Measuring battery voltage with ESP8266

If you run IoT devices on batteries then it would be nice to know how much capacity is left on the battery, so you can change it before the device dies. Easiest way to do this is measuring battery voltage. Esp8266 has adc, but it's only from zero to one volt. The correct way would probably be to use Non-Inverting Op-Amp Level Shifter, but that would require many components and negative voltage. Easier approach is to just use voltage divider and just get the maximum voltage to acceptable range.

Using a calculator we get resistor values of 120kOhm and 39kOhm for getting 4.1 volts to 1 volt. For a low voltage of 3 volts these resistors would give 0.736 volts. So we are using 0.264 of esp8266 adc range of 0 - 1 volts. Adc is 10 bits so we have 0.264*1024=270 different values, even with some noise that should be enough for our purpose.

I made a small lua script that reads adc every minute and then posts the result with mqtt. Then used mosquitto_sub from Mosquitto to subscribe to that topic and log the messages.
mosquitto_sub -h roope.local -t /esp/battery | while read line; do echo -n $(date -Ins); echo -e "\t$line"; done | tee esp_battery.log Where roope.local is the mqtt broker machine.

Measurements and calculations can be found here https://docs.google.com/spreadsheets/d/1t18V1jrTRbj8nI5lrpctb8Fs9KzmpQ9O05dR5AeF37k/edit?usp=sharing. Esp_battery tab has raw data and formula has cleaned up data and formula.

I used google sheets linest function to calculate a function to get voltage from adc values, voltage=0.003539154715*adc+0.3756228338.

adc	voltage
1000	3.914777549
900	3.560862077
800	3.206946606
700	2.853031134
600	2.499115663

One problem for voltage divider is that it uses some power all the time. I should probably check how much that is, and if it considerably affects how long the battery will last. But for my next battery powered project, it will most likely report battery status also.

Code can be found here https://github.com/mika-koivusaari/check_battery.

You have mail, power usage 2

Still no luck, batteries died over the weekend.

One of my thoughts was that if something goes wrong, for example we don't get wifi connection, then the device just sits there sucking power. So i added a timer just after we wake up that waits for 10 seconds and then goes to sleep, 10 seconds should be plenty time to get a connection and send data.

Also added was error handling to wifi and mqtt connections, if we don't get a connection we go to sleep and try again.

I also added a feature that we only send messages when state changes, and we only wake up the modem when we need to send something. For this I needed a new firmware that has rtcmem module. But the master had changed SDK version and I couldn't get the new firmware to work. I tried erasing (http://nodemcu.readthedocs.io/en/latest/en/flash/#upgrading-firmware) flash but still couldn't get it to work. I had to get the excellent Nodemcu-build docker image and use it to build a previous version. Which worked fine. I tried using the lowest power option in deep sleep, but there seems to be some problem with it, and I couldn't get it to work https://github.com/nodemcu/nodemcu-firmware/issues/1225.

On thing that also was on my mind was the really small antenna that came with the Esp-201 modules. Most likely a better antenna would give less trouble. I had some old adsl-modems laying around that had built in wifi with external antennas. I salvaged an antenna from one of them, and now I get better reception.

You have mail, power usage.

After a less than 24 hours my mail sensors died. Batteries where around 1 volt, so it had drained them pretty quickly, something had to be done.

First I started to measure how many mA my circuit uses. But I was faced with a continually resetting Esp8266. At first I thought it had somehow messed the program in it's flash because of low voltage situation, but after a while of debugging I got the idea to measure the voltage, and it turns out that when measuring mA my multimeter drops the voltage from 3.3 to 3.1, and that makes the Esp8266 go crazy. Then I moved amperage measuring before the dc-dc converter. After that the circuit worked fine. There is a nice article about this issue here http://www.eevblog.com/files/uCurrentArticle.pdf

The following amperage readings are before the dc to dc converter when I powered the circuit from usb at 5 volts. When Esp8266 was active the circuit drawed around 40 - 100 mA of current. In deep sleep it took about 8 mA, which was quite high. After a bit of googling I found that the power led was using significant amount of power, so I took it out. Power went down to 3.4 mA, which is still quite high. After some thinking I started looking at how much TSL257 draws current, and it was typical 1.9 mA and max 3.5 mA according to datasheet. After moving TSL257 power to gpio, so it's only on during mail check, deep sleep current is 0.5mA, which is pretty much all taken by the dc-dc converter. Which I can't do anything about, without buying a new converter, so I'll settle for this now. If aaa batteries are about 1000 mAh then they should last about 1000/0.5/24=83 days, so over two months. Which at least gives me some time to think what I will do.