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'[EE] Re: Detection of Alkaline batteries in a char'
2005\08\15@222821 by Vasile Surducan

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On 8/15/05, Vasile Surducan <spam_OUTpiclist9TakeThisOuTspamgmail.com> wrote:
> On 8/15/05, Wouter van Ooijen <.....wouterKILLspamspam@spam@voti.nl> wrote:
> > > Anyone know of a method of detecting if a battery that is
> > > inserted into a charger is alkaline or nicad/niMH?
> >
> > apply a square-wave load, observe the voltage difference, estimate
> > source resistance? AFAIK the source resistance of NiCad is *much* lower
> > than a compareable alkaline. But you might get introuble distinguishing
> > a very small NiCad from a large alkaline.
>
>   Apply a square wave when ? after a complete charge-discharge cicle
> of the NiMh when it's fully charged, after 30 minutes with heavy load
> on the battery, or when the battery is completely discharged? When it
> have 60C or when have -10C ?
> Source resistance of a good NiMh or NiCd is modifying with about 5-10
> x between these situations. So everything is pure fantasy.

Let me detail a little. A battery is not a passive component. You
can't measure it's internal resistance r (that one gaven by Ohm low I
= U /( R+r)) considering it a passive R. It's a dinamic resistance
which depends on many factors, the heaviest one is the load current
and internal temperature (which is much higher than the one measured
by the external temperature sensor of  the charger).
The r variation is so high, that you even can't compare a bad
rechargeable battery from a good one if both of them passed the
regeneration cycle (3 time charge-discharge with no more than 0.2C and
charging discharging time more than 10-12 hours).
The principle used for measurement is r = du/di as Wouter said, but
as many current range you'll chose for i, as many r you'll measure.
So, after a bunch of measurements stored on your computer you have to
average them in a convenient way.

If you are a researcher and need urgent to publish an issue into a
paper, then you could claim you've measured the internal resistance of
a NiMH cell with an accuracy of 0.1%. If you don't do this, no one
else from the comunity believes you. Lucky with the reference which
believes you without testing...or beying far away from the problem.

If you are a battery producer, then you can't lie with more than 3%,
because the competitors will show at you with fingers and laugh about
you.

Finally if you are indeed a technician and you don't want to lie
yourself, nor the friends, you say the truth: internal resistance of a
battery  can be measured with an accuracy of about 20% and this if you
choose enough restrictions for the load and temperature margins.

best regards,
Vasile

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