Re: My Battery Monitor

["Eric Chang" <ericchang@my-deja.com>]

I tried something like this.  It was not so bad.  The main problem was
that the batteries that it identified as poor could be returned for
a while, but soon the vendor would only take a battery back if it was in
really bad shape.  Eventually, I had a reversal in one of the
batteries being monitored.  Most of these oscillator circuits do not
mind input being reversed, but they do mind power being reverse
polarized.  Accuracy, stability, and all kinds of other small issues
went out the window since the affected module never worked again,
even after the (only temporarily) reversed battery was replaced.

>The idea is sound; convert voltage into frequency, which is easy to
>optically couple, and easy to measure with a microcomputer. However, the
>choice of parts is poor.

>The 555 is a good general-purpose timer IC, but not good for precision
>measurements. There is a lot of variation from IC to IC, and they drift
>a lot with temperature. There is no voltage reference in the circuit;
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ yes there is
 it
>depends on the power supply, so it would have to be a precise
                                 ^^^^^^^^^^^^^ not necessarily
 voltage.

>And, the capacitors and resistors would have to be accurate and
>temperature-stable. This will be particularly hard for the 4.7uF
>capacitor.

You are right about this one.  There is no need for a 4.7uF
capacitor in this circuit, and I don't see why anybody would use one





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