In Search Of: NiCd Minus Delta V -- Lost and Found

MrAl

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Hello again,

Recently i revived a set of sub C NiCd's i havent used
for maybe a year or so, and decided to recharge them and
take a look at how well they performed as well as to
look for their minus delta V. As some of you already
know, the minus delta V is sometimes used to trigger
the end of charge time for a NiCd cell.
There are various debates about using the minus delta V
method as an end of charge indicator which involve
whether or not the battery is already charged too far
ahead of the indication and so waiting for the minus
delta V condition is not good for the cell.

Im presenting the results of the test i did myself to
try to understand the pro's and con's a bit better.
The test was performed using two sub C NiCd's wired
in series.

TEST #1

Starting conditions:
The cells were sitting around for a year or so discharged
to about 0.8v each. Pumping them up with 300ma for seven
hours to make sure they were fully charged. The discharge
consisted of one 5 ohm resistor across the two cells.
From the discharge time and the cells terminal voltage
the ampere hour rating of the two cells was estimated
at 1.24 ampere hour.


TEST #2

The cells were totally discharged from the first test so
now they were connected to a constant current source for
charging. The constant current source puts out 1.045 amps
regardless of voltage and maintains that level within about
+/- 0.1 percent or less.
Right after the cells were connected their series voltage
went up to 2.75v, then gradually rose up to 3.00v, 3.10v,
etc., until some 2 hours later the voltage was up to
3.30v exactly. Funny thing is, NO MINUS DELTA!!
Since the cells were rated at 1.2 to 1.3 and they had been
charging for at least 2 hours they were getting hot. I had
to stop the charging after about 2 hours and 10 minutes.

TEST #3

This turned out much more interesting results.
The cells were discharged at about 500ma for
3/4 of an hour (45 mins). This means they need
about 25 mins of charging at 1 amp constant to get
the charge back in, so connecting the constant current
again i monitored the cell voltage of each cell
individually with two digital voltmeters set on their
2.000v scales. AMAZINGLY, this time the minus delta V
showed up! The cell voltages continued to decrease
until it was decided they were overcharged slightly.
Even more interestingly, just before the voltage of each
cell topped off, the voltage shot up very quickly.
To get a better idea of this phenomenon take a look
here:
http://hometown.aol.com/xaxo/page6.html

This pic clearly shows the quick rise as well as
the minus delta.

TEST #4

I havent done this test yet. What i'd like to do is
find out why the delta V didnt show up in Test #2.

Any ideas?

Take care,
Al
 

jamaica

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Mr Al,
I've long enjoyed your posts.

1) I've not heard of that jump up just before the -delta(V) your webpage shows, but it looks like an awfully great, conservative point to terminate charge, doesn't it?

2) What's charge efficiency of NiCad at 1C rates? (It takes 14hours @ 0.1C to fully charge a cell, but I think the efficiency is /higher/ at faster charge rates. Not sure about this though.) If you consider this factor, however, the cells in your test #3 may not have been fully charged.

3) I have a RayOVac 1hr charger (PS4?). Others have reported that this charger cooks cells, but I found it to have excellent manners with the 1,600mAH and other various cells I put in it. Top temps were balmy, reaching about 35deg.C at end-of-charge. That was true /until/ I tried some of the Sanyo/Energizer 2,300 NiMH's. Those, on their first cycle, got hot enough to start smelling, perhaps 50C(?) to the touch, and I jerked them out while still under full charge. Clearly, the charger had been fooled. I mention this because your data, the experiences reported elsewhere on CPF, and mine suggest that NiMH cells' delta(V) rate varies with the number of cycles, state-of-charge, internal impedance, or some other factor. Wonder what this could be...

Just a few ideas to ponder.

Jamaica
 

MrAl

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Hey thanks jamaica :)

Yes, the jump 'up' sure is interesting and like you said
it does look like a good point to terminate charge, as
right at the end just as the curve goes through zero
slope the charge accumulated matches the calculation for
a full charge, off by only two minutes! I sure wouldnt
mind that. Also, what's not plotted there is the
temperature, which begins to rise right about at the zero
slope point. Halting the charge there would mean very safe
charging for a 1C charge rate.

As for the charge acceptance with charge rate,
at 1C charge rate i use 1.1 and at 10C i use 1.4 .
In between i simply extrapolate.

After seeing how the Test #2 curve came out i can see why
this method sometimes 'cooks' cells, because had the cells
been in an automatic minus delta V detection circuit they
would have charged for hours without end because, after
all, the delta V never occurred during that run.
And yes, i have to agree, it might have something to do
with state of charge prior to charging -- test #2 had cells
that were charging from zero charge state, while the cells
in test #3 were only partially discharged.

I guess one more test is in order here :) where the
cells are AGAIN discharged all the way down, then
put on charge and watch for minus delta V again.

I was guessing that the delta V needs to be measured at
the end of a current pulse, where the pulse is long
relative to the 'off' (and measurement) period, but the
phenomenon showed up so well i cant see this being true
really.

Oh, BTW, these were actual NiCd's, not NiMH cells, and
i cant be sure if NiMH would have that great of an
increase as obtained with the NiCd's. I've read the
NiMH minus delta V is much lower.

I guess the plateau could be useful too.

Take care,
Al
 

eluminator

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The voltage during charging seems a bit high, but maybe that will vary depending on the charging current. In my charger, the voltage never goes over 1.50 volts. It uses negative delta V, and also delta V squared over delta t squared as charge termination criteria. At least that's what the manual says.
 

MrAl

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Hello there,

Delta V squared over Delta t squared sounds like the
second derivative, and the second derivative is used
to determine inflection points along the charge curve,
and that technique was introduced in 1994 i think.
You're lucky because that's one of the better charging
methods from what i've read. It detects more then one
point along the curve and so reduces termination time
error. Also from what i've read, supposedly you can
charge batteries in 15 minutes using that method because
the termination time is accurate enough.

I've done some more tests on the same set of NiCds
and some other sets also, and i've found that the
minus delta V isnt always present. It seems to
be absent when the cells are charged from zero
charge but present when they havent been run down
all the way. Im not sure how far they have to be
run down before the minus delta disappears yet, but
i've found that if the cells charge current is shut
off for a period of time (say two minutes) in between
charging then the minus delta shows itself. If it's
shut off too soon in doesnt work, but if done toward the end
it brings the minus delta V back. Since it's not really
possible to tell when it's near the end, periodic shutoffs
should work to bring the minus delta v back.

A few of the cells i tests ALWAYS show the minus delta,
which is strange, because they very old and dont hold
much charge. The newer ones are the ones that dont show
a minus delta when run down all the way and then charged
continuously with 1 amp constant current (1.2ah ratings).

Take care,
Al
 

SilverFox

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Hello Al,

This seems strange...

Do you have any brand new cells to try as well? I would think the charge pattern should be consistent between cells. There must be something else going on here.

Any ideas?

Tom
 

MrAl

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Hi there Silver,

No i dont have any brand new cells but i guess i could
pick up a couple and test them. The original cells i
tested werent exactly used much though, maybe run down
10 to 20 times, that's about it, they were just sitting
on a shelf for over a year or so before using again.

What i think is going on now is that the pressure (or
just simply the presence of gasses) is masking the true
voltage curve so it doesnt appear. Given a few minutes
for the gasses to recombine, the expected curve shows up
after only a few minutes!
If i can find a few new cells locally i'll test them
and see what happens.

I've heard about something like this before, and also
about people's chargers cooking their batteries too,
and i can see why now. If the minus delta V doesnt
show up even ONE time for ONE charge cycle, the cell(s)
will cook until the user gets around to taking them
out of the charger.
I would bet that manufacturers have realized this at
some point and are using backup methods to terminate
charge if they do use minus delta V still.

One thing i would really like to do is monitor temperature
because as soon as the cell gets near it's required charge
termination time (calculated) the temperature starts to
rise. This would make an excellent way to stop the charge,
except that temperature isnt exactly that easy to measure.
Sure, a small, cheap thermistor would do the job, but how
to make sure it continues to make good contact with
the cell, and sometimes the cells have plastic and/or
heavy paper jackets which dont conduct heat very well.
The negative terminal of the battery would be a good
place to make contact, but sometimes it has leads soldered
to it or a tab or whatever so that might be kind of
tricky too.

Take care,
Al
 

SilverFox

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Hello Al,

That is very interesting.

I noticed that during the first charge on my NiMh D cells, the cells got hotter than I expected. Subsequent charges have not repeated this. The cells still get warm, but not hot.

I did not take any temperature measurements, but did contact the vendor about a possible malfunction. This only occurred during the first charge and I have wondered about the safety of unattended charging.

It would appear that the automatic shut off timer that some chargers have may be an important safety device.

Tom
 

jamaica

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Location
HB, SoCal
[ QUOTE ]
One thing i would really like to do is monitor temperature
because as soon as the cell gets near it's required charge
termination time (calculated) the temperature starts to
rise.

[/ QUOTE ]

Amen brother! It looks to me like temperature is the only reliable, dead-sure indication that a cell is full.

I've noted others' speculation that -delta(V) is a result of temp rise; I don't know if that's true.

I really wonder how a useful a -delta(V) signal a unit with two cells in series can reasonably expect. One cell will almost always finish first, and its -delta(V) would be masked by the other's +delta(V). I suppose that's where the 2nd derivative comes in: stop when the rise in voltage slows, and you've stopped when the fastest charging cell is done.

Overall I much like the notion of temp. sensing, at the cathode end of the can, have wanted this for years, yet still no one seems to do this. (Hmmm ... there's one possibility: anyone know how the Energizer 30min charger terminates charge?) It's almost enough to make me want to build my own charger. Almost.

Jamaica
 

MrAl

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Hello again jamaica,

Yes temperature, but now how to measure reliably :)

Yes, it's quite possible that with two cells in series
one's minus delta v could be masked by the other's
plus delta v if one gets done before the other, and that's
why i measure each cells voltage individually even though
they can be charged from the same current source.
Charging with a constant current source ensures that each
cell will always get 1 amp (or whatever) regardless of
what it's terminal voltage decides to do.

The second derivative is supposed to simply add accuracy
to the determination of the termination time thus making
it possible to charge in a shorter time then would be
required for minus delta v, because supposedly the only
danger is overcharging, but it's more of a risk with
higher current levels (like 6 amps or so). In any case,
each cells voltage has to be measured independantly,
which wouldnt be too easy with a battery pack of say 16
nicds in series.

I'd like to measure temperature at the negative side of
the can too, but im not sure how easy it will be to
attach a thermistor there, especially since it should
really be one thermistor per cell. Have to measure all
the cells in the pack individually.

Yeah, im thinking about building something up too,
simple yet good enough to catch the full charge time
every time not just 'most' of the time.

Take care,
Al
 

MrAl

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Hello again,

I managed to find a couple 1600mah NiCds at a local
RS and tried the same test with them (charging at
1 amp constant current).
The minus delta showed up on the very first charge :)
but unfortunately i didnt know the state of charge
before the start of the charge cycle, so next test
i'll run them down (and check capacity) and do the
test one more time.
The temperature increase started just after the
minus delta v appeared in these brand new cells.


Take care,
Al
 

SilverFox

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Hello Al,

I believe most rechargeable batteries are shipped from the factory at around 60% - 80% charge.

It will be interesting to see what happens on a full discharge/charge cycle.

Tom
 

MrAl

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Location
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Hello again,

Well, i discharged the cells all the way down and found
that they showed a 1.9ah rating, a little higher than
1600mah.
On charge, they both showed their minus delta v's at
about the calculated time for a 1.9ah cell charging at
1.038 amps constant, ahead by only a few minutes each.

Im now thinking that perhaps cells manufactured say 10
years ago wont show the same results as those made
more recently with regard to the minus delta v, but
i'd still make sure i had a back up detection plan
just in case minus delta v didnt catch the correct stop
time. If the charge wasnt terminated soon after the
minus delta appeared the cells would have gotten
too hot.

Take care,
Al
 

Ginseng

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Re: In Search Of: NiCd Minus Delta V -- Lost and F

Silver,

The battery mannufacturers may ship at 60-80% but what they have when you finally buy them is another matter. I've bought various types of cells and they've come with 5%-90% capacity.

This makes a diagnostic conditioning process very important.

Wilkey
 

SilverFox

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Re: In Search Of: NiCd Minus Delta V -- Lost and F

Hello Wilkey,

That's right. I forgot that there might be some time between manufacture, purchasing, and putting into use. The cells are self discharging the whole time.

Do you agree with Al that battery chemistry has changed from 10 years ago?

I have some old NiCd's and will have to dig them out and watch them closely while charging. My charger (Vanson) has a shut off timer, but I think it relies upon delta V monitoring to end the charge.

Tom
 
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