Check Battery Level

ljbeng · Joined: 10 Feb 2004 Posts: 205

Any ideas how a guy could check the battery level of 2 AA batteries from the A/D port of a 16F628? The 2 AA's are the only power supply in the circuit. No regulator. I want to flash an LED if they are low. (Even though this means faster battery drain.)

Guest · Posted: Wed Feb 09, 2005 9:21 am

rt(f)m

Guest · Posted: Wed Feb 09, 2005 9:22 am

and maybe use a resistor divider to lower the voltage to a measurable value.

Humberto · Posted: Wed Feb 09, 2005 9:35 am

I would use a Micro-Power Rail-to-Rail CMOS Comparators with Open-Drain/Push-Pull
Outputs like National LMC7215 ($0.54) or similar wich will run down to 2V of power supply,
detect the minimum allowable voltage, switch-off the microcontroller and slow blink a LED
until batt is completelly die. My $0.02 contribution.

Humberto

ljbeng · Joined: 10 Feb 2004 Posts: 205

I will try the MCP100-270DI/TO-ND from Digikey. It goes low <2.7V. Adds 45ua to supply.

dyeatman · Joined: 06 Sep 2003 Posts: 1933 Location: Norman, OK

In answer to your original question: Yes it can be done.

Two AA cells would be a max of about 3.2 volts which could be monitored directly by the PIC ADC. You will need to have some kind of circuit isolation due to the low impedence of the battery. A low cost op-amp and a few resistors would be what you need in that case.

To learn how to code for using ADCs to monitor voltage, search for ADC in this forum. There is a huge amount of info here.

That being said, the most cost (and time) effective approach is the one being suggested above using a battery monitor chip.

valemike · Guest

I had a 9V battery which i wanted to monitor if it got to 7V (low batt).

I went with someone's advice of using a MAX666 voltage regulator with lowbatt detector (the lowbat level was configured using an xMohm resistor voltage divider). This method is costly, but saves a lot of current (standby of 5uA or something like that).

As for your 2AA method, then i really don't see how a MAX666 could apply. Might want to check the lowest possible Vout of the MAX666 and the lowest possible Vdd of the PIC. This would be overkill because you then wouldn't need a voltage regulator.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Is the problem that your batteries are also the reference for the A/D? If so then as the batteries drop both the reference and the divider output drop together so the A/D reading doesn't change.
If you can power a 1.25V reference chip from a port pin that will give you something to compare to. Look at a MAX6190 or simillar. As the batteries fail and the A/D Vref drops the 1.25V will look like it is getting bigger.
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