Single AA Battery Low Detection

[Urahara]

Display a battery low warning message prompting the user to change to a fresh battery.

 

[cct314]

I had a problem like this on a (different) PIC where the supply was not well-regulated (it didn't need to be!) and I used a bandgap voltage reference diode - essentially a 1.2 volt Zener, although they come in different voltages and are well-behaved with respect to temperature. It was connected like a Zener, with a resistor at the "top" end but the "bottom" end of this diode was connected to an I/O input of the PIC that, when not in use, was set high so that the diode did not conduct current, reducing battery drain. The problem with these devices is that they are not necessarily "dirt cheap."

In theory, one could use an ordinary diode for this, but these drift all over the place with temperature. Similarly, a cheap, old-fashioned "dim" red LED will have a fairly consistent voltage drop as well (1.8 volts or so, varying with the device) - a bit less temperature dependent than a normal diode, but still pretty bad - but it may be "good enough." (And before anyone asks, no, it won't be affected by ambient light by more than a couple A/D counts...)

Knowing the voltage across the diode, an A/D reading was made of this diode, then an A/D reading made of the voltage source and the actual voltage of that source determined ratiometrically.

Many newer PICs (but not the one that you chose - at least as far as I can tell from a quick review of the data sheet) have these bandgap references built into them already, to allow one to do exactly this, saving the need to tie up a few I/O pins in the process!

 

[Tony Stewart]

Here is a discrete low battery detector with Vbat logic level out.

Threshold set by R ratios e.g. 9.6K sets threshold just above 1.15V with 0.05 mV of hysteresis with noise filter C.

Motorboating will be common as battery voltage recovers from load.

https://www.falstad.com/circuit/#$ 0 5.0E-6 10.20027730826997 50 5.0 50 v 160 400 160 128 0 3 76.0 0.05 1.15 0.0 0.5 w 160 128 272 128 0 t 272 304 384 304 0 1 0.02553347401501327 0.5056109476279457 100.0 r 272 128 272 224 0 10000.0 w 272 128 384 128 0 r 384 128 384 288 0 10000.0 w 384 400 160 400 0 r 272 304 272 400 0 9590.0 r 384 320 384 400 0 1000.0 w 272 224 272 304 0 w 272 400 384 400 0 w 384 288 432 288 0 w 544 272 592 272 0 r 544 304 544 384 0 1000.0 r 544 112 544 272 0 10000.0 t 432 288 544 288 0 1 -0.16984635719260888 0.4972826188803099 100.0 w 544 112 384 112 0 w 384 112 384 128 0 w 544 384 544 400 0 w 544 400 384 400 0 r 496 256 320 256 0 300000.0 w 496 256 544 256 0 w 544 256 544 272 0 w 320 256 272 256 0 w 272 256 272 304 0 c 224 304 224 400 0 1.0000000000000001E-7 0.5663351303886637 w 224 304 272 304 0 w 224 400 272 400 0 o 0 64 0 290 2.187250724783012 1.7087896287367282E-4 0 -1 o 12 64 0 290 2.5 9.765625E-5 0 -1

 

[blueroomelectronics]

Many newer PICs (but not the one that you chose - at least as far as I can tell from a quick review of the data sheet) have these bandgap references built into them already, to allow one to do exactly this, saving the need to tie up a few I/O pins in the process!

The 18F1220 does have a 1.2V reference built in for the LVD. Chapter 18 of the data sheet.

 

[cct314]

From what I can tell from the data sheet, the PIC18F1220's 1.2 volt reference is not accessible to the user in any direct way - that is, it cannot be routed to an A/D input for a ratiometric reading.

If one wants to read the output voltage of the converter, the LVDIN could be useful, but if the stated intent is to detect when the voltage of the single AA/AAA cell powering the entire device is <= 1.25 volts, I don't see how the LVD helps: As noted in a previous posting its range (1.99V-5.15V) is wholly outside the range of a single typical Alkaline/NiMH cell.

True enough, the output of the boost converter will start to sag when its input voltage gets low, but this is a bit of a "cheesy" way to ascertain a cell's voltage as this will vary tremendously with the load on the converter's output once it drops out of regulation, also varying with temperature of the device itself and unit-to-unit variation.

One possible point of concern of measuring accurately is that if the cell's voltage is too low when the converter starts to drop out, there is the chance that it will be unable to reliably start again, once powered down, if called on to do so as the voltage of the cell decreases and its impedance increases, but this may not be of critical importance in this application: A cell voltage of 1.25 volts (for Alkaline) represents a reasonably healthy amount of remaining energy/low impedance whereas 0.9 volts does not.

 

[Dr_Doggy]

HI, i found this clever little circuit helped me with a similar project, cause a diode always stays at 0.7v, but beware as vcc goes down adc goes up!
Also i forget what i used for R1, somewhere between 1-10k i think

 

[Tony Stewart]

I use a sawtooth to represent a battery from 1 to 1.6V and set the threshold with Rb ratio to just below 1.2V

How do you want to indicate ok?