Single AA Battery Low Detection

[Urahara]

Hi

Am designing a small project using a single AA 1.5V alkaline battery boosted to 3.3V to power a 18F1220. I would like to have a way to detect a low battery situation which it reaches around 1.25V or lower (not obsessed with accuracy).

What is the best way to achieve this? One way I read involves the use of an IC (eg TSM9118 or STM1601) but detecting single battery low voltage at 1.25V seems to be outside their range.

Am looking for a design involving the least parts and cost involved.

Thanks!

 

[JimB]

At a quick look at the datasheet, the 18F1220 has an analogue to digital converter.
Use one of the analogue inputs to read the battery voltage and set a low alarm at your required low volt point.

JimB

 

[Urahara]

Hi Jim

Thank you for the prompt response!

However, I read that this may not be workable as the MCU is powered by the same battery it is trying to measure. So as the battery depletes, its voltage drops, the ADC is also affected.

Is my understanding wrong in this case?

Thanks!

 

[misterT]

However, I read that this may not be workable as the MCU is powered by the same battery it is trying to measure. So as the battery depletes, its voltage drops, the ADC is also affected.

You said that the battery voltage is boosted to 3.3V. If that voltage is regulated, then you can use it as reference voltage for the ADC and then measure the battery voltage correctly.

Also, Neat way to measure battery voltage is to set the battery voltage as the voltage reference and then measuring the internal reference voltage. Not sure if this is possible in PIC.
For 10 bit conversion the battery voltage is then: Vbat = INTERNAL_REF*1024 / ADC_READING

 

[Ian Rogers]

Use a power supply booster chip with a low battery output pin. Microchip's own does the job

 

[ronsimpson]

You might be able to measure the duty cycle of the power supply. When it is boosting to the max is a good time to shutdown.
The output of the error amp can also indicate the amount of boost.

 

[Urahara]

thanks folks! will try them out.

 

[Daniel Wood]

I have been using the LT1307 that has a built in module that does the trick. Check out their datasheet on page 13 to see if its what you need.

 

[dr pepper]

Good idea Ron, I'll remember that one.

Daniel your not far down the road from me, I'm in Preston.

 

[Ian Rogers]

dr pepper. He sits next to me at work!!

 

[dr pepper]

I was trying to remember what other member was from rochdale.

 

[Urahara]

Reviving this thread as I put this off until now.

Have decided to use the ADC pin of the MCU to determine the voltage of the AA battery using the regulated 3.3V Power to the MCU as the reference.



I plan to measure the battery level occasionally but am wondering if there's a simple way to prevent the battery from "leaking" when there is no need to measure it.

Thanks!

 

[MikeMl]

Are you worried about the input impedance of the ADC input pin when it is in input mode?

 

[Urahara]

When the ADC pin is not measuring the battery (not in input mode I suppose) would there still be current from the battery going into the pin? My primary objective is to conserve the limited power of the AA battery since I want the circuit to run as long as possible.

Must admit I do not quite understand your question (still a newbie ) but is it something I should be worried about?

Thanks Mike!

 

[ronsimpson]

I have not looked at your data sheet but most of those ADC input pins have a worst case leakage current of 10uA and a typical leakage current is very very low.

There is a time (fraction of second) when the battery is 1.5 but Vcc is less than 1.5 because the boost up supply is just starting up. I am thinking about the diode from input to VCC in side the micro. I would break open the wire from micro-input to battery and add a 1k to 10k resistor. This should not change the readings but will limit current through the protection diode.

 

[blueroomelectronics]

Use the LVD detect, it has a fixed 1.2V reference and a programmable trip point so it won't vary with VCC. Also uses zero I/O pins.

Chapter 18 of the 18F1220/1230 data sheet.

 

[Urahara]

Ron, thanks!

Bill, the lowest voltage that can be measured by the LVD module is 1.99V according to the datasheet. This is still above the 1.5V of the battery. Are you suggesting I measure the 3.3V going into the MCU itself?

 

[ronsimpson]

The LVD is not measuring the 1.5 battery but the 3.3v.
When the 3.3V gets too low you get a signal.
You are watching when the power supply can't do its job any more.

 

[Urahara]

Ok, fair enough. Here's the range of the tipping points found on the datasheet

LVDL3:LVDL0: Low-Voltage Detection Limit bits
1110 = 4.04V-5.15V
1101 = 3.76V-4.79V
1100 = 3.58V-4.56V
1011 = 3.41V-4.34V
1010 = 3.23V-4.11V
1001 = 3.14V-4.00V
1000 = 2.96V-3.77V
0111 = 2.70V-3.43V
0110 = 2.53V-3.21V
0101 = 2.43V-3.10V
0100 = 2.25V-2.86V
0011 = 2.16V-2.75V
0010 = 1.99V-2.53V
0001 = Reserved
0000 = Reserved

Between 0110 or 0101, which one would be a better choice?

 

[blueroomelectronics]

Correct, it compares VCC to a built in voltage reference.
That PIC is still functional down to 2V if I recall, so unless your peripherals would suffer I'd pick the lower. What do you plan for it to do when low voltage occurs?