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'[EE] Charging NiMH batteries'
2009\01\29@041538 by Forrest W. Christian

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I'll admit it.

Any circuit which charges batteries scares me.  Perhaps I've put too
much stock in those warnings on batteries and chargers, or seen the
effect of an overcharged battery one too many times.   So, if a product
design required charging batteries, I put it on the shelf for later,
hoping that sometime I would conquer my fear.

Well, I guess that there is no better time than now to conquer my fears.

I have an application where I need to end up with a ~15 cell (18V) AA
NiMH battery pack.  And I need to charge it from a automotive power
source.   And preferably relatively quickly, like a 1 hour charger or
similar.

Oh, and I'd prefer to keep melting, flame, and explosions to a minimum.  :-O

I've looked at a lot of charger circuits and I'm still scared.    What
I'd really like is for someone (or someones) on the list to tell me how
paranoid I am being (or not paranoid) and sort of give me some pointers
of what to watch for.
(Note I'm not suggesting LiIon... :)

-forrest

2009\01\29@044436 by solarwind

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On Thu, Jan 29, 2009 at 4:15 AM, Forrest W. Christian
<spam_OUTforrestcTakeThisOuTspamimach.com> wrote:
{Quote hidden}

You're being pretty paranoid.

--
solarwind

2009\01\29@044756 by Nate Duehr

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On Jan 29, 2009, at 2:15 AM, Forrest W. Christian wrote:

> I have an application where I need to end up with a ~15 cell (18V) AA
> NiMH battery pack.  And I need to charge it from a automotive power
> source.   And preferably relatively quickly, like a 1 hour charger or
> similar.
>
> Oh, and I'd prefer to keep melting, flame, and explosions to a  
> minimum.  :-O

Can you just design your gadget with a removable pack that can plug  
into a good high-quality charger made by someone else?

Just a thought.  Not as much of a challenge, perhaps -- but there are  
some charger companies out there who make excellent products.  With  
the right keyed connector on their charger, and your pack... problem  
solved.

And you can move on to more interesting things that don't keep you  
awake at night.

Nate

2009\01\29@053909 by solarwind

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Not to mention that if your product sets someone's car on fire during
a charge, you're sued :)

--solarwind

2009\01\29@053951 by Jake Anderson

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Forrest W. Christian wrote:
{Quote hidden}

We use(ed) to use nimh and nicad in robot wars. We would typically suck
a pack down 80% in 3 minutes
We would then give it about 15 minutes of forced air cooling (the cells
would typically be 70C minimum, we saw 120c more than once, but that
caused the cells to leak some)
then stick it on charge for 40 minutes or so at 5 amps or so (~3Ah
cells) with the cooling running, let that charge finish, then do another
charge at ~1A or so for another 20 minutes to "top it off"
after another 10 minutes or so they were cooled down enough to go back
in the bot.

Needless to say we were fairly abusive of the packs.
For all that only one person had a major problem, his pack shorted every
pair of battery's to the frame of his (aluminium) bot.
These cells were good for 200A pulse discharge. Lets just say his bot
got rather warm. And more than a little smokey.
and then his battery compartment filled up a little with some boiling
electrolyte.

We stopped the match and got an IR thermometer onto it, he managed to
hit 160C or so then he pulled it all apart.
washed his bot out, (the electrolyte is pretty nasty stuff to metals),
then rinsed his hands again with soap after I told him to ;->
He almost managed to keep his finger prints ;-> (after about 20 minutes
of immersion, no permanent harm, keep it in mind if you want to pull off
a bank job ;-P)

He still uses the pack as a dummy pack I believe for making sure his
bots drive around etc, its pretty screwed in terms of supplying current.


My point is, you don't need to be that paranoid, these were C and larger
cells with 100-200A pulse discharge and sometimes 10A(4C) charge rates
If your worried put a temperature sensor in the pack, Its also a good
idea as a charge termination (if you can pick up the temperature spike
as the cells are about to hit charged its actually a better measure than
-dv/dt sensing)

We have mainly gone to lithiums now, in a similar environment,
Personally we use A123 packs (taken from dewalt drills) because they
don't do that whole "supply their own oxygen to a lithium fire" thing
and are sposed to be more robust than the li-polys.
That said, the same guy who melted his pack last time uses li-polys. We
are mainly concerned about physical damage to those, but personally I'd
use one of those li-poly charging bags if i was running those.



A thought for you is to perhaps look at using an unmodified drill pack
in your device if you can, that way people can easily buy batteries for
it (they do die over time) and as a result can quick swap with a few
spare packs.


2009\01\29@061925 by solarwind

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Also keep in mind that NiMH have the ability to discharge far more
rapidly than other types such as nickel cadmium.

2009\01\29@110446 by Harold Hallikainen

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I did a simple NiMH charger for a veterinary thermometer. The thermometer
has a 5 cell NiMH pack in it. It charges with a 12VDC "wall wart" power
supply. Because I already used an LTC regulator in the device, I used
another one as a charger switch (set feedback input voltage to zero so the
regulator just saturates when enabled). I use a resistor between the
incoming DC and the switch, which then drives the battery. Not quite
constant current charging, but close enough. During charging, a PIC
watches the battery voltage. If it stops rising for a few minutes, I
assume the battery is charged and turn off the charger. If the battery
voltage falls below some threshold, I start the charge cycle again. This
is "zero delta V charge termination." The PIC handling the charger is the
same one that does everything else.

I'm currently working on another project that requires charging of two AAA
cells from the AC line. I've decided to just use an external charger
(http://www.powerstream.com/NiMHWMm.htm) for this one.

Harold




--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\01\29@110934 by John Ferrell

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More information needed.....
Is this a one off personal project or a proposed product for consumer use?
AA NiMh implies 2100 to 2800 mah cells. 2 amp charger minimum.
If they are to be charged in series at least 24 to 30 volts dc will need to
be developed.

A quick & dirty solution for loose cells would be a kludge consisting of 5
four cell camera battery chargers plugged into a power strip (squid is
better) and a commercially built inverter.

You have $50 worth of batteries involved. If you need to charge them in
series some sort of balancing charger will save money in the long haul.

Please note that the new NiMh technology that has the low self discharge
rate also has a much higher internal impedance. Don't expect to be able to
get the peak power levels of the older NiMh.
Great for cameras though!

John Ferrell  W8CCW

"All that is necessary for the triumph of evil is for good men to do
nothing." -- Edmund Burke
http://DixieNC.US


{Original Message removed}

2009\01\29@112229 by Dr Skip

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Harold Hallikainen wrote:
> watches the battery voltage. If it stops rising for a few minutes, I
> assume the battery is charged and turn off the charger.

> is "zero delta V charge termination."

What percentage 'full' are they at that point? Have you had a chance to test
their capacity vs rated capacity after several cycles like that?

-Skip

2009\01\29@113739 by Harold Hallikainen

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>
>
> Harold Hallikainen wrote:
>> watches the battery voltage. If it stops rising for a few minutes, I
>> assume the battery is charged and turn off the charger.
>
>> is "zero delta V charge termination."
>
> What percentage 'full' are they at that point? Have you had a chance to
> test
> their capacity vs rated capacity after several cycles like that?
>
> -Skip
>

No, I haven't done tests on it, but it seems to work. The zero delta V
seems to be the suggested method for NiMH. NiCd often uses negative delta
V, but apparently NiMH doesn't go very negative.

There seems to be a lot of nice single chip lithium ion chargers (I've
used some LTC and Maxim ones), but for NiMH, they generally require
external transistors and external charge termination detection. That's why
I'm going external on the latest project. The whole charger is not much
more than a switching wall wart.

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\01\29@125244 by Marcel Duchamp

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Forrest W. Christian wrote:
{Quote hidden}

Something that will complicate your design here is the fact that you are
stacking 15 cells.  As the cells age they will charge at differing
rates.  This has the effect of hiding any end-of-charge behavior.  For
example, nicads peak and then droop slightly as they become full.  If
all were in sync, it would be easy to detect as the effect would be 15
times larger than for a single cell.  But when they get old, they all do
this at differing times and the effect is washed out.  And for cells
that lose capacity in old age, they charge quicker since they hold less
and then are subject to overcharging while the good cells are still
coming up.

Nate and others advice of a removable pack and oem charger is good
advice although probably more expensive.

BTW is there any good reason to choose NiMH?  Lithium technologies beat
NiMH in just about every category.

2009\01\29@165200 by apptech

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> A thought for you is to perhaps look at using an unmodified drill pack
> in your device if you can, that way people can easily buy batteries for
> it (they do die over time) and as a result can quick swap with a few
> spare packs.

I'm sure Gargoyle knows and might even tell me - but can you advise what
sort of  voltage, capacity, number of cells, and price are the A123 drill
packs? I haven't used LiFePO4 as yet, but it's looking attractive. Not
overly available here yet AFAIK (which may more reflect my state of knowing
than their state of availability).




           Russell

2009\01\29@165201 by apptech

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While we have all the NimH experts gathered ... .

Has anyone got figures for NimH energy recovery efficiency?
This is not something that is much reported on or even cared about usually.

"Current recovery efficiency" is reasonably good - you typically charge to
110% of rate mAh - although higher and much higher figures are suggested by
some.
But energy recovery =  Sum/(Vout.Iout.dT) / Sum(Vin.Iin.dT) is another
matter.
This will vary with brand and load and cell body sizeand capacity,
temperaure,  and ....

For AA NimH Vout_average is around 1.2V at loads << C and about 1.1V at
loads approaching C.

Vin varies with internal impedance, state of charge, charge rate, brand ...
Charging even < C/5 near full capacity gives Vin around  1.4 - 1.5 V for AA
cells.

That suggests efficincies of around (1.2V) / ( 1.5 x 110%/100) = ~~ 70% at
end of charge for light loads and higher at earlier stages of charge.

But nobody seems to publish the nice curves sets you see for other battery
parameters.

Any thoughts?


           Russell

2009\01\30@102828 by Bob Axtell

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We use NiMH extensively. Our packs are 12V and 18V at 3500mAH. While
lithium packs DO have power advantages, they have terrible
restrictions when shipping (your product is then  considered a
hazardous shipment). Lithiums are also VERY unforgiving when charging.
Our chargers have NO trouble seeing the "end charge" dip.

more comments below.

On Thu, Jan 29, 2009 at 10:52 AM, Marcel Duchamp
<.....marcel.duchampKILLspamspam@spam@sbcglobal.net> wrote:
{Quote hidden}

An 18V pack requires a 24VDC supply to be properly charged. You must
first BUCK-UP the automotive voltage to 24V then charge your pack.

{Quote hidden}

We have not noticed this from quality pack suppliers.

>
> Nate and others advice of a removable pack and oem charger is good
> advice although probably more expensive.
>
> BTW is there any good reason to choose NiMH?  Lithium technologies beat
> NiMH in just about every category.

Actually, extensive improvements have recently been made in NiMH technologies,
and I am totally sold on NiMH. Ask anybody that owns a Prius.

-Bob

2009\01\30@111617 by Sean Breheny

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I agree with Bob. NiMH has *somewhat* caught up to Lithium. It used to
be that Lithium was around 3x higher in energy per weight, but now
NiMH is about 2x where it was 8 years ago, so the ratio is now around
1.5x (i.e., Lithium's energy per mass is only about 1.5x that of NiMH
now)

NiMH is still considerably cheaper than Lithium type batteries. NiMH
CAN be unforgiving with regard to charging (especially repeated
overcharging, even if very slow), but the consequences of damaging an
NiMH pack are less, cost wise and potentially safety wise, than a
Lithium pack.

Lithium does have a longer service life (in terms of cycles) and a
MUCH lower self discharge rate than NiMH. However, NiMH can be stored
fully charged whereas Lithium does not like to be left above 80% SOC
for long periods of time. Also, Lithium cells cannot be charged at as
high a rate as NiMH.

By Lithium here I mean Lithium Ion/Lithium Ion Polymer. I have no
experience with Lithium Iron Phosphate yet.

Sean


>> BTW is there any good reason to choose NiMH?  Lithium technologies beat
>> NiMH in just about every category.
>
> Actually, extensive improvements have recently been made in NiMH technologies,
> and I am totally sold on NiMH. Ask anybody that owns a Prius.
>
> -Bob
> -

2009\01\31@095104 by apptech

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>I agree with Bob. NiMH has *somewhat* caught up to Lithium. It used to
> be that Lithium was around 3x higher in energy per weight, but now
> NiMH is about 2x where it was 8 years ago, so the ratio is now around
> 1.5x (i.e., Lithium's energy per mass is only about 1.5x that of NiMH
> now)

Related ramblings.
Many may be more or less correct.
Some may not be.

I noted today that people had AA LiIon rated at 780 mAh. That's about the
same energy density as a 2500 mAh NimH.

> NiMH is still considerably cheaper than Lithium type batteries.

I'd be interested in any volume pricing of reputable cells of both sorts
around the 2 - 8 Wh range.

> NiMH CAN be unforgiving with regard to charging (especially repeated
> overcharging, even if very slow),

Modern NimH tend to have zero long term trickle charge specifications.

> but the consequences of damaging an
> NiMH pack are less, cost wise and potentially safety wise, than a
> Lithium pack.

> Lithium does have a longer service life (in terms of cycles)

I'm not certain of that.
Formal LiIon specs are typically 300 to 500 cycles to 70%.
LiFePO4 is around 1000.
The OLTPC people claim 2000 cycles with liFePO4 with suitable care - mainly
omitting top and bottom of charge cycle I think.

> MUCH lower self discharge rate than NiMH.

Yes.

> However, NiMH can be stored
> fully charged

BUT older style ones will not stay that way.

> whereas Lithium does not like to be left above 80% SOC
> for long periods of time.

I believe that optimum storage point is the constant current to constant
voltage transition point which is AFAIUI at about 2/3 charge.

> Also, Lithium cells cannot be charged at as
> high a rate as NiMH.

Standard, no. Many variants are claiming some stunningly high rates. One
Japanese maker claimed a new variant with a ?5 minute? charging time within
the last year.

> By Lithium here I mean Lithium Ion/Lithium Ion Polymer. I have no
> experience with Lithium Iron Phosphate yet.

Nor I. Yet.
Lower energy density. Longer cycle life.

>>> BTW is there any good reason to choose NiMH?  Lithium technologies beat
>>> NiMH in just about every category.
>>
>> Actually, extensive improvements have recently been made in NiMH
>> technologies,
>> and I am totally sold on NiMH. Ask anybody that owns a Prius.

LiIon charge control is extremely simple for most purposes. NimH is far more
of a black art. Getting LiIon wrong can get extremely exciting.
LiIon can (almost) be managed with:

Below Vx attempt to trickle or small pulse up with care.
Above Vx and below Vmax charge at C (or less)
When Vmax is reached, charge at Vmax until current tapers to Icutoff.
Vmax 4.3V max bravery.
Vmax 4.2V mx capacity
Vmax 4.1V longer life.
Vmax 4.0V few do = very good life effects.
Charge termination at C/20 = max charge.
Charge termination at C/10 = longer life.
Charge termination at C/5 = even longer life.

_____

Empty to Vmax at C takes about 1 hour and installs about 0.66C.
Vmax taper to C/20 takes about 2 hours and adds the extra 0.33C.
Store at Vmax with no taper for best lifetime.

LiIon starts dying as soon as first used. A cell with almost no use will die
in notionally about 3 years.
Some user experience reports longer lives at low capacity.


        Russell McMahon










>
> Sean
>
>
>>
>> -Bob
>> --

2009\01\31@123147 by olin piclist

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apptech wrote:
>> By Lithium here I mean Lithium Ion/Lithium Ion Polymer. I have no
>> experience with Lithium Iron Phosphate yet.
>
> Nor I. Yet. Lower energy density. Longer cycle life.

Another important advantage is higher current.  This is a major reason they
are making inroads in electric vehicles where satisfying peak power demand
is often the tougher requirement compared to long term average power.  A123
for example is specifically targeting this market.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\01\31@143422 by John Ferrell

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Here are a few A123 pointers. Note that not all LiFePO4 are A123 quality...
http://www.rcuniverse.com/forum/forumid_450/tt.htm


http://www.slkelectronics.com/DeWalt/index.htm



http://www.modelelectronicscorp.com/products/manuals/A123_SPT_Pack_Instructions.pdf
Solderless Power



http://www.bphobbies.com/view.asp?id=A0320075&FormPost=yes&SearchText=A123&ManufacturerDBID=

I bought 4 of the 2300 packs at $17.05 each, have not put them in service
yet.



http://www.rclipos.com/A123_Cells.htm

no tabs on these



http://www.rcgroups.com/forums/showpost.php?p=8663228&postcount=110

Charging with zip cord as a dropping resistor tread carefully here...





DeWalt uses the 2300 cells and Black & Decker has the smaller cells. The B&D
cells were in Walmart stores and then disappeared.

Youtube search on A123 will amuse you for quite a while...



John Ferrell  W8CCW

"All that is necessary for the triumph of evil is for good men to do
nothing." -- Edmund Burke
http://DixieNC.US


{Quote hidden}

> --

2009\01\31@203926 by Dave Lagzdin

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2009/1/30 Bob Axtell <engineerspamKILLspamcotse.net>:
> We use NiMH extensively. Our packs are 12V and 18V at 3500mAH. While
> lithium packs DO have power advantages, they have terrible
> restrictions when shipping (your product is then  considered a
> hazardous shipment). Lithiums are also VERY unforgiving when charging.
> Our chargers have NO trouble seeing the "end charge" dip.
>

What would your charge currents be Bob?
Keeping the current high enough to see the dip yet not cook the cells
is like threading a needle IMHO.

2009\01\31@221841 by apptech

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>> ... Lithiums are also VERY unforgiving when charging.

Can you expand on that.
They generally seem to be considered rather easier to implement well than
NimH.


 Russell

2009\01\31@231742 by William \Chops\ Westfield

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On Jan 31, 2009, at 6:50 AM, apptech wrote:

> Vmax 4.1V longer life.
> Vmax 4.0V few do = very good life effects.
> Charge termination at C/5 = even longer life.

How much do you pay in capacity for going with these "long life"  
options?

BillW


'[EE] Charging NiMH batteries'
2009\02\01@003013 by Forrest W. Christian
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I had a couple of busy days and got behind on my list mail... so I'll
try to reply to some of the questions/suggestions with my thoughts in
relation to what I am looking for.

One of my primary concerns with this whole thing is flaming or
detonating batteries.   That is a primary reason why I was staying away
from the Lithium chemistry.  From what I can tell, NiMH is pretty
forgiving as far as flame goes ...  Li not so much.  I also would like
these field replaceable, so I was sort of leaning towards the >2000mAh
NiMH's.  Although the A123 products are interesting as well.

Just curious, what really is likely to happen to a NiMH cell when you
say charge it at 2C "forever"?  Does it just fail, or does it fail in a
way you wouldn't want it near your person?   I think for the most part,
we know this answer for most of the Li chemistries...but charging a Li
battery at a lower terminal voltage to provide a higher safety margin is
rather interesting.

-forrest

2009\02\01@015923 by Sean Breheny

face picon face
Do you really mean "2C"?! I guess if so, you are talking about a
failure mode of your charger. I have not tried exactly what you are
describing, but I'd guess that what would happen is that the cell
would get hot enough to melt plastic and would then "vent" (i.e., ooze
a large quantity of hot electrolyte). I don't think there would be any
fire, but there would likely be damage to the device it is in from the
temperature and the electrolyte.

Sean


On Sun, Feb 1, 2009 at 12:29 AM, Forrest W. Christian
<.....forrestcKILLspamspam.....imach.com> wrote:
> Just curious, what really is likely to happen to a NiMH cell when you
> say charge it at 2C "forever"?  Does it just fail, or does it fail in a
> way you wouldn't want it near your person?   I think for the most part,
> we know this answer for most of the Li chemistries...but charging a Li
> battery at a lower terminal voltage to provide a higher safety margin is
> rather interesting.
>
> -forrest
> -

2009\02\01@024916 by Jake Anderson

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apptech wrote:
>>> ... Lithiums are also VERY unforgiving when charging.
>>>      
>
> Can you expand on that.
> They generally seem to be considered rather easier to implement well than
> NimH.
>
>
>   Russell
>
>  
lithium polymer cells are prone to "emit flame" when overcharged.
and they are physically fragile to boot.

Basically the problem is a) lithium is flammable, b) the "electrolyte"
material produces oxygen when heated over 300C or so (from memory)

so if you over charge it and plate out some metallic lithium, it can
punch through the electrolyte and create a short, which heats to > 300C
producing oxygen which burns the lithium etc etc > boom

google lipoly fire for some pretty explosions.

The reason they are easier to implement is lithium charge is CC/CV or
basically a current limited power supply. no precision voltage detection.

2009\02\01@025511 by Jake Anderson

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apptech wrote:
>> A thought for you is to perhaps look at using an unmodified drill pack
>> in your device if you can, that way people can easily buy batteries for
>> it (they do die over time) and as a result can quick swap with a few
>> spare packs.
>>    
>
> I'm sure Gargoyle knows and might even tell me - but can you advise what
> sort of  voltage, capacity, number of cells, and price are the A123 drill
> packs? I haven't used LiFePO4 as yet, but it's looking attractive. Not
> overly available here yet AFAIK (which may more reflect my state of knowing
> than their state of availability).
>
>
>
>
>             Russell
>
>  
Typically in robots we buy Dewalt 36volt LiFePo packs (2.2Ah ~$120US
now, sometimes get them for ~$80) 10 cells to a pack off ebay, dewalt
want $400 new in Australia (or did last time i looked) you can buy loose
cells from united hobbies for $16 per cell and from A123 racing.
I wouldn't recommend using those in any gadget you give to somebody else
they are way to tricky in terms of plugging and charging etc.
They have a brick of some kind of epoxy coated electronics in them to
connect to the dewalt charger and manage the balancing.

Make sure your charger supports LiFePo cells, their CV point is 3.6 not
4.1 or whatever the lipo cells are. Do use a balancer, One of the guys
here has killed his cells and we think its due to running without the
balancer rather than the running them at 90C 8 times a day ;->.


2009\02\01@144153 by Forrest W Christian

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Sean Breheny wrote:
> Do you really mean "2C"?! I guess if so, you are talking about a
> failure mode of your charger.
Yes, I would say a failure mode.   I'm not necessarily worried about
being able to charge the batteries correctly if everything is fine, but
instead what I'm going to catch fire if something fails.   I would
envision something like a charging FET failing shorted or similar.
> I have not tried exactly what you are
> describing, but I'd guess that what would happen is that the cell
> would get hot enough to melt plastic and would then "vent" (i.e., ooze
> a large quantity of hot electrolyte). I don't think there would be any
> fire, but there would likely be damage to the device it is in from the
> temperature and the electrolyte.
I can live with "it's broken, and a melted blob".   Not so much with "it
caught on fire and burnt the building to the ground".

-forrest

2009\02\01@215500 by apptech

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>> Vmax 4.1V longer life.
>> Vmax 4.0V few do = very good life effects.
>> Charge termination at C/5 = even longer life.
>
> How much do you pay in capacity for going with these "long life"
> options?

Realtively liitle, from the graphs etc I've seen. I've yet to confirm it in
practice.

It would seem that stunning life extension MAY be available at 20% loss,
good extension at 10% loss and some at 5%.

Those who want real capacity tend to do topping charges at the end etc to
cram in more coulombs and drain out more life.

>From what I've read so far, for laptop users who want long battery cycle
life and who don't want to shave off every last gram, charging only to the
end of the current ramp at the point when constant voltage mode commences
(at about 2/3 capacity) seems the way to easily maximise longevity. I have a
reasonably bottom end dual core AMD equipped Compaq with a 12 cell battery
and battery-mode throttled down nearly as far as it can go (5% on one cpu
etc). I haven't yet found the battery life - not up to VAIO 10 hours plus
but probably 5+. Another 12 cell battery may happen for lonnng flights and
all day sessions.



   R


2009\02\01@220620 by apptech

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>> Do you really mean "2C"?! I guess if so, you are talking about a
>> failure mode of your charger.

> Yes, I would say a failure mode.   ...

> I can live with "it's broken, and a melted blob".   Not so much with "it
> caught on fire and burnt the building to the ground".

It's extremely easy to try this on some real cells (you don't even need to
look in the "Route" menu.)

Modern higher energy density NimH (say 2000 mAh+ in an AA form factor)
usually allow of zero trickle charge so 2C for any time will definitely
damage them. Given how hot they get at 1C at end of charge I imagine 2C
would not take long to do them major damage.

But, my reasonably extensive experience with NimH to date (not at 2C)
suggests that they tend to die by losing capacity and possibly physically
venting in relatively mild ways. While no doubt it can happen, I've not yet
had a NimH melt, flame, spoke, explode or anything equally exciting. At 2C
something like melting could be hoped for. A 2000 mAH AA has perhaps a
kiloWatt second of energy in it (at 50% nominal) so a bit of melting seems
entirely fair.

For those wishing to live at  a faster pace, LiIon batteries or tantalum
capacitors are recommended. (I've had the latter squeal, smoke, stink, flame
and explode all in the same session.)


Russell

2009\02\02@001929 by William \Chops\ Westfield

face picon face

On Feb 1, 2009, at 7:06 PM, apptech wrote:

> I've not yet had a NimH melt, flame, spoke, explode or anything  
> equally exciting.

NiMH internals are inherently water-based, so about the worst I'd ever  
expect is a steam-pressure explosion and spewing of nasty caustic  
solution around.

The dangers of Li-ion, as I understand them, are threefold:
1) The electrolyte is a flammable organic.
2) Lithium is flammable.
3) The Cobalt Oxide (cathode material?) in combination with metallic
   lithium, forms a nice thermite-like mixture.  Get it above the
   appropriate activation temperature, and it will "burn' even in the
   absence of other oxygen, until the resulting heat splits the casings
   and vents the flammable lithium and/or electrolyte to the  
atmosphere,
   where it will happily burn some more...

BillW


2009\02\03@162630 by Bob Axtell

face picon face
Hi, Dave, sorry about being late to reply. Answers below.


On Sat, Jan 31, 2009 at 6:39 PM, Dave Lagzdin <EraseMEdtekk.comspam_OUTspamTakeThisOuTgmail.com> wrote:
> 2009/1/30 Bob Axtell <engineerspamspam_OUTcotse.net>:
>> We use NiMH extensively. Our packs are 12V and 18V at 3500mAH. While
>> lithium packs DO have power advantages, they have terrible
>> restrictions when shipping (your product is then  considered a
>> hazardous shipment). Lithiums are also VERY unforgiving when charging.
>> Our chargers have NO trouble seeing the "end charge" dip.
>>
>
> What would your charge currents be Bob?

We charge at 2.5A to 3A charge rate. That corresponds to about 0.8C
charging rate.

> Keeping the current high enough to see the dip yet not cook the cells
>  is like threading a needle IMHO.

Well, we use a quality cell (French-made). Testing showed this charge
rate as working well in our setup.

> -

2009\02\03@164101 by Bob Axtell

face picon face
We deliberately overcharged a pack just to see what would happen.

It seemed to hold together until about 90C, then several cells began
venting and two (of 10) actually lost end caps, so the electrolyte got
loose. There was a lot of heat, but no explosion.
The electrolyte smells, but not badly. The electrolyte is similar to
"DrainO", a product designed to clear clogged pipes, so it is
corrosive.Burt after being cleaned up, the battery brackets were
perfectly usable again.

--Bob A

On Sun, Feb 1, 2009 at 8:06 PM, apptech <@spam@apptechKILLspamspamparadise.net.nz> wrote:
{Quote hidden}

> -

2009\02\03@190536 by Harold Hallikainen

face
flavicon
face

> We deliberately overcharged a pack just to see what would happen.
>
> It seemed to hold together until about 90C, then several cells began
> venting and two (of 10) actually lost end caps, so the electrolyte got
> loose. There was a lot of heat, but no explosion.
> The electrolyte smells, but not badly. The electrolyte is similar to
> "DrainO", a product designed to clear clogged pipes, so it is
> corrosive.Burt after being cleaned up, the battery brackets were
> perfectly usable again.
>
> --Bob A

Many years ago, a guy I knew worked on the design of a medical device (a
surgical bone saw) that used NiCd cells. The charge termination method
used there was a thermostat in the battery pack. When it opened, the
batteries were charged! As I recall, the -dV/dt is due to the temperature
rise, so just sensing the temperature rise might be a more direct method
of doing charge termination.

Harold



--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\02\03@194218 by Jake Anderson

flavicon
face
Harold Hallikainen wrote:
{Quote hidden}

The dv/dt is actually a result of the chemistry within the cell.
However, I believe manufacturers are now recommending dv/dt (IE watch
for a heat spike) for charge termination.

2009\02\03@200749 by Sean Breheny

face picon face
Hi Bob,

By 90C do you mean 90 degrees Celsius or do you mean 90C charge rate? (!)

Also you mention that your supplier is French. Is it Saft by any chance?

Sean


On Tue, Feb 3, 2009 at 4:34 PM, Bob Axtell <KILLspambob.axtellKILLspamspamgmail.com> wrote:
{Quote hidden}

2009\02\04@085454 by Alan B. Pearce

face picon face
The OP may like to look at the Linear technology LTC6802-1 Multicell Battery
Stack Monitor. Primarily designed for use in electric/hybrid vehicles, but
would be usable in many applications.

http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P86662

2009\02\04@105935 by Bob Axtell

face picon face
Hi Sean,

Yes, we are using SAFT now, as our new packager would not use the old
vendor (Tynergy).

They rose to 90Centigrade. We know this because we had installed the
maxim 1-wire thermometer for.

On Tue, Feb 3, 2009 at 6:07 PM, Sean Breheny <RemoveMEshb7TakeThisOuTspamcornell.edu> wrote:
{Quote hidden}

> -

2009\02\04@144534 by Michael Algernon

flavicon
face
>
> On Feb 4, 2009, at 6:55 AM, Alan B. Pearce wrote:
>
> The OP may like to look at the Linear technology LTC6802-1 Multicell  
> Battery
> Stack Monitor. Primarily designed for use in electric/hybrid  
> vehicles, but
> would be usable in many applications.
What does "OP" mean ?
>
>
> http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P86662
>

2009\02\04@150807 by olin piclist

face picon face
Michael Algernon wrote:
> What does "OP" mean ?

Original Poster.  The person that started the mail list thread.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\02\04@164833 by Russell McMahon

face
flavicon
face
> The dv/dt is actually a result of the chemistry within the cell.
> However, I believe manufacturers are now recommending dv/dt (IE watch
> for a heat spike) for charge termination.

Typo above I think.
dT/dt maybe.
dT/dt is always positive as far as I know

Absolute temperatur wrt ambient is a good metric for charge termination for
moderately high charge rates. ie C/10 or less may not be so good.



  Russell

2009\02\04@164834 by Russell McMahon

face
flavicon
face
> Yes, we are using SAFT now, as our new packager would not use the old
> vendor (Tynergy).

Can I ask why? (answer on or odd list OK).
Tenergy LOOK quite good.
They appear to be a repackager of various Chinese factories products BUT
they also seem to know how to get a quality product.



  Russell

2009\02\04@170433 by Dave Schmidt

flavicon
face

> Can I ask why? (answer on or odd list OK).
> Tenergy LOOK quite good.
> They appear to be a repackager of various Chinese factories products BUT
> they also seem to know how to get a quality product.
>  
I've been an end user of Tenergy stuff mostly for RC use and I find that
their mAH ratings are seriously overrated and diminishes quickly with
10's of cycles.  Their 2600mAH AA's come up ~ 1700mAH, their sub C
3800mAH cells measure ~ 2200 - 2600mAH.  This is as measured on my CBAII
battery analyzer.

Dave

2009\02\04@171619 by Funny NYPD

picon face
I am just curious how long those secondary battery last in a lifetime? 500 time recharge? or more?

Funny N.
Au Group Electronics, http://www.AuElectronics.com




________________________________
From: Russell McMahon <TakeThisOuTapptechEraseMEspamspam_OUTparadise.net.nz>
To: Microcontroller discussion list - Public. <RemoveMEpiclistspamTakeThisOuTmit.edu>
Sent: Wednesday, February 4, 2009 4:43:26 PM
Subject: Re: [EE] Charging NiMH batteries

> Yes, we are using SAFT now, as our new packager would not use the old
> vendor (Tynergy).

Can I ask why? (answer on or odd list OK).
Tenergy LOOK quite good.
They appear to be a repackager of various Chinese factories products BUT
they also seem to know how to get a quality product.



  Russell

2009\02\04@171827 by Alan B. Pearce

face picon face
>What does "OP" mean ?

It has been used here a lot to mean 'Original Poster' ...

2009\02\04@185716 by Jake Anderson

flavicon
face
Russell McMahon wrote:
>> The dv/dt is actually a result of the chemistry within the cell.
>> However, I believe manufacturers are now recommending dv/dt (IE watch
>> for a heat spike) for charge termination.
>>    
>
> Typo above I think.
> dT/dt maybe.
> dT/dt is always positive as far as I know
>
> Absolute temperatur wrt ambient is a good metric for charge termination for
> moderately high charge rates. ie C/10 or less may not be so good.
>
>
>
>    Russell
>
>  
yeah, read what I mean not what i say dammit ;->
nimh have a pretty rapid temperature spike when they hit charged. Even
at moderately low charge currents, though perhaps not C/10, though i'd
expect a degree or 2 even then.

2009\02\05@010409 by Dave Lagzdin

picon face
2009/2/3 Bob Axtell <bob.axtellEraseMEspam.....gmail.com>:
> Hi, Dave, sorry about being late to reply. Answers below.
>
>
> On Sat, Jan 31, 2009 at 6:39 PM, Dave Lagzdin <EraseMEdtekk.comspamgmail.com> wrote:
>> 2009/1/30 Bob Axtell <RemoveMEengineerEraseMEspamEraseMEcotse.net>:
>>> We use NiMH extensively. Our packs are 12V and 18V at 3500mAH. While
>>> lithium packs DO have power advantages, they have terrible
>>> restrictions when shipping (your product is then  considered a
>>> hazardous shipment). Lithiums are also VERY unforgiving when charging.
>>> Our chargers have NO trouble seeing the "end charge" dip.
>>>
>>
>> What would your charge currents be Bob?
>
> We charge at 2.5A to 3A charge rate. That corresponds to about 0.8C
> charging rate.
>
That matches our conclusions/experiments.

>> Keeping the current high enough to see the dip yet not cook the cells
>>  is like threading a needle IMHO.
>
> Well, we use a quality cell (French-made). Testing showed this charge
> rate as working well in our setup.
>

We initially tried a very popular RC charger which only used the dip.
Inner cells got hot enough to melt the sleeves , short themselves and
their neighbors.
Built our own after that.

>> --

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