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'[EE] Charging NiMH'
2005\10\24@093309 by

This question probably falls into the category of things that are so obvious
that they are never spelled out.

Up until now I have built several gel battery chargers, always using
current-limited constant voltage charging (in other words a single 317).

However, now I need to build a NiMH battery charger and need to build a
constant current charger (with temperature control).  I haven't been able to
find a specification of the range of voltages I can use.  Measuring an
ordinary, cheap charger gives me 2 volts per cell.
On the other hand, I have seen schematics for chargers which say they
will take a range of voltages (for example 7.2 to 14.4 using a 30 volt
supply).

It would be particularly helpful if I could charge a range of 4.8 to 12 volt
battery packs with the same charger.

Bruce Douglas

NIMH is the most complicated of the common chemistries, to recharge.

You normally charge them by applying a constant current, and watching
the voltage and temperature. You terminate on rise of more than 1 degree
C / min or fall of some small amount per minute.  Now it begins to get
complicated.
If the battery is below a critical voltage, you need to apply a lower
current until it comes up.  If the battery is below a critical
temperature, you need to apply a very low current until it warms up. If
the battery is above a critical temperature, you shouldn't charge it at
all.
While charging, there are spots where the battery may develop one of the
termination criteria without really being charged, so you need to apply
lockouts so that condition X won't cause termination if it happens at
this point in the charge cycle..   You also need to run a timer, set to
the point where the battery has received about 120% of it's charge, as a
backup to all the above.

Some NIMH cells will tolerate a C/10 Trickle charge, some won't.

The Panasonic and Sanyo data sheets, plus information you'll find on the

Bruce Douglas wrote:
> However, now I need to build a NiMH battery charger and need to build
> a constant current charger (with temperature control).  I haven't
> been able to find a specification of the range of voltages I can use.

NiMH cells go from about 900mV at empty to about 1.5V when nearly full and
charging, maybe a bit more under high charge current.  Design your current
source to be able to produce 1.7V per cell and you should be fine.  By the
way, it's not a good idea to charge a bunch of cells in series unless they
are built into a pack that way.  If you have individual cells, they should
be charged that way.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

Olin Lathrop wrote:
> it's not a good idea to charge a bunch of cells in series unless they
> are built into a pack that way.  If you have individual cells, they should
> be charged that way.

I was intending to make up packs of up to 8 cells in series, perhaps with
the thermal sensor shrink-wrapped to the batteries as shown in the
Panasonic literature.  Is there something I need to take into account?

Bruce Douglas

2005\10\24@200333 by
Does anyone have any advice for charging very small capacity NiMH cells?
I've got these 1.2V coin cells (yes, they really are NiMH) and would rather
not deal with explosions ;-)

Can I just trickle charge with a few mA until they get warm?

-marc

On 10/24/05, Bruce Douglas <bruce_listtutopia.com.br> wrote:
{Quote hidden}

>
Bruce Douglas wrote:

> Olin Lathrop wrote:
>
>> it's not a good idea to charge a bunch of cells in series unless they
>> are built into a pack that way.  If you have individual cells, they
>> should
>> be charged that way.
>
>
> I was intending to make up packs of up to 8 cells in series, perhaps with
> the thermal sensor shrink-wrapped to the batteries as shown in the
> Panasonic literature.  Is there something I need to take into account?
>
> Bruce Douglas
>
Olin is right. When packmakers shrink-wrap cells in series, they have
equipment
that carefully matches the characteristics of each cell. The main
measurement
is impedance, but peak voltage is another. You will not be able to do
this at home.

The reason this is needed is that unless they are matched exactly, one
cell may be
overcharged, while another is undercharged. These mismatches weaken the pack
and reduce its life to an unacceptable level.

--Bob

--
Note: To protect our network,
attachments must be sent to
attachengineer.cotse.net .
http://beam.to/azengineer

On 24 Oct 2005 at 17:18, Bob Axtell wrote:
> When packmakers shrink-wrap cells in series, they have
> equipment
> that carefully matches the characteristics of each cell. The main
> measurement
> is impedance, but peak voltage is another.

I probably don't need to point out that you don't always get this level
of testing quality when buying battery packs.

Cheerful regards,

Bob

Got any recommended generic Battery Charger brands or models which
follow the recommended procedures? Or is it better to buy the chargers
which are produced by the battery manufacturers?

I bought a Sony 1-hour quick charger and find out the batteries
are not really charged very well and the charging current may well
be too high. My Nikon Digital Camera comes with a charger as well
and the quality is also not very good. I am afraid that the
chargers are killing the rechargeable batteries. Or is it better
to throw away batteries every two or three years?

Regards,
Xiaofan

{Original Message removed}

> Got any recommended generic Battery Charger brands or models which
> follow the recommended procedures? Or is it better to buy the chargers
> which are produced by the battery manufacturers?

The first thing to do is to make sure that your batteries are top
quality.

Chargers vary widely.  I don't know which consumer chargers would be

Maxim makes multi chemistry charger chips. I have used MAX 712 and MAX713.

http://pdfserv.maxim-ic.com/en/ds/1666.pdf

Regards,

--
Chetan Bhargava
Web: http://www.bhargavaz.net
Blog: http://microz.blogspot.com

On 10/24/05, Bruce Douglas <bruce_listtutopia.com.br> wrote:
{Quote hidden}

> Maxim makes multi chemistry charger chips. I have used MAX 712 and
MAX713.

Yecchhh.. Too much trickle current for most NIMH cells, and a lot of
other features missing.   I'd give them a D+ on NIMH.

Any recommended good Nigh batteries for general use?
Any recommended battery charger ICs for the good batteries you recommend?

Regards,
Xiaofan

-----Original Message-----
From: piclist-bouncesmit.edu
Sent: Tuesday, October 25, 2005 10:52 AM
To: Microcontroller discussion list - Public.
Subject: RE: [EE] Charging NiMH

The first thing to do is to make sure that your batteries are top
quality.

Chargers vary widely.  I don't know which consumer chargers would be
I've been using both energizer and Duracell rechargeable NiMH since they
were only 1200 mAh.  Lately, I have been leaning towards the Duracell
because they were up to 2300 mAh last time I bought them (and probably
on sale).

Cheers,

Ian

Chen Xiao Fan wrote:

{Quote hidden}

Bruce Douglas wrote:
> I was intending to make up packs of up to 8 cells in series, perhaps
> with the thermal sensor shrink-wrapped to the batteries as shown in the
> Panasonic literature.  Is there something I need to take into account?

Yes, that it's a bad idea.  The cells won't be matched.  They will be
difficult to charge evenly, and there is no way to avoid them discharging
unevenly.  Manufacturers can get away with packs of 3 or sometimes even 4
cells, but 8 is way too much.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Marc Nicholas wrote:
> Does anyone have any advice for charging very small capacity NiMH cells?
> I've got these 1.2V coin cells (yes, they really are NiMH) and would
> rather not deal with explosions ;-)
>
> Can I just trickle charge with a few mA until they get warm?

This should, of course, be specified in the data sheet.  Why are you asking
here?

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Chen Xiao Fan wrote:
> Got any recommended generic Battery Charger brands or models which
> follow the recommended procedures? Or is it better to buy the chargers
> which are produced by the battery manufacturers?

Many consumer battery chargers badly abuse the cells.  Consumers seem to buy
on shortest charge time rating and don't take into account how this might
hurt the cells.  The best thing is to look at the data sheet and make or buy
a charger that follows the recommended charging routine.

I made a PIC controlled NiMH charger for 3-cell 400mAH packs a few years
ago.  It worked great on those packs, but were something like 4-5 different
discharge/charge phases.  Properly charging NiMH cells is more complicated
than for most other battery types.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
On 10/25/05, Olin Lathrop <olin_piclistembedinc.com> wrote:
>
> Marc Nicholas wrote:
> > Does anyone have any advice for charging very small capacity NiMH cells?
> > I've got these 1.2V coin cells (yes, they really are NiMH) and would
> > rather not deal with explosions ;-)
> >
> > Can I just trickle charge with a few mA until they get warm?
>
> This should, of course, be specified in the data sheet. Why are you asking
> here?

Because they don't come with a datasheet, nor am I able to get one as they
come from a "surplus" supplier.

-marc
Olin Lathrop 13:25 2005-10-25:
>Bruce Douglas wrote:
>>I was intending to make up packs of up to 8 cells in series, perhaps
>>with the thermal sensor shrink-wrapped to the batteries as shown in the
>>Panasonic literature.  Is there something I need to take into account?
>
>Yes, that it's a bad idea.  The cells won't be matched.  They will be
>difficult to charge evenly, and there is no way to avoid them discharging
>unevenly.

This is the manin problem.  Even when you can use tricle charge to top every cell up, they will not be discharged evenly, because of both different capacity and different internal leakage.

>  Manufacturers can get away with packs of 3 or sometimes even 4
>cells, but 8 is way too much.

Pellenc electic cutters use 20 cells in series.  I rebuilt one using NiMH and still (2 years) no complaint.  Charge mechanism was 1/6 C until thermostat opens, then trickle charge.

Getting back to the problem of cell matching, in an attempt to be creative i ponder about winding a toroid with one winding for each cell, which through a schottky charges the cell.  This can probably NOT be used as main charge principle as a full charged hot cell have lower voltage than a cold not-full cell.  The idea is instead to drive that transformer just by a small power by a small oscillator in flyback mode when the pack is discharged, powered by total cell voltage.  Thereby we can

1) detect when any one cells voltage drops.  (flybac voltage drops)

2) the weakest cell(s) are helped, so total usable energy is higher.

Anothe idea is to sense each cell trough mux and/or resistors optpcoupler or whatever, byt all solutions i have sketched have some problem, especially at 20 Volts and I want them not to consume power at rest.

/Morgan

--
Morgan Olsson, Kivik, Sweden

Olin Lathrop 13:27 2005-10-25:
>>Can I just trickle charge with a few mA until they get warm?

Problem with charging just a little means they do not get very warm.

>This should, of course, be specified in the data sheet.  Why are you asking
>here?

It is often very hard to find decent datasheet.  I am speaking dervice/one-off business here.  It is anothe rmatter when designing for series production where you have to chech this, and your budget alows that work, and the business is enough money that the supplier wants to help...

/Morgan

--
Morgan Olsson, Kivik, Sweden

Marc Nicholas wrote:
> Because they don't come with a datasheet, nor am I able to get one as
> they come from a "surplus" supplier.

Then there's no way to know the right answer.  I've worked with some NiMH
button cells where the manufacturer specifically said it was OK to trickle
charge at a certain rate indefinitely.  For others, the manufacturer
specifically says not to do that.  Look at a bunch of representative data
sheets and take your best guess.  If the cells get damaged, remember what
you paid for them.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Maybe this has been mentioned already, but Linear has
a nimh charging chip LTC4060

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1037,C1078,C1088,P7601

-Mark

Hi Chen:

I recently have done a lot of experiments charging Ni-MH batteries.  Ni-Cad batteries have a chemistry that does not get hot when fully charged, but Ni-MH batteries will get very hot at the end of charge.  This overheating of the battery will degrade it's future performance.  One way of avoiding overheating in fast chargers is to monitor the battery temperature with a thermistor built into the battery pack.  Another way is to use a "Burp" charger that every now and then provides a strong discharge pulse.

A series string of batteries is very similar to a chain.  The amp hours you can pull from the battery is the same as the amp hour capacity of the weakest cell.  But this does not mean that you can not make up your own long strings.  I'm having good results with strings of 10 cells.  For more see my web pages:
http://www.pacificsites.com/~brooke/BatChg.shtml#Burp = Burp Charging

Have Fun,

Brooke Clarke, N6GCE
--
w/Java http://www.PRC68.com
w/o Java www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

Date: Tue, 25 Oct 2005 09:43:36 +0800
From: Chen Xiao Fan <xiaofansg.pepperl-fuchs.com>
To: "'Microcontroller discussion list - Public.'" <piclistmit.edu>
Subject: RE: [EE] Charging NiMH
Content-Type: text/plain;
charset="iso-8859-1"
MIME-Version: 1.0
Precedence: list
Reply-To: "Microcontroller discussion list - Public." <piclistmit.edu>
Message: 24

Got any recommended generic Battery Charger brands or models which
follow the recommended procedures? Or is it better to buy the chargers
which are produced by the battery manufacturers?

I bought a Sony 1-hour quick charger and find out the batteries
are not really charged very well and the charging current may well
be too high. My Nikon Digital Camera comes with a charger as well
and the quality is also not very good. I am afraid that the
chargers are killing the rechargeable batteries. Or is it better
to throw away batteries every two or three years?

Regards,
Xiaofan

For my autonomous R/C car, I'm using 8 packs of 6 cells in series (48
cells). The motor needs 14.4V. The speed controller has two connectors that
expects 7.2V. It [the speed controller] serializes the two inputs internally
to generate the 14.4V, so in summary, the packs are organized like this: 2
big packs of 7.2V in series, being each big pack made of 4 parallel packs of
6 cells in series.
I'm charging packs two by two in series using a supposedly intelligent
charger (Triton) at 1.35C. So far so good.

Out of curiosity, what other alternatives are there if one needs lots of mAh
but doesn't have much payload capacity? I've looked at lipoly's, but they
were too expensive for the amount of power I needed to buy (and supposedly
they have the same matching problems too right?)

Finally, what about using a small and cheap PIC that controls the amount of
current being drained from each individual cell of a battery pack, so one
doesn't need to worry about matching cells?

Cheers

{Original Message removed}
Brooke Clarke 18:52 2005-10-25:
> Ni-Cad batteries have a chemistry that does not get hot when fully charged

Of course they will if you pump energy into it when it is fully charged.

You can not destroy energy, so whatever you pump energy in that is not used for other things (such as the electrochemical storage in the cell, or at worst abuse gassing or explosion...) is bound to be heat.

/Morgan

--
Morgan Olsson, Kivik, Sweden

Hi Morgan:

Yes, but it's a matter of how hot.  The Ni-Cad chemistry is what's
called endothermic and the Ni-MH is what's called exothermic.  This
means more heat is produced at the end of charge with the Ni-MH battery
and amounts to about 30 degrees C more than the Ni-Cad (100 F vs 150
F).  Reference: Handbook of Batteries, chapter 29 (portable sealed Ni-MH
Batteries, paragraph 29.5.1 Charging sealed Ni-MH batteries, general
principles.  Figure 29.16b is a plot comparing battery temperature vs %
charge for Ni-Cad & Ni-MH.  Fig. 29a compares cell voltage vs. % charge
and shows the the -delta V for Ni-MH is smaller than for Ni-Cad so some
chargers designed for Ni-Cad may not sense the -delta V on Ni-MH.

A clear example are the military BB-590/U and BB-390/U batteries.  The
earlier BB-590/U is a Ni-Cad battery and has no provision for
temperature monitoring.  The newer BB-390/U has built-in temperature
sensors needed for charging.

I discovered "burp" charging when testing a surplus charger and it did
not overheat my Ni-MH battery.  The Maha C777+ terminates charging my
Ni-MH battery in the over temperature protection mode and shows an error
number leaving the battery over 140 deg F.  There seems to be some
controversy about "burp" charging but I have seen it work
experimentally.  A burp fast charger will not overheat a Ni-MH battery
and does not require a temperature sensor.  Battery Space now offers one.

Have Fun,

Brooke Clarke, N6GCE

--
w/Java http://www.PRC68.com
w/o Java www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com

{Quote hidden}

Brooke Clarke wrote:
> Yes, but it's a matter of how hot.  The Ni-Cad chemistry is what's
> called endothermic and the Ni-MH is what's called exothermic.  This
> means more heat is produced at the end of charge with the Ni-MH battery
> and amounts to about 30 degrees C more than the Ni-Cad (100 F vs 150
> F).

No matter what chemistry goes on inside, you can't escape basic conservation
of energy:

HeatOut = PowerIn - PowerStored

Perhaps power is stored/released under different conditions for NiMH than
for NiCd, but the above equation has to still be true.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
> Yes, but it's a matter of how hot.  The Ni-Cad chemistry is what's
> called endothermic and the Ni-MH is what's called exothermic

I've found that the reputable cells like Panasonic and Sanyo, have very
sharp thermal excursions at end of charge, where the cheap Chinese cells
are much more linear.

correct!

talking more heat, should relate to Ni-MH has higher, often
4 time more, mAh.

temperature method is not better way! problem is
lower accuracy of measurring T, than measurring V!
A little T change, means lots mAh.

{Original Message removed}

> I progressed in -dV method and
> already achieved very satisfied results, after I found
> a better algorithm. Before, I was totally dislike -dV.

If the better algorithm you found was public domain, would you
mind posting a pointer to it?

Brian Aase

Everyone is making this more complicated than is needed.

Simply put, a pause-charge method with minimum 10bit ADC for peak voltage
detection is equal to commercial charge management solutions available
today.

Because batteries are chemical components in electrical designs, fixed point
methods of battery management can never adapt to changes in battery
charge/discharge efficiencies as the batteries age or are under non-typical

I'll post a Basic Stamp version shortly, and soon a C++ version.

Steve Halla

{Original Message removed}
The code would be interesting to see.

Thanks,

On 10/28/05, Steve Halla - LEAP/Rutgers <shallacamden.rutgers.edu> wrote:
>
> I'll post a Basic Stamp version shortly, and soon a C++ version.
>
> Steve Halla

--
Chetan Bhargava
Web: http://www.bhargavaz.net
Blog: http://microz.blogspot.com

It is from myself and based on rate of V reducing.
Not only -V.

I found the critical point related with V sesoring is
you need reliable hardware! With hardware
functional, firmware is easy, but unique!

{Original Message removed}

On Thu, 27 Oct 2005, Olin Lathrop wrote:

{Quote hidden}

Yes but "PowerStored" could be in the form of unwanted products (e.g.
hydrogen gas) so although you are not seeing any more useable stored
electrical energy it would be possible (depending on battery chemistry)
NOT to see an increase in heat output (or a disproportionately small
increase). I can also imagine conditions where unwanted products cause
changes in the thermal system of the battery (e.g. complexes with higher
latent heat of solution or fussion).

Regards
Sergio Masci

http://www.xcprod.com/titan/XCSB - optimising PIC compiler
FREE for personal non-commercial use

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