Square. Whatever it uses, it works. You can charge anything from one AAA cell to 4 D cells and it always gets it right. You can charge NiMH or NiCad. The only restriction is that all cells charged together should be the same. I think the negative delta V is only used to switch from top-off to trickle.

I've had a couple of these chargers for 5 or 6 years and it's the only charger I use. I like the fact that it analyzes the cells, and I always do that before I charge them. The LCD display shows what's going on when charging or discharging on analyzing. Actually it's a good idea to test the cells before charging in case the cells have different voltages. Because it charges cells in parallel, the voltage of each cell should be similar.

This charger has "soft start" so it will charge cells no matter how much they are discharged. They claim it also prevents the generation of damaging heat. The charger also has "negative pulse" which they claim prevents crystallisation and reduces gas bubbles which increases charging efficienty.

The charger has four phases. Soft start, fast charge, top off, and trickle. None of these phases are timed and I don't think they depend on absolute voltage either. They are all based on feedback from the battery. When I charge different size cells or cells with a different initial charge, the duration of each phase (except trickle) is different and the amount of charging current during the various phases is different also. You can't fool this charger. No matter what I put in it, it handles the situation. Before it starts charging, it analyzes the cells for ten seconds and then starts the charge.

Here's the manual for the Saitek. It's no longer available as far as I can tell but the C. Crane is functionally equivalent except it doesn't charge 9 volt batteries.
http://www.hypercon.net/~blisscomm/P...artCharger.pdf

[NiOOH]

Hello NiOOH,

I remember you bringing this up, and I think I gave you a bad time about it. It looks like your observations were correct.

I have seen termination voltages above and below 1.47 volts, but there does seem to be a lot of charges that end right at 1.47 volts.

Tom

Hello Tom.
Yes, I remember it also. In fact, it is possible to observe cut-off voltages belov 1.47 V. I have a set of cheap, low capacity NiMH cells that came with a multiband receiver. The celles are labeled at 1300 mAh. When charged at 1000 mA the voltage peaks at 1.45-1.46 V. In this case the -dV kicks off and the charger terminates at voltages lower than 1.47 V. On all other cases the termination occurs shortly after the display shows 1.47 V. During the top-off, the voltage could rise above 1.47, but I haven't seen terminating the fast charge above 1.47.

[Mr Happy]

Yes, I remember it also. In fact, it is possible to observe cut-off voltages belov 1.47 V. I have a set of cheap, low capacity NiMH cells that came with a multiband receiver. The celles are labeled at 1300 mAh. When charged at 1000 mA the voltage peaks at 1.45-1.46 V. In this case the -dV kicks off and the charger terminates at voltages lower than 1.47 V. On all other cases the termination occurs shortly after the display shows 1.47 V. During the top-off, the voltage could rise above 1.47, but I haven't seen terminating the fast charge above 1.47.

This is exactly the same as I observe. The cheap Chinese cells in my test above terminated on the -dV signal below 1.47 volts. However if you watch the charger with cells like the Eneloops, Done appears in the display as soon as the voltage rolls over 1.47. Following this the voltage may creep up to 1.48 or 1.49 during top off, but this is always after Done has appeared and never before.

I have version number 0G0D01. It is possible that the first firmware version of the C9000 is different in this regard to the later one.

[bob_ninja]

Wow, very nice work Mr. Al
Thank you very much

I remember the termination problems in early C9K versions and debates about difficulties in identifying a reliable end of charge signal, especially for older cells. I am only speculating here: Could this max voltage 1.47V end of charge trigger be the latest firmware update resulting from early problems? Perhaps they added this max voltage end of charge signal as a safer alternative to other methods to stop those runaway charges that kept pumping many Ah into cells. Just wondering.

The interesting outcome of stopping at 1.47V is that the old problem of cooking cells if now gone and C9K is actually cooler than BC900. Funny reversal.

After the inflection point temperature starts to rise much faster, so it is probably the best point to stop. However, there is only about 100 mAh between it and the 1.47V "signal", or 5.5% difference. The important point is that the remaining 200 mAh delivered by top off charge is done at a much lower rate (100 mA instead of 1600 mA in your example). Thus it still squeezes those last few mAh without heat buildup. I'd say that is a damn well way to ensure virtually 100% full charge without much complexity.

Also, I am not sure what is the point of trying to squeeze in every last mAh anyway as SD is highest at 100% SOC, even for LSDs. The car batteries are charged only up to 80% SOC for longevity. In this instance, the original 1.47V charge termination resulted in 94% SOC which is pretty close.

Once again I am impressed by this charger. Great stuff.

[Mr Happy]

Wow, very nice work Mr. Al

But...but...it was me that did the testing...

Thank you very much

You are welcome. It was fascinating to look at the detail of exactly what happens with this charger.

[wptski]

Somewhere in the early C9000 threads is a post of mine with a temperature graph of two cells. One is charging inserted in the unit and the other is connected outside the unit. If I remember correctly, the cell outside the unit runs >20F cooler, so the built in PS is generating much of the cell's heat. So how to you enter that into the equation? Some slots run warmer than others too.

[Mr Happy]

I did wonder about heat from the C9000 itself. I tried to minimize that by testing one cell in isolation.

During the charging process the C9000 did not feel very warm to the touch, but I know that with four cells at once at high charge rates it does tend to get a bit warm. In the test with the Chicago Electric cell that got hot, the heat was definitely generated inside the cell.

[wptski]

So this method is for charging a single cell at a time?

[Mr Happy]

As with other methods like -dV it applies to a single charging channel of a possibly multi-channel charger. I wouldn't know how effective it might be when charging two or more cells in series such as in a battery pack.

What I was saying above was that to minimize the heating effect of the charger and adjacent cells on the temperature measurements, I just tested one cell by itself.

[NiOOH]

I remember the termination problems in early C9K versions and debates about difficulties in identifying a reliable end of charge signal, especially for older cells. I am only speculating here: Could this max voltage 1.47V end of charge trigger be the latest firmware update resulting from early problems? Perhaps they added this max voltage end of charge signal as a safer alternative to other methods to stop those runaway charges that kept pumping many Ah into cells. Just wondering.

The interesting outcome of stopping at 1.47V is that the old problem of cooking cells if now gone and C9K is actually cooler than BC900. Funny reversal.

After the inflection point temperature starts to rise much faster, so it is probably the best point to stop. However, there is only about 100 mAh between it and the 1.47V "signal", or 5.5% difference. The important point is that the remaining 200 mAh delivered by top off charge is done at a much lower rate (100 mA instead of 1600 mA in your example). Thus it still squeezes those last few mAh without heat buildup. I'd say that is a damn well way to ensure virtually 100% full charge without much complexity.

Also, I am not sure what is the point of trying to squeeze in every last mAh anyway as SD is highest at 100% SOC, even for LSDs. The car batteries are charged only up to 80% SOC for longevity. In this instance, the original 1.47V charge termination resulted in 94% SOC which is pretty close.

Once again I am impressed by this charger. Great stuff.

This is exactly what I think too. IMO, MAHA engineers pressed with time, did this quick and safe fix.. Unfortunately, my BC900 broke so I cannot directly compare charge completeness between it and c9000. For someone with these two chargers, can you make the experiment lkike charging the same cell at the same rate on both chargers and discharge on the MAHA to get the available capacity

[bob_ninja]

But...but...it was me that did the testing...


You are welcome. It was fascinating to look at the detail of exactly what happens with this charger.

Ohh, shoot I got mixed up. Sorry ment thank you Happy Face

[bob_ninja]

This is exactly what I think too. IMO, MAHA engineers pressed with time, did this quick and safe fix.. Unfortunately, my BC900 broke so I cannot directly compare charge completeness between it and c9000. For someone with these two chargers, can you make the experiment lkike charging the same cell at the same rate on both chargers and discharge on the MAHA to get the available capacity

Do you think that there would be much of a difference? Do you think it is worth an experiment?

I thought BC900 also charged bit below 100%

I have both so could try it I suppose. Using Eneloops? Using like 0.5C, so 1000 mA charge and 500 mA discharge? That sort of thing?

I don't like using more than 1A charge rate on BC900, so ....

[wptski]

Discounting topoff or trickle charge. If one thinks a charger terminates at peak or zero Delta, it would charge to a less degree of fullness then -DeltaV. This inflexion method would be even less.

[NiOOH]

Do you think that there would be much of a difference? Do you think it is worth an experiment?

I thought BC900 also charged bit below 100%

I have both so could try it I suppose. Using Eneloops? Using like 0.5C, so 1000 mA charge and 500 mA discharge? That sort of thing?

I don't like using more than 1A charge rate on BC900, so ....

I think that would be great thanks in advance. Just make sure that you always discharge on the same charger, I'd guess it ought to be the C9000. It would be interesting to see the results Maha C9000 shortly after done vs BC900 vs Maha C9000+ 2 hours top off.

[bob_ninja]

C9000 0G0E01, BC-900, Eneloop HR-3UT6

Charge 1A, discharge 0.5A, min 2 hour rest between each cycle
Used C9000 for discharge for both tests

C9000

Charge:
57 min 868 mAh 1.4V
88 min 1335 mAh 1.41V
113 min 1712 mAh 1.45V
C1 116 min 1763 mAh 1.46V
C2 115 min 1748 mAh 1.46V
left C1 for 2 more hours, removed C2 when done without delay

Discharge:
C1 1812 mAh
C2 1732 mAh

Both cells were very close (such as same voltage) so same number for both.
I tried using a thermometer for my kid, so it would display error for any temperatures that are too far from normal for humans. It displayed errors, so temperature was below 34C, fairly cool.

BC900

Charge:
93 min 1540 mAh 1.42V 34.3C
102 min 1700 mAh 1.45V/.44V 35C/34.4C
109 min 1810 mAh 1.48V/.46V 35.3C/34.5C
120 min 2 Ah 1.52V HI/38.4C

C1 122 min 2.03Ah 1.53V
C2 125 min 2.08Ah 1.53V

left C2 for 2 hour trickle charge at 59mA, removed C1 without when done
(got mixed up so removed the wrong cell, still they are close)

Discharge (on C9000):
C1 1811 mAh
C2 1868 mAh (trickle charge)

This time cells did register human-like temps. Just before the end C1 registered a HIGH temp error, so likely around 40C. Overall temperatures were higher but still fine.

BC900 shows trickle charge rate. Interesting that its "trickle charge" rate is 59mA, close to C9000's "topoff charge" rate of 100 mA.

C9000 stopped close to 1.47V (I probably missed it so registered 1.46V)
While it stopped short of nominal 2Ah capacity, it is interesting that C2 pumped out almost exactly what was put in (in 1748 out 1732). Seems it is very efficient in putting energy in and just the right amount.

On the other hand BC900 kept going longer. When C9000 stopped (around 115 min) at the point BC900 was still going and cell temperature started rising faster. While BC900 pumped in about 250 mAh more, it only got out about 50-80 mAh more.

Anyway I should probably stop rambling now
Bottom line is C9000 is more conservative and (for my own taste stops right around the point where cell temperature starts to rise, so gives up the last 5%-10% or thereabouts. BC900 kept going to a higher voltage so got bit more in.

[unattended]

So where are the inflexion point chargers?

you have to pay for a hobby charger to get that - and even there the inflexion method is pretty rare.
i bought the "pulsar 2+", they have the even more expensive pulsar 3 and there are one
or two other manufacturers with inflexion chargers.

[Hedles]

+1
Yeah, Why?

After reading SilverFox's excellent reference application note on the ST6210 charger chip above: (From Nickel-Cadmium To Nickel-Hydride Fast Battery Charger - by: J. NICOLAI, L. WUIDART), I now want a charger that stops on the inflection method! Give me a smoothed first derivative curve of V = f(t) and a charger that stops the fast charge before the battery starts heating up and damaging itself! (I wish that I had read this one month ago... I'm now saying: "D'oh!" for my four recent charger purchases: MH-C9000, Lacrosse BC-900, MH-C800S and MH-C401FS.) Are there any decent (kinder, gentler) NiMH chargers out there (inexpensive or otherwise) that use the ST6210 IC and its three termination methods?

Or is it just that folks are more interested in cramming every last mA into every charge (even though that extra last bit of charge may be damaging to the cells)? Or was the article just plain wrong and charging NiMH to the -dV/dt point is not actually harmful? (I find it difficult to believe that Maha would choose a default method that would be in any way harmful to the battery cells!)

Hey SilverFox, what's your take on this?

The article linked above and in the referenced post by SilverFox, is no longer available at the linked address but it can be downloaded from http://application-notes.digchip.com/005/5-10683.pdf. Perhaps a moderator can change the link in the original posts if judged appropriate. This post would then be obsolete and could be removed.

[SilverFox]

Hello Hedles,

Welcome to CPF.

I edited my post to look to your post for the current information. Thanks for finding this.

Tom

[Power Me Up]

As far as I'm aware, the only charger that uses Inflection termination is the UltraSmartCharger - I'm happy to be corrected on this though.

Disclaimer: I wrote the firmware for the UltraSmartCharger!

[Benediction]

As far as I'm aware, the only charger that uses Inflection termination is the UltraSmartCharger - I'm happy to be corrected on this though.

Disclaimer: I wrote the firmware for the UltraSmartCharger!

I agree, but there might be a new competitor on the block. Since it appears you can't get any more UltraSmartChargers (design Paul Allen!, coding by PowerMeUp) I will mention the Panasonic BQ-CC55 charger - their name for Inflection appears to be “peak sensing technology” -

[uk_caver]

If so, it seems a poor description, as 'peak sensing' rather suggests 0dV, or a sensitive -dV.