ESP8266 Community Forum

- Wed Jan 29, 2020 3:29 pm #85474 Hello all,

I am wondering if there are any benefits of using MAX471 voltage/current sensor on Analog 0 on a Wemos D1 mini versus using a resistor 220k perhaps to measure a 3.7 Li-on battery. I don't need to measure current just voltage?

So far I am thinking a resistor is better because it takes up less space which is important to my project- the smaller the better.

This is the Max471 board I have: https://www.amazon.com/gp/product/B01N6 ... UTF8&psc=1

I ask because I don't have a 220k resistor on hand and am eager to finish my project. Or maybe I should go dismantle some old electronics in search of the resistor?

- Wed Jan 29, 2020 6:04 pm #85477 Just make sure whatever circuit you use doesn't draw current continuously, or you'll drain your LiPo for no reason

I've built an INA219 (now INA223 for 2.8V operation) onto my board to do the same thing - works well.

- Wed Jan 29, 2020 6:41 pm #85478 Good point!
Looks like a resistor option will draw current even when Wemos D1 is powered off. Which is no good. Have to double check on the MAX471.
I've seen a lot of people use INA219, I will consider it soon.

- Thu Jan 30, 2020 4:52 am #85487 The D1 has a 220K / 100K divider on the ADC input to allow 3.2V full range (1V inot A0). THis means it will draw 10uA at full scale.

If you use a resistor to drop further from a battery then you can choose to use a value to give say 0.6V at A0 for 4.2V battery voltage. Now the draw will be 6uA.

The D1 even in deep sleep draws significantly more current than the esp8266 chip on its own with the USB serial and the regulator. The 6uA extra (or even 10uA) will be insignificant compared to these and will have very little impact on overall battery life.

If you really want to optimise then you need to use a raw module like ESP-12 without serial. You can then use a very low quiescent regulator and use your own resistors for the ADC. I tend to use 1M / 220K for battery monitoring which means extra current is about 4uA.