newbee - battery level

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

We have one product in which I check for a low battery voltage
and display the "LOBAT" symbol on the LCD if it's low.
On the 16F628, I used the comparator to check the voltage
of a 9v battery. The circuit consists of a IRF7306 FET with
one Source pin going to the 9v battery. The two A & B drain
pins go to a voltage divider (with 3.9K on top and 2.2K on the
bottom). The gate on the FET is driven by the Collector pin of
a 2N3904 transistor with a 3.9K to the 9v battery. The emitter
goes to ground, and the base connects to a port B pin on
the PIC, with a 3.9K series resistor. The FET is turned on
for just a few microseconds to check the battery voltage.
You could check it once every second, or once every minute,
or longer. It doesn't really matter because the test is done
so quickly that hardly any power is wasted.
The code is in the post below.
http://www.ccsinfo.com/forum/viewtopic.php?t=2236

guest · Guest

What is your PIC doing when it is not checking the battery? Is it sleeping? Is it powered off? Is it polling a UART or waiting for a button press?
If all you are doing is testing a bit the resource cost is very low. You could check it every button press or incomming serial character. If you decided to use an 8 bit counter and only check the battery every 256 characters the overhead of incrementing the counter would probably be as much as that of checking the battery and there would be no savings.

Sherpadoug

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

It's doing the multitasking described in this thread. One of the tasks
is called check_battery().
http://www.ccsinfo.com/forum/viewtopic.php?t=17189

Guest · Guest

Guest · Guest

Sorry for such dumb questions but im newbee in this...

TO PCM'S EXAMPLE

Realy newbee question:

BATTERY_TEST_FET_ON_PIN_B5 ---> IS THIS V P P IN YOUR CASE
BAT_LEVEL_PIN ---> IS THIS V D D IN YOUR CASE

?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Guest · Guest

I need a lobat but i have some problem with understanding...

Teoreticly:
If u would have just one pin ( let's say pin 21 ) that i want to test, and
lets say i have value of 1 on this pin, so long until battery falls under 50%.
After the battery falls under 50% i have 0 on this pin.

I want so check each time, when button will be clicked, the value of that pin.
If its 0 show LOBAT.

How would you do that?
Do i need comparator and can this be this done on this way?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The design that I posted uses the internal comparator in the 16F628.
I don't want to design a new low battery circuit. The design that I
posted uses very little of the battery power.

Here's an example of a design that wastes the battery.
http://www.circuitcellar.com/library/print/0798/May96/Fig2.htm
Scroll down to the 9v battery input on the left side of the page.
Notice how they have a voltage divider consisting of two 5.1K
resistors connected across the 9v battery. The voltage divider
is always in the circuit. There's no way to disable it. It wastes
0.9ma (or less, as the battery ages) per hour.

You are free to design your own circuit with an external comparator
or whatever you want.