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'[EE]: Anyone got NiMH charge profile data?'
2006\06\27@173114 by Philip Pemberton

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Hi,
  Has anyone here got a recorded charge profile from a NiMH battery? What I'm
after is - in effect - a spreadsheet with three columns:
  - Time
  - Voltage
  - Current (optional)

Does anyone have anything like this kicking around their HDD? The battery
manufacturers seem to be quite happy to give out pretty pictures of their
charge profiles, but don't seem to publish actual numbers.

I want to play around with charge termination detection algorithms for my
(half-done) NiMH charger, and I don't have any test data to feed into my
detection algorithms. I figure while I'm waiting for the parts for the
discharger circuitry to turn up (a DAC, a voltage reference and a few
op-amps), I might as well get some of the software algorithms tested and
tweaked for use on a PIC.

What's bugging me is that the manufacturers are saying one thing (wait for dV
to hit -5mV per cell) while all the books and websites I've been scanning
through are saying different things. One suggested "zero voltage delta" (wait
for dV to get close to zero) and another suggested "inflection point sensing".
The latter involved calculating the first or second-order derivative of dV/dt
and looking for the point where the derivative crosses zero. Surely that would
produce the same result as zero-delta sensing though?

The main thing is that -dV is reputed to slightly overcharge the cells (which
produces heat, which is bad) and 0dV allegedly stops that. But if you get the
trip point wrong, you end up undercharging the cells, which is also bad.

As far as preferences go, data for any cell manufacturer is fine, but Sanyo
(or anyone who rebrands Sanyo cells - Kodak, Energizer or Duracell) or another
name-brand (Uniross excepted) would be preferred. Same for cell count - given
the number of cells in a pack, I can get the per-cell voltage back pretty easily.

Thanks.
--
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2006\06\27@175233 by Robert Young

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{Quote hidden}

Phil -

While I don't have any data on my hard drive, I can collect the data
(charge and discharge) curves using my CBA (Computerized Battery
Analyzer) from West Mountain Radio
(http://westmountainradio.com/CBA.htm).  Not terribly expensive piece of
equipment and very useful for characterizing batteries and charging
circuits.

Right now all I have handy are some 6 cell NIMH packs and a "low"
current charge design (charger pushes maximum of 750mA, battery packs
are rated at 2000mAhr) so it doesn't charge terribly quickly.

Rob

2006\06\28@010833 by Martin Klingensmith

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face
I found this website:
<http://www.mpoweruk.com/chargers.htm>
I don't know if you're going to find the spreadsheet you're looking for.
There are many methods of charging batteries. What you may need to do is
some experimentation.
You mention that the -dv method slightly overcharges cells, creating
heat. It was my impression that NiMH cells heated up as a result of
becoming fully charged.
My AA cells get pretty warm in the charger. There was an EV (the RAV4?)
that had to run the A/C when charging, to keep the huge NiMH cells at a
reasonable temp.
--
Martin K


Philip Pemberton wrote:

{Quote hidden}

--
Martin Klingensmith
http://wwia.org/
http://nnytech.net/

2006\06\28@030718 by John Chung

picon face
Philip,

Do you measure the tempature of the battery while
charging it? If so what method are you using?

John

--- Martin Klingensmith <.....martin-distlistsKILLspamspam.....nnytech.net>
wrote:

{Quote hidden}

> --

2006\06\28@031540 by Philip Pemberton

face picon face
John Chung wrote:
>  Do you measure the tempature of the battery while
> charging it? If so what method are you using?

I'm not doing it yet, but the plan was to use a thermistor. Cheap, and faster
than a DS18S20. Only problem is, I don't think I've got any spare thermistors...

Thanks.
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2006\06\28@051959 by Philip Pemberton

face picon face
Robert Young wrote:
> While I don't have any data on my hard drive, I can collect the data
> (charge and discharge) curves using my CBA (Computerized Battery
> Analyzer) from West Mountain Radio
> (http://westmountainradio.com/CBA.htm).  Not terribly expensive piece of
> equipment and very useful for characterizing batteries and charging
> circuits.

Looks like a neat bit of kit. I was going to build something similar, but
simpler. Basically a voltage/current logger with an RS232 interface. The
problem was I wanted a floating input, which isn't exactly hard to do, just a
bit expensive.

> Right now all I have handy are some 6 cell NIMH packs and a "low"
> current charge design (charger pushes maximum of 750mA, battery packs
> are rated at 2000mAhr) so it doesn't charge terribly quickly.

Like I said, I'm not in a mad rush. The voltage reference ICs arrived this
morning (Maxim MAX6034AEXR41-T; 4.096V in an SC70 package) so I'm going to try
and hack together a constant-current pulse charger in a bit. Hopefully I'll be
able to get some decent test data out of that, assuming I can find a
discharged NiMH AA in my "Big Box O' Batteries".

Thanks.
--
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2006\06\30@134253 by Howard Winter

face
flavicon
picon face
Phil,

On Tue, 27 Jun 2006 22:30:59 +0100, Philip Pemberton wrote:

>...
> What's bugging me is that the manufacturers are saying one thing (wait for dV
> to hit -5mV per cell)

I'd be careful with this - I have a feeling that a worn-out cell doesn't go into negative dV, and you end ep
cooking it.  I can't say for sure, but I've had very sophisticated chargers end up frying cells that I
believed to be a bit dodgy, but wanted to see if they'd charge...

And I think you need to be even more careful if you're charging a battery rather than individual cells - one
cell's +dV may mask another's -dV !

Cheers,


Howard Winter
St.Albans, England


2006\06\30@135442 by David VanHorn

picon face
>
>
> I'd be careful with this - I have a feeling that a worn-out cell doesn't
> go into negative dV, and you end ep
> cooking it.  I can't say for sure, but I've had very sophisticated
> chargers end up frying cells that I
> believed to be a bit dodgy, but wanted to see if they'd charge...


I would terminate high rate (or not start high rate) on one of several
conditions: (1C charge assumed)
T < 0C
T>40C
dT > 2 degreesC/min UNLESS the cells are below room temperature.
dV = 0 for 30S
dV negative, with a 10 min lockout
Pack voltage exceeding 1.5V/Cell

I measure at the end of short pauses that happen once a second.




And I think you need to be even more careful if you're charging a battery
> rather than individual cells - one
> cell's +dV may mask another's -dV !


Indeed they do.  Very good idea not to make up packs from cells with
dis-similar histories

2006\06\30@144230 by Philip Pemberton

face picon face
Howard Winter wrote:
> Phil,
>
> On Tue, 27 Jun 2006 22:30:59 +0100, Philip Pemberton wrote:
>
>> ...
>> What's bugging me is that the manufacturers are saying one thing (wait for dV
>> to hit -5mV per cell)
>
> I'd be careful with this - I have a feeling that a worn-out cell doesn't go into negative dV, and you end ep
> cooking it.  I can't say for sure, but I've had very sophisticated chargers end up frying cells that I
> believed to be a bit dodgy, but wanted to see if they'd charge...

That's why I'm planning to use 0dV or Inflection Point sensing.
What really bugs me is that all the cell manufacturers are harping on about
negative-delta ("has the voltage dropped by more than N millivolts below the
peak?") charge termination, yet most of the reference material says that the
negative delta occurs when a cell goes into overcharge.

I've pretty much figured out how to implement a "homebrew hack" of Galaxy
Power's Quicksaver algorithm, which seems to consist of:
  - Make sure there's a battery connected to the charge output
  - Pre-charge. Ramp the charge rate up from C/10 to 1C over the course of
two minutes
  - Fast charge. Charge the battery at 1C until a termination threshold is
detected (e.g. inflection point)
  - Top-off. Two hours at C/10 to finish charging the battery

It seems that the reason the batteries run cooler when they've been charged
with an ICS170x chip is because it's designed to cut off early, and top the
battery up. Inflection-point sensing trips at the point where the delta-V
starts to drop - at that point, the battery is 90% charged. Follow that up
with a C/10 top-off charge which still overcharges the battery, but does it at
a rate that the cell can handle without venting.

Seems GPI have disappeared, which is a shame. Their website has been gone
since March last year (but it's still in Wayback) but all the data I've seen
on the street mapping sites and the stuff on PA state's website suggests
they're still in business... Anyone know for sure?

> And I think you need to be even more careful if you're charging a battery rather than individual cells - one
> cell's +dV may mask another's -dV !

Another reason to use inflection-point or zero-delta sensing.

I notice the Maxim MAX712 datasheet suggests that zero-delta sensing is only
suitable for NiMH packs, but negative-delta is suitable for all battery packs.
Most peculiar.

I'm just trying to solve what appears to be the last of the Nasty Design
Issues (tm) now. I want a decent amount of resolution out of the A/D
converter. I've got ten bits (1024 steps) and a reference of 2.048V. That
gives 2mV per step.
That's fair enough for a single cell, and leaves a good bit of overflow.
Problem is, if I want two cells, I have to halve the input voltage. I still
keep the same per-cell resolution, but then if I go back to a single cell I
can only get 4mV per step resolution.
It gets worse when I design for four cells - with a single cell, I only get
8mV resolution, which is nowhere near sufficient to pick up the negative
delta-V...

So far the best idea I've come up with is to have swappable battery holders
and a voltage divider. The pulldown resistor is in the charger, the input
resistor is in the battery holder. A five-pin connector connects the pack to
the charger (2 pins for +/-, one pin for sense, two pins for telling the
charger how many cells the holder is designed for.

Or I could redesign the voltage amp using a digital potentiometer or
something. Divide it down as if there was a four-cell pack connected, then use
the digipot to vary the gain of an op-amp stage... Decisions, decisions.

But at least the other 80% of the hardware is finished. Rather typically it's
the last 20% of the design that takes 80% of the time. Then there's the
software...

Thanks.
--
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2006\06\30@145632 by Philip Pemberton

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David VanHorn wrote:
> I would terminate high rate (or not start high rate) on one of several
> conditions: (1C charge assumed)
> T < 0C
> T>40C
> dT > 2 degreesC/min UNLESS the cells are below room temperature.

I don't have a temperature sensor yet. It's on the drawing board though, just
as soon as I get my mitts on a few LM35 temperature sensors.

> dV = 0 for 30S
> dV negative, with a 10 min lockout

A 10min lockout? What, you ignore the dV for the first ten minutes of the
charge cycle?
I guess the negative dV is the delta from the peak voltage, i.e. Vpeak - Vnow.

> Pack voltage exceeding 1.5V/Cell

Got that (or something close to it anyway). If Vcell > 1.7V then the charge
algorithm fails with an "EOVERVOLT" error (maximum safe voltage exceeded).

> I measure at the end of short pauses that happen once a second.

Seems everyone does that. One second charge, 4ms rest, 5ms discharge, 4ms
rest, 16ms acquisition time (16 samples at 1ms intervals, averaged out to get
rid of any noise).

The plan so far is to have a PIC16F88 running as a charge manager (the "PMU",
power management unit), which runs the charge algorithms and reports back to
the second PIC. The second PIC will probably be an F628 or another F88, and
will be responsible for dealing with the user-interface aspect of things.

It really all boils down to the fact that I'd rather have two PICs working
together than try and implement the UI code and the charging / PWM / etc. code
on one chip. PICs aren't exactly expensive (16F88s seem to run about £1.50
each), so using two of them isn't really a big deal.

> And I think you need to be even more careful if you're charging a battery
>> rather than individual cells - one
>> cell's +dV may mask another's -dV !
>
>
> Indeed they do.  Very good idea not to make up packs from cells with
> dis-similar histories

I'm using Kodak 1600mAh AAs and Energizer 2500mAh AAs - I've got three sets of
four of the Kodaks (plus a set of three), and one set of four of the
Energizers. Note that these are loose AAs - I wanted them to use in my Gameboy
(don't deny and old man his few pleasures in life) and my digital camera
(which consumes Duracells at a stupid rate).

In its absolute simplest form, this whole project is one giant example of NIH
(Not Invented Here). But it's a fun example of NIH :)

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2006\06\30@150536 by David VanHorn

picon face
>
>
> That's why I'm planning to use 0dV or Inflection Point sensing.
> What really bugs me is that all the cell manufacturers are harping on
> about
> negative-delta ("has the voltage dropped by more than N millivolts below
> the
> peak?") charge termination, yet most of the reference material says that
> the
> negative delta occurs when a cell goes into overcharge.



Correct.  The problem with the zero delta is that it's tricky to pick up.
You can have false inflection points.
The thermal plume is more obvious, and hard to fake except for cold cells
coming into a warm environment.  At the point where you are done charging
(more or less) the input energy begins to be put into heat, and making H2
and O2.  The recombination of H2 and O2 by the catalyst (YOU HOPE!!!) is
also exothermic.



{Quote hidden}

Right, and know that a lot of information out there is still talking nicad.
NIMH is a bit different.
You can also to the topping charge at C/2


>
> I notice the Maxim MAX712 datasheet suggests that zero-delta sensing is
> only
> suitable for NiMH packs, but negative-delta is suitable for all battery
> packs.
> Most peculiar.


Wrong, actually.  I mean yes you can use it, but it's overcharging.
Some cells are more tolerant of that than others, but the free O2 tends to
combine with other things it shouldn't and the H2 leaks past the seal, and
so you end up with degradation.  Also, at the higher voltages, you can get
into other undesired reactions that pollute the chemistry.


> It gets worse when I design for four cells - with a single cell, I only
> get
> 8mV resolution, which is nowhere near sufficient to pick up the negative
> delta-V...


Worked ok for me on a 6 cell pack with a 10 bit ADC and 4.096V ref, divided
down just enough to keep the 9V+ peak pack voltage from railing the ADC.


> But at least the other 80% of the hardware is finished. Rather typically
> it's
> the last 20% of the design that takes 80% of the time. Then there's the
> software...


And when you run into cheap cells and don't know it, 6 months + of putzing
around trying to fix something that isn't broken.....  !@#!@#!$#!$%

2006\06\30@151019 by David VanHorn

picon face
>
>
> I don't have a temperature sensor yet. It's on the drawing board though,
> just
> as soon as I get my mitts on a few LM35 temperature sensors.


I used 10k thermistors, worked nicely.


> A 10min lockout? What, you ignore the dV for the first ten minutes of the
> charge cycle?
> I guess the negative dV is the delta from the peak voltage, i.e. Vpeak -
> Vnow.


Yup,  Packs can lower their impedance during the first part of the charge
cycle.

>
> Got that (or something close to it anyway). If Vcell > 1.7V then the
> charge
> algorithm fails with an "EOVERVOLT" error (maximum safe voltage exceeded).


Check your cell specs, I remember 1.5-1.6 depending on who I was reading.


> It really all boils down to the fact that I'd rather have two PICs working
> together than try and implement the UI code and the charging / PWM / etc.
> code
> on one chip. PICs aren't exactly expensive (16F88s seem to run about £1.50
> each), so using two of them isn't really a big deal.


Hmm.. I had that, plus 500 kHz PWM, plus clocked serial communications with
the host, and still had plenty of time to spare.  Very little really happens
during charging, except the 10mS current control loop.



> I'm using Kodak 1600mAh AAs and Energizer 2500mAh AAs - I've got three
> sets of
> four of the Kodaks (plus a set of three), and one set of four of the
> Energizers. Note that these are loose AAs - I wanted them to use in my
> Gameboy
> (don't deny and old man his few pleasures in life) and my digital camera
> (which consumes Duracells at a stupid rate).


It's fun to compare cell curves. Very interesting.  The good cells give a
thermal SPIKE at the end, the cheap ones more of a ramp with a hill at the
end.
You can also cycle them into memory effect without too much work, though
it's pretty underwhelming when you see it in person.

2006\06\30@155708 by Robert Rolf

picon face

Philip Pemberton wrote:
> So far the best idea I've come up with is to have swappable battery holders
> and a voltage divider. The pulldown resistor is in the charger, the input
> resistor is in the battery holder. A five-pin connector connects the pack to
> the charger (2 pins for +/-, one pin for sense, two pins for telling the
> charger how many cells the holder is designed for.

Can't you tell that just from the value you set for the reference voltage?
Seems to me 3 pins is all you need since the number of cells also determines
the reference voltage you want.

> Or I could redesign the voltage amp using a digital potentiometer or
> something.

Or something. 1% resistors attached to tri state pins on PIC to change
values in binary. If you're clever about the R values you can probably
get away with ONE pin. Low, Tristate, High for  1, 2, 4 cells.

> Divide it down as if there was a four-cell pack connected, then use
> the digipot to vary the gain of an op-amp stage... Decisions, decisions.

See above.
You can use diodes to give you different R values for pull up and
pull down, if you don't mind their change in voltage drop with temperature
(which can be compensated by a matching diode elsewhere in the circuit).

R

'[EE]: Anyone got NiMH charge profile data?'
2006\07\08@105549 by Philip Pemberton
face picon face
Robert Rolf wrote:
> Can't you tell that just from the value you set for the reference voltage?
> Seems to me 3 pins is all you need since the number of cells also determines
> the reference voltage you want.

The charger has a fixed reference voltage (2.048V -- a little close to the
PIC's 2V minimum, but it seems to work).

One NiMH cell has a maximum safe voltage of 1.5V, but I'd like to give the
circuitry the ability to measure up to 2V (just for the sake of having a bit
of a safety margin)

> Or something. 1% resistors attached to tri state pins on PIC to change
> values in binary. If you're clever about the R values you can probably
> get away with ONE pin. Low, Tristate, High for  1, 2, 4 cells.

Hm, not in an opamp feedback circuit. I could replace the pulldown resistor
with a permanent pulldown and two switchable parallel resistors.. Good idea,
thanks. 5p worth of resistors vs. a £5 digital potentiometer IC...

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