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'[PIC]: Monitoring battery status.'
I have a PIC16F873 that sends through the USART some analogic
Now I want to watch the status of the battery (Ni-Mh or Alcaline) that
powers the pic.
I suppose to:
- measure the voltage between the battery pin with one of the ADC to get
- measure the voltage between the cap of a shunt resistor in series with
one pole of the battery to get the intensity.
Is it correct?
Have you any suggestions?
Does exist any chip from maxim or analog (you know) that does it and has
an I2C output?
And last, if my solution above is correct how I can get the reference
for those ADC?
Thanks a lot,
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Alan B. Pearce
>I have a PIC16F873 that sends through the USART some analogic
>acquisition. Now I want to watch the status of the battery
>(Ni-Mh or Alcaline) that powers the pic.
>I suppose to:
>- measure the voltage between the battery pin with one of the
>ADC to get voltage and
>- measure the voltage between the cap of a shunt resistor in
>series with one pole of the battery to get the intensity.
There is a sneaky way to do this.
Use the internal reference for the ADC
Use an external reference that is a lower voltage then the low voltage drop
out point of the battery. Many of the 2.5V reference chips are probably
suitable for this. One like the Analog Devices AD580 is what I have in mind,
as it has an internal constant current supply from the battery, but you may
wish to hook that to a PIC pin to turn on when you want to take the
measurement, otherwise you have a constant drain of 1.0-1.5mA with this
device, that is doing nothing for you for most of the time. also check the
minimum operating voltage (4.5V in this case) for the current source to stay
stable. Other devices may give you better specs than these.
Now to measure the battery voltage, actually measure the voltage of the
reference diode as though it is an external voltage. Note that the value the
ADC will give you is going to rise as the battery voltage falls - think
about the relationship between the reference voltage (which is what is being
measured) and the battery voltage (which is being used as the reference).
----- Original Message -----
From: "Alan B. Pearce" <rl.ac.uk> A.B.Pearce
To: "Microcontroller discussion list - Public." <mit.edu> piclist
Sent: Wednesday, February 02, 2005 11:11 AM
Subject: Re: [PIC]: Monitoring battery status.
You may be able to use the same PIC pin that you use for the analog input to
power the reference chip. Simply connect a diode, anode to PIC pin, cathode
to Vdd pin of reference chip. Then connect a capacitor to the Vdd pin of the
reference and ground. Now you simply set the PIC to be a digital output, set
it high for a while, and then set it back to analog input to read the
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