Post by thread:[PIC]: Monitoring battery status.

Hey everyone, 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. 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, Massimo. -- ____ ____ ____ _ _ / ___)| __ \/ ___)/ /| | Massimo Gaggero (spam_OUTmaxTakeThisOuTcrs4.it) | (___ | /\___ \\__ | Net & System Management \____)|_|\_\(____/ |_| Phone: +39 070 9250 329

>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" <.....A.B.PearceKILLspam@spam@rl.ac.uk> To: "Microcontroller discussion list - Public." <piclistKILLspammit.edu> Sent: Wednesday, February 02, 2005 11:11 AM Subject: Re: [PIC]: Monitoring battery status. {Quote hidden}> >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. 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 reference. Bob Ammerman RAm Systems