Subject:
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RE: Low battery detection
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Wed, 25 Mar 1998 11:04:11 GMT
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Original-From:
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Brian Lavery <blavery@computer.orgAVOIDSPAM>
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Viewed:
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1496 times
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> > Is it possible for a handy-board program (either 'C' or assembler) to
> > detect a low battery condition?
========================================
> Sure:
> Solder a wire to the "BATTERY +", add 2 resistors to make a voltage
divider
> (to divide the battery ABSOLUTE MAX condition, say about 12V, down to
about
> 4 volts for the analog 0-5v range), and feed it into an ANALOG input
point.
> Put a multimeter on the battery to measure its volts in a LOW charge
> condition, and then in a HIGH charge condition. Read the HB analog
input
> value for both those conditions, and assume a linear connection between
bat
> volts and analog reading.
===============================================
> I slowly start to grasp some of the idea about
> voltage dividers but still don't understand.
> Won't this setup drain the battery? How does
> one calculate and wire such a thing - in this
> particular case?
==============================================
Hi Jaron,
I took the PCB off the battery case, and connected a wire to the +ve
terminal of the battery pack. Then I connected two resistors like this,
and brought the wires out, and plugged into the Analog5 connector:
BAT --------R1---------*--------- Analog5 Input
+ve (10K) | (Active)
R2 (4K7)
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*--------- Gnd (at Analog5
Connector)
(There is an inbuilt 47K resistor from Analog5 Active to +5 in the HB
itself, but ignore that for the moment.)
I use the following code to read the battery voltage:
float bvolts(void)
/* BATTERY VOLTS MEASUREMENT - USES ANALOG 5 */
{
return .1 * (float) ((((analog(5)-17)*75)/57 + 2)/2) ;
}
int main()
{
while (1)
{
printf("V =%f\n",bvolts());
if (bvolts()>12.2) /* 8 cells x 1.525V/cell */
beep();
sleep(2.0);
}
return 0;
}
This program I leave running while the battery is charging, and the
beeper starts when the voltage gets too high. For your purposes, you
will need a suitable test to check for falling volts as the battery goes
flat!
Why do I use such a complicated function to calculate BVOLTS() ? Because
it rounds off the result to 1 decimal place for me, and that is easier to
read on the LCD panel. A simpler calculation is:
<bigger>0.06579*(float)analog(5) - 1.0184
if no rounding is needed. (Check the algebra of that, I think I got the
simplification right!)
In any case, the reading should be linear for different battery voltages,
and the "-1.0184" bit is a correction for the 47K inbuilt resistor on the
HB.
Will that flatten the battery? Well I find the NiCd battery on its own
leaks away in a few weeks or so. With the resistors added like above, it
flattens in a fortnight or two <<g>. Let's look at a rough
calculation... With 10K resistor, say it drains 1 mA, with 10 volts
across it. What capacity NiCds? - say 600mAh. Very roughly that gives
about 600 hours to drain all away - and 600 hours is about 3 or 4 weeks -
so that seems to add up.
</bigger>Good luck
Brian Lavery
Sydney
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