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PICList Thread
'[PIC]:PWM Battery Charger ?'
2002\02\07@061556 by kben

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Hi Everyone,
I am attempting to build a smart battery charger for
NiMH batteries (8-AA), with a F877. I am going to
use PWM to control a LM317, and sample the voltage
with the ADC to look for the delta V.
I was planning on: (I got these from Panasonic)
trickle charge .05C which would be 5% duty cycle
Soft charge    .20C which would be 20% duty cycle
fast charge    .75C which would be 75% duty cycle
I have a couple questions:
1.) What frequency should the PWM be ?
2.) Anybody know a simple circuit to control the
LM317 with the PWM PIC pin ?
3.) How often should I use the ADC to look for the
delta V to turn off or switch to trickle charge ?
Any help would be appreciated.

Thanks,
       Kevin
PS. I am a programmer not an EE, if you couldn't tell from above :)

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2002\02\07@074017 by Simon-Thijs=20de=20Feber?=

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Hi Kevin,

Check this site :

http://www.angelfire.com/electronic/hayles/charge1.html

He did something same.
I am designing a similar circuit and have not even
asked tehse questions myself so i am interested in
ideas of others / your results.

grtz

Simon

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2002\02\07@090046 by David VanHorn

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At 04:25 AM 2/7/02 -0500, Kevin A. Benedict wrote:
>Hi Everyone,
>I am attempting to build a smart battery charger for
>NiMH batteries (8-AA), with a F877. I am going to
>use PWM to control a LM317, and sample the voltage
>with the ADC to look for the delta V.
>I was planning on: (I got these from Panasonic)
>trickle charge .05C which would be 5% duty cycle

This is rather hot for NIMH batteries. C/300 is what you want.

>Soft charge    .20C which would be 20% duty cycle

This is ok for a timed top-off, but don't plan on detecting peak.

>fast charge    .75C which would be 75% duty cycle

This may cause problems with the peak as well. C is better.

>I have a couple questions:
>1.) What frequency should the PWM be ?

Fast enough for your inductor.

>2.) Anybody know a simple circuit to control the
>LM317 with the PWM PIC pin ?

Why use the 317? Just do a PWM buck regulator.
With a nice P channel mosfet, and 2N3904 pre-driver, it will run basically
cold.

>3.) How often should I use the ADC to look for the
>delta V to turn off or switch to trickle charge ?
>Any help would be appreciated.

This will depend on charge rate, but once a minute is good.
You want better than 10mV resolution.

You also want to watch absolute, and delta temperature on the pack.  Fast
charging above or below temperature limits is bad, and a certain delta T /
delta time is indicating the beginning of overcharge.

I'm doing the same thing with an AVR Tiny-15.
Targeting a 30 minute charge.

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2002\02\07@090258 by Milan Pavlica YU7AEC

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part 0 44 bytes
his is a multi-part message in MIME format.
part 1 1848 bytes content-type:text/plain; charset=koi8-r (decoded 7bit)

I am also interested...
maybe we can join and use both EE and programmer experience...
See ya...

Simon-Thijs de Feber wrote:

{Quote hidden}


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(decoded 7bit)

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title:Chief
fn:Milan Pavlica
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part 3 144 bytes
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2002\02\07@124446 by bblick

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> I am attempting to build a smart battery charger for
> NiMH batteries (8-AA), with a F877. I am going to
> use PWM to control a LM317, and sample the voltage
> with the ADC to look for the delta V.

No, that will not work. It may work a few times, but with 8 cells you will
get a good peak only when the cells match perfectly. They won't do that
very often. Also delta-V on NiMH is pretty mild. Works OK on NiCd.

Cheerful regards,

Bob Blick

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2002\02\07@125247 by David VanHorn

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At 09:42 AM 2/7/02 -0800, bblick wrote:
> > I am attempting to build a smart battery charger for
> > NiMH batteries (8-AA), with a F877. I am going to
> > use PWM to control a LM317, and sample the voltage
> > with the ADC to look for the delta V.
>
>No, that will not work. It may work a few times, but with 8 cells you will
>get a good peak only when the cells match perfectly. They won't do that
>very often. Also delta-V on NiMH is pretty mild. Works OK on NiCd.

You need to feed the batts a constant current, and look at both dV/dT and
dT/dT (temperature)

NIMH dosen't necessarily give you a peak or droop, so much as flatten to
zero delta for a while.

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2002\02\07@132719 by Milan Pavlica YU7AEC

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part 0 44 bytes
his is a multi-part message in MIME format.
part 1 1213 bytes content-type:text/plain; charset=koi8-r (decoded 7bit)

Hey, Bob Blick, i have used your Basic Stamp sch, - i tried to do it with
PIC16F84 & PICBASIC...
Can i send you a software & schematic to check is it OK???
It cold be nice charger - basic for further development...

David VanHorn wrote:

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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 3 144 bytes
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