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'Sensing Low Battery Condition (Thanks!!)'
1997\10\14@155957 by Ricardo Seixas

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       I'd like to thanks all that replyed to my question.
       In this circuit I'll use the method described by Mike Smith
to store "battery used time" since I'm using nicad.
       Using one or two EEPROM locations I can create a counter that
increment every minute and, when a determined count (time) is reached
I can display a message with a safety margin to recycle the battery.
       When the battery is recharged I can press a button before turn-on
the device then, at power-up the start routine check the button and if
pressed reset the counter.
       Another thing that flashed my mind is to use the EEPROM counter as a
safety device so, when a preset used time (or number of power-up's)is
reached the user at the next power-up MUST press a pre-determined sequence
in the buttons, otherwise the device becomes useless.
       To increment the counter on EEPROM every minute is no problem since
1x10^6 cycles (EEPROM safety margin)  = 694 days, 24 hours a day and, the
circuit is used 2 or 3 times a day (one hour max.).
       Note, this is note a comercial device.
       Now, another question ...

       Do nicads self-discharge when not in use ?
               If yes Then
                       this method becomes half useless
               End If
       Loop

Thanks again.

Ricardo Seixas

1997\10\14@163705 by WF AUTOMA‚̀O

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Ricardo Seixas wrote:
{Quote hidden}

Seixas,

O gravador PICgrammer vai indo bem? Qual m‡quina voc est‡ utilizando? Um Pentium?

Miguel.

1997\10\14@172432 by Steve Baldwin

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>         Do nicads self-discharge when not in use ?
>                 If yes Then
>                         this method becomes half useless
>                 End If
>         Loop

Just happen to have the right book next to me.

"Nicads have a self discharge rate of about 0.5% per day at 23 deg C when
near full charge and retain 20% to 40% capacity after 5 months. At 30 deg
C, 505 to 80% of
the charge is retained after 30 days."

Steve.

1997\10\15@012712 by tjaart

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Steve Baldwin wrote:
>
> >         Do nicads self-discharge when not in use ?
> >                 If yes Then
> >                         this method becomes half useless
> >                 End If
> >         Loop
>
> Just happen to have the right book next to me.
>
> "Nicads have a self discharge rate of about 0.5% per day at 23 deg C when
> near full charge and retain 20% to 40% capacity after 5 months. At 30 deg
> C, 505 to 80% of
> the charge is retained after 30 days."

This is true. We also use NiCds and have found the only way of getting
any meaningful indication is to measure the voltage difference between
discharging the battery for 10s at around 0.1C The voltage means
*nothing*.
Even the ambient temperature affects the discharge rate quite a bit.

--
Friendly Regards

Tjaart van der Walt
spam_OUTtjaartTakeThisOuTspamwasp.co.za
_____________________________________________________________
| WASP International http://www.wasp.co.za/~tjaart/index.html |
|       R&D Engineer : GSM peripheral services development    |
|   Vehicle tracking | Telemetry systems | GSM data transfer  |
|    Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973    |
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1997\10\15@042552 by tjaart

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Scott Walsh wrote:
{Quote hidden}

That is also a possibillity (the negative dV/dT method). This is a good
way
to decide when to stop fast charging. However, if you just need to know
the
current charge, do the following :
1) Measure V
2) Discharge for 10s at 0.1C
3) Measure V
The difference voltage as a percentage of 1) is a good indication of
current
charge level. (not perfect by any means, but good)

Don't do this often though, because you loose charge, and you may
deteriorate
the capacity through the 'memory' effect. This memory effect is claimed
to be
something of the past, but it is not (anyone with a cellphone should
know this).

The two greatest enemies of NiCd's :
1) Heat (ambient or from charging) - permanent loss of capacity
2) Cold - charge loss

--
Friendly Regards

Tjaart van der Walt
.....tjaartKILLspamspam@spam@wasp.co.za
_____________________________________________________________
| WASP International http://www.wasp.co.za/~tjaart/index.html |
|       R&D Engineer : GSM peripheral services development    |
|   Vehicle tracking | Telemetry systems | GSM data transfer  |
|    Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973    |
|              WGS-84 : 26010.52'S 28006.19'E                 |
|_____________________________________________________________|

1997\10\15@074059 by Mike Smith

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-----Original Message-----
From: Ricardo Seixas <rseixasspamKILLspamCICLONE.COM.BR>
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Date: Wednesday, 15 October 1997 5:35
Subject: Sensing Low Battery Condition (Thanks!!)


{Quote hidden}

Yes, somewhat, but the specs sheets should give you enough data to include
this effect.  Use a sleep system / RTC to keep track of time.

MikeS
<mikesmith_ozspamspam_OUTrelaymail.net>

1997\10\15@082910 by tjaart

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Scott Walsh wrote:
>
>      Doing the measurement every 10s or so at 0.1C discharge, is not a
>      problem for me. I have a 650mAh battery pack being discharged at 60mA
>      is this 0.09C ?
>
>      So if you do use this method, what is the change in voltage between
>      each 10s sample that you use to indicate a battery that is failing?
>      Eeek, hold on, I know this depends on the number of cells in your
>      battery, but hit me with the info anyway? I have three NiMH cells!
>
>      kind regards,
>      SW.

Whoa! NiMH is not NiCd! Therefore the charge/discharge curves are not
the
same.

To give you some indication for NiCds, if you take the difference as a
percentage of the starting voltage, over 95% is full, and below 50%
becomes dicey.

I haven't tried it on NiMH cells, but it would be interesting to see how
well it works.

--
Friendly Regards

Tjaart van der Walt
@spam@tjaartKILLspamspamwasp.co.za
_____________________________________________________________
| WASP International http://www.wasp.co.za/~tjaart/index.html |
|       R&D Engineer : GSM peripheral services development    |
|   Vehicle tracking | Telemetry systems | GSM data transfer  |
|    Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973    |
|              WGS-84 : 26010.52'S 28006.19'E                 |
|_____________________________________________________________|

1997\10\15@120628 by Mike Keitz

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On Tue, 14 Oct 1997 17:58:00 -0300 Ricardo Seixas
<KILLspamrseixasKILLspamspamCICLONE.COM.BR> writes:
>
>        I'd like to thanks all that replyed to my question.
>        In this circuit I'll use the method described by Mike Smith
>to store "battery used time" since I'm using nicad.

I believe in the original post you said the project was a portable
measuring device.  What does your device do as the battery runs down?
Does it (a) just quit all of a sudden, or (b) give inaccurate readings
when the battery voltage is low?  The latter condition is very bad.
Others have proposed schemes to give an early warning that the battery is
failing.  This is somewhat difficult on NiCds and may not be of much use
anyway.  Users will correctly deduce that a mode (a) condition is a dead
battery.  But you need the circuit designed to guarantee that
mis-operation in mode (b) will not happen.  Regardless of the battery
type, this is done with a simple absolute voltage detector or design of
the circuit so it always goes to mode (a) first.

>        Using one or two EEPROM locations I can create a counter that
>increment every minute and, when a determined count (time) is reached
>I can display a message with a safety margin to recycle the battery.
>        When the battery is recharged I can press a button before
>turn-on
>the device then, at power-up the start routine check the button and if
>pressed reset the counter.

This assumes that the battery will be fully recharged every time.  The
time needs to be decreased as the battery loses capacity due to age, or
you could set it rather short from the outset so significant capacity
will be sure to remain.  This will lead users to ignore the warning
though.

Users often only charge enough to get a little more use.  This is rough
on the battery, but what do they know?  Also they may have to if they
can't afford the time to take the device out of service for a full
recharge.  They may ignore the warning and continue using until the
battery is dead.  That's why it needs to be designed to stop gracefully
when the battery expires.

1997\10\16@072953 by Mike Smith

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-----Original Message-----
From: Mike Keitz <RemoveMEmkeitzTakeThisOuTspamJUNO.COM>
To: spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Date: Thursday, 16 October 1997 1:37
Subject: Re: Sensing Low Battery Condition (Thanks!!)


{Quote hidden}

My suggestion was to use a dedicated battery minder chip - these monitor
degradation of battery with time, and measure discharge via a series
resistor temp transducer.  (they also keep battery charged nicely:)

MikeS
<mikesmith_ozEraseMEspam.....relaymail.net>

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