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'[PIC]: NiMH battery chargers'
2001\04\26@144629 by Lawrence Lile

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Bob,
I noted you have a nice schematic on your page for a NiMH battery charger.
I'm trying to figure out the theory on these.  You seem to advocate having a
temperature sensor on the battery (this is possible, since my batteries will
be built in to the product)  and checking for excessive temperature rise.
Charge at a constant current, then quit at a fixed temperature rate of rise.
Not too tough, on the face of it.

Other sources I've seen seem to indicate monitoring battery voltage - when
the voltage doesn't rise so much for a constant amperage, you're charged,
and you can quit.

What's the pros and cons?

Some chargers go into trickle charging after this, but another site
recommends not using trickle charging, saying it shortens battery life.  Any
thoughts?


-- Lawrence Lile

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2001\04\26@152121 by Eisermann, Phil [Ridg/CO]

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Panasonic has charging information on their website for NiMH (and other
types). Look for their NiMH handbook. If you can't find it, i can email you
a copy privately.

Among other things, the handbook says that trickle charging using larger
than recommended current  (1/30 to 1/20CmA), or overcharging even by trickle
charge, deteriorates battery characteristics. By overcharging with trickle,
i think they mean a length of time. It's in the book somehwere, i'm sure.



{Original Message removed}

2001\04\26@152750 by Milan Pavlica (YU7AEC)

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part 0 44 bytes
his is a multi-part message in MIME format.
part 1 2117 bytes content-type:text/plain; charset=koi8-r (decoded 7bit)

Also, i have some project of NiMh chrger with PIC....
If someone is interested i can send a copy of schematic and software..
Can you please email me privately copy of that book?
Thanks!

"Eisermann, Phil [Ridg/CO]" wrote:

> Panasonic has charging information on their website for NiMH (and other
> types). Look for their NiMH handbook. If you can't find it, i can email you
> a copy privately.
>
> Among other things, the handbook says that trickle charging using larger
> than recommended current  (1/30 to 1/20CmA), or overcharging even by trickle
> charge, deteriorates battery characteristics. By overcharging with trickle,
> i think they mean a length of time. It's in the book somehwere, i'm sure.
>
> {Original Message removed}
part 2 201 bytes content-type:text/x-vcard; charset=koi8-r;
(decoded 7bit)

begin:vcard
n:Pavlica;Milan
x-mozilla-html:FALSE
org:SuperSonic Systems
adr:;;;;;;
version:2.1
email;internet:.....mpavlicaKILLspamspam@spam@ptt.yu
title:Chief
fn:Milan Pavlica
end:vcard


part 3 105 bytes
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2001\04\26@155855 by Lawrence Lile

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In case Bob hasn't beat me to the punch,


http://www.bobblick.com/bob/stamp/charger/

-- Lawrence Lile


----- Original Message -----
From: "Duane Brantley" <.....Duane.BrantleyKILLspamspam.....genband.com>
To: "'Lawrence Lile'" <EraseMEllilespam_OUTspamTakeThisOuTtoastmaster.com>
Sent: Thursday, April 26, 2001 1:50 PM
Subject: RE: [PIC]: NiMH battery chargers


> What is the URL to this site?  I was looking for just a circuit.
>
> Thanks,
>
> Duane
>
> {Original Message removed}

2001\04\26@160335 by Lawrence Lile

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Looks like the handbook is at:

ttp://www.panasonic.com/industrial_oem/battery/battery_oem/chem/nicmet/nicme
t.htm


-- Lawrewnce Lile
Lile's Integrated 'Lectronic Engineering Services

----- Original Message -----
From: "Eisermann, Phil [Ridg/CO]" <peisermaspamspam_OUTRIDGID.COM>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, April 26, 2001 2:08 PM
Subject: Re: [PIC]: NiMH battery chargers


> Panasonic has charging information on their website for NiMH (and other
> types). Look for their NiMH handbook. If you can't find it, i can email
you
> a copy privately.
>
> Among other things, the handbook says that trickle charging using larger
> than recommended current  (1/30 to 1/20CmA), or overcharging even by
trickle
> charge, deteriorates battery characteristics. By overcharging with
trickle,
> i think they mean a length of time. It's in the book somehwere, i'm sure.
>
>
>
> {Original Message removed}

2001\04\26@183841 by Bob Blick

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Hi Lawrence,

Only chargers that do fast or quick charging need this.

Both nicad and nimh batteries show a peak in the voltage at full charge,
then drop. In nimh it is about half as noticeable. If you have just a few
cells and they are well matched, you can detect the peak and use that as
your indicator, because it will come before temperature rise. You
absolutely must measure temperature because the peak method is not
infallible. And temperature is the biggest killer of nicad and nimh
batteries.

I didn't have much luck doing peak detection on a 10 cell nimh pack. One
thing that I did recently was measure the battery and also the ambient
temperature, quit charging when the difference was greater than 6 deg C.

If you do the peak detection, you should probably switch to trickle, if
you use temperature, they're definitely full and probably don't want any
more charge.

Keep them as cool as possible, if your charger itself produces heat, try
to prevent it getting transmitted to the battery.

Cheerful regards,

Bob

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2001\04\27@094007 by Lawrence Lile

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This is really valuable info, Bob!

I'm thinking of just pasting a dallas 1-wire temperature sensor on my
battery pack, and another on the circuit board, which will be remote.  That
will get me a measure of the battery vs ambient temperature.

I'm going to have a 7.2 volt stack, several in parrallel, so I'll stay away
from using the peak voltage method, as you said.

How well does the voltage into a charging NiMH need to be regulated?  I am
planning on using an unregulated 12V supply only regulating the current ,
not the voltage.    The rated amp-hour capacity of my battery stack will be
about 5-6 amp-hours.  I understand that is also the maximum quick-charge
current.

Then I'll have a pass transistor that can turn off the power either briefly
( to measure actual battery voltage) or for good.  Won't need trickle
charging, this device will be charged, then taken to the field, then brought
back for charging.

I could also charge off some solar cells.  These will have a max. capacity
of only an amp at best.  I'm not sure how this lower-current charger would
work, though.  Would the temperature method still work at 1/6th the
battery's mAH rating?  Or do I also need to use the voltage method?  Seems
like if you pump in some current, and deltaV is less than zero, zero, or
just a little above zero, then you can't add any more juice.  If you could
hit the flat spot just before it peaked, by stopping at a certain minimum
slope of voltage, that might save the battery from early death??


Lawrence Lile
Lile's Integrated 'Lectronic Engineering Services



{Original Message removed}

2001\04\27@112357 by Bob Blick

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> I'm thinking of just pasting a dallas 1-wire temperature sensor on my
> battery pack, and another on the circuit board, which will be remote.  That
> will get me a measure of the battery vs ambient temperature.

One of these days I'll get some of those, they sound real cool and I'm
tired of thermistors :-)

> How well does the voltage into a charging NiMH need to be regulated?  I am
> planning on using an unregulated 12V supply only regulating the current ,
> not the voltage.    The rated amp-hour capacity of my battery stack will be
> about 5-6 amp-hours.  I understand that is also the maximum quick-charge
> current.

That's fine, although it does mean you'll get less efficient charging(but
it's how almost everyone does it except laptops that already have a
regulated power supply). Figure out what your peak current is and make
sure the battery and your connectors are rated for it.

> Then I'll have a pass transistor that can turn off the power either briefly
> ( to measure actual battery voltage) or for good.  Won't need trickle
> charging, this device will be charged, then taken to the field, then brought
> back for charging.

You can also measure the peak-to-peak voltage and get an idea of what's
going on as the unregulated supply goes on and off 120 times per second.

> I could also charge off some solar cells.  These will have a max. capacity
> of only an amp at best.  I'm not sure how this lower-current charger would
> work, though.  Would the temperature method still work at 1/6th the
> battery's mAH rating?  Or do I also need to use the voltage method?  Seems
> like if you pump in some current, and deltaV is less than zero, zero, or
> just a little above zero, then you can't add any more juice.  If you could
> hit the flat spot just before it peaked, by stopping at a certain minimum
> slope of voltage, that might save the battery from early death??

At 1/6 you still get temperature rise although it will not get more than a
few degrees over ambient.

One thing I noticed about voltage, during the first 15 minutes of charging
you get a peak effect. That was another reason the peak effect didn't work
for me, it seemed like I'd spend a whole lot of effort trying to
characterize a battery, only to find it's really a moving target. I think
I have something perfect and then it stops charging after 10 minutes, or
charges until thermal overload.

One important thing I learned was that RA4 on a pic, being an open-drain
output, has no diode to V+, so you can use it as an output to drive a PNP
transistor or P-channel MOSFET on a high side up to 12 volts. This works
great for about a week or so, then RA4 doesn't work any more. I have a
feeling it's because the input circuitry is still connected and nasty
things happen over time. So don't do it :-)

Cheerful regards,

Bob Blick

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2001\04\27@123446 by David Cary

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Dear battery charger people:

[When does this topic move to [EE]: ?]

Bob Blick <RemoveMEbobTakeThisOuTspamTED.NET> on 2001-04-27 10:22:28 AM had several good points, then
mentioned something that triggered an old memory:
> You can also measure the peak-to-peak voltage and get an idea of what's
> going on as the unregulated supply goes on and off 120 times per second.

Measuring the voltage across the battery this way gives you
the ESR (effective series resistance) of the battery.
ESR = (charging_voltage - no_load_voltage) / ( charging_current ).

I hear that some battery chargers periodically interrupt charging to measure the
(no-load) voltage of the battery, then *discharge* the battery for a few dozen
milliseconds at some fixed current (or some precision resistance), measuring the
voltage-under-load of the battery. This also gives the ESR of the battery. (same
equation, but plug in a negative charging_current ... ESR still ends up
positive.).

I know that both kinds of ESR are very low in a freshly-charged battery, and a
completely drained battery has a very high ESR (both kinds). They are not
exactly the same, but I don't know why.

From the point of view of using a battery under high-current conditions (RC auto
racing sprints), the "ESR while discharging" is crucial -- that's the main
bottleneck in sucking the energy out of the battery as quickly as possible and
turning it into SPEED.

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David Cary

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2001\04\27@130458 by Bill Westfield

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Is there any way to keep NiMH (or NiCd, for that matter) batteries 'topped
off' so that I can pull them out of a charge and get a full XXX mAH out of
them WITHOUT overheating/overcharging/etc and descreasing overall expected
lifetime?

BillW

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2001\04\28@025253 by Damon Hopkins

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Lawrence Lile wrote:
<SNIP>
> How well does the voltage into a charging NiMH need to be regulated?  I am
> planning on using an unregulated 12V supply only regulating the current ,
> not the voltage.    The rated amp-hour capacity of my battery stack will be
> about 5-6 amp-hours.  I understand that is also the maximum quick-charge
> current.

Not necessarily.. that's be a 1 C charge rate.. My R/C Car batteries
charge up in 15 minutes for a 1.7Ah pack at 6A thats a charge rate of
1.5C
Not sure how that affects the battery life though :) it IS better to
stick to 1C or better I think most manufact. recommend something like
1/2 C.

{Quote hidden}

or never charge the battery up all the way.. the DeltaV for NiMH
batteries is something like 10x smaller than for NiCads. I think it's
something like 1 mv vs. 10 mv but I can't remember.

               Damon Hopkins

> Lawrence Lile
> Lile's Integrated 'Lectronic Engineering Services
>
> {Original Message removed}

2001\04\28@031559 by Terry

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Bob, did you try using a string of 1N4148 diodes in the packs as
temperature sensors?

At 03:22 PM 4/27/01 +0000, you wrote:
{Quote hidden}

brought
{Quote hidden}

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2001\04\28@031615 by Terry

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I'd try Periodic Current Reversal for charging and dropping the current for
trickle. Pure DC is great for growing dendrites that'll short out NiCd cells.

At 10:03 AM 4/27/01 PDT, you wrote:
{Quote hidden}

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2001\04\28@124425 by Bob Blick

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At 03:13 PM 4/28/2001 +0800, you wrote:
>Bob, did you try using a string of 1N4148 diodes in the packs as
>temperature sensors?

I haven't used diodes for a long time, mostly because even with 5 of them
you only get 10mV/deg. Thermistors have such a big change it makes them
resistant to noise, but they are so nonlinear. A digital output sensor is
very appealing. The Dallas ones seem good for remote applications(never
used them) and National make a frequency output one(never used them). Both
look to be pretty useful, if a bit expensive.

Cheerful regards,

Bob

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2001\04\28@171346 by Bill Westfield

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    Thermistors have such a big change it makes them
   resistant to noise, but they are so nonlinear.

If you're looking for a significant change in dT/dt to detect end-of-charge,
why do you need "linear" ?

BillW

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2001\04\28@180416 by Bob Blick

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At 02:12 PM 4/28/2001 PDT, you wrote:
>     Thermistors have such a big change it makes them
>    resistant to noise, but they are so nonlinear.
>
>If you're looking for a significant change in dT/dt to detect end-of-charge,
>why do you need "linear" ?

Because depending on the temperature, 1 degree can be 100 ohms or 1000. The
thermistors I and most others use are 10000 ohms at 25 deg C. They change
-3.87% per degree C. If you use a pic with A/D and put the thermistor to
ground and use a pullup resistor then things don't work out so bad
voltage-wise, but it's very luxurious to think about using a sensor that
just tells you the temperature. So far I have not done designs with that
much $$ to throw away, doing it in software is easier and a $0.20
thermistor is the right fit.

Cheerful regards,

Bob

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'[PIC]: NiMH battery chargers'
2001\05\01@104549 by Lawrence Lile
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National has a whole set of app notes devoted to NiMH chargers:


http://www.national.com/pf/LM/LM3647.html

There are several good app notes:


http://www.national.com/pf/LM/LM3647.html#Applications


And You can buy a demo board all built up for $59  (add up your time
soldering)
http://store.national.com/natsemi/lm3647eval.html

that will implement a battery charger with fast charge, reverse battery
protection, top-off charge, sensing of shorted battteries, and a number of
other cool features.


I'm warming up my soldering iron now...


-- Lawrence Lile

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