Ahhhh!

     I am also trying to define a way of implementing a 'low battery'
     detector.

     I have set up a GPIB system to make LOTS of measurements on a battery
     discharging, and it is becoming clear that just measuring an absoulte
     voltage to determine that the battery is about to fail is pretty bad.
     The behaviour of one battery pack to the next means that if using a
     voltage threshold as the decision for sensing the battery low
     condition that I will get from 1.5 minutes to 10minutes difference.

     I have come to the conclusion that meauring the rate of change of
     voltage drop is a MUCH better way. And is this what you are saying?
     Measure the voltage every 10 or 20 seconds or so and look at the
     difference between samples. This way you are going to 'quite' reliably
     going to find the 'knee' in the discharge curve of the battery just
     before it starts plumeting and your supply voltage becomes
     unregulated.

     regards,
     SW.


______________________________ Reply Separator _________________________________
Subject: Re: Sensing Low Battery Condition (Thanks!!)
Author:  tjaart@wasp.co.za at INTERNET
Date:    15/10/97 07:28


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
mailto:tjaart@wasp.co.za
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