Battery Shield Questions: cutoff voltage?


#1

Will the Bluz work down to the CR2032’s 2.0V cutoff? Is there a boost converter to get this to 3.3V for the system, or is it generally pretty happy between 2.0 and 3.3V?

Is it possible to measure battery life on the bluz itself by typing VIN to an analog pin and doing analogRead, or will the changing system voltage affect the analog reference (is the analog read reference voltage stable? What is it?)

I ask because the batteries I bought were reading 2.8V new and unloaded, so they had clearly self-dischagred some on the shelf. It’d be nice if I could add a Particle.publish notification for a “low battery” event, or even if the battery shield had a low battery indicator on it that could be tied to a Bluz interrupt.


#2

Great question, and this is actually already on the roadmap. The nrf51 is capable of measuring the voltage of VDD without any external components, so we will add this is a feature soon.


#3

So can I expect the battery shield to work down to 2.0 volts, using almost all of the CR2032’s capacity? How does that work if the chip is a 3.3V chip?


#4

The system won’t work down to 2.0V. As the battery drains, the voltage drops and the device will stop working in the 2.5V range. 2.8V is perfectly fine, but if you start to get below 2.7V, you can start to have issues.


#5

So what does the ADC use for reference? I’m assuming Vin(aka 3.3 with USB, and vbat under battery)? And if so, what’s the best way to do an analogRead with a varying ref voltage?


#6

The nrf51822 uses a voltage bandgap reference of 1.2V, this should never vary based on input voltage or temperature. The ADC then pre-scales the input down into this range, and compares it to this value. So varying input voltage wouldn’t make a difference.


#7

I found this article on measuring Battery:

https://devzone.nordicsemi.com/blogs/30/measuring-lithium-battery-voltage-with-nrf51/

Is there some other way to read battery without the voltage divider and capacitor built into the nrf51?