Battery level monitor

[iw2nzm]

Hi,

What could be the minimal configuration to monitor the level of a battery?

In detail: I'd like to know if the Vbat is above or below a defined threshold. The PIC is powered @ 5V and the battery voltage is about 12V.

In other projects I used the internal ADC and an external op-amp to reduce both the voltage to max 5V and the source impedance.

But now I want use the internal comparator. I need a resistor divider but the source impedance must be < 10 kohm... thus the battery draws always several mA.

May you suggest me a tricky way to do this using few external components (no op-amp) ?

Thanks
Marco / iw2nzm

[SherpaDoug]

Since you are only looking for consistency, not true accuracy, you can cheat quite a bit. Also you only need an impedance below 10K for the brief duration of the A/D measurement.

The 10K figure comes from making sure the input changes less than 1 LSB when loaded by the A/D. Since we are measuring a DC voltage we can use a capacitor to stop the input from drifting when loaded at no DC current cost. Using C=T/R with t=10us and R=10K gives 0.001uF for a single time constant. For any accuracy you need at least 5 time constants, and if you use 0.1uF you will be very safe.

So if you use a divider of say 700k and 500k with a 0.1uF cap to ground you will have a battery drain of only 10uA and a reading that is consistent within 1LSB. Note you should wait at least 250ms from when you first apply battery power till you take your first reading to let the cap get its initial charge. T=5RC=5*(500k//700k)*0.1uF

Have I lost you yet?...Just try it, it will work.
_________________
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[iw2nzm]

It's very interesting!

Just a question: is the capacitor connected between the A/D input and ground, right?

I'll try it of course!

Ah, btw, what's the tradeoff of this method? What cannot I do?

Thanks
Marco / iw2nzm

[SherpaDoug]

Yes, the cap goes from the A/D input to ground.

[iw2nzm]

Thanks!!!