Battery percentage level varying often for each uplink when b-l072z-lrwan1 board powered with batteries.

[Amotu.1]

Running " i-cube-lrwan - End node" firmware on board. We are expecting consistent battery percentage but observing inconsistent in battery percentage.

e.g. At first when battery is fully charged, it should show 100%. Later on as battery started draining percentage should decrease linearly.

But we are observing non linear change in battery percentage. Like

1. 87%
2. 87%
3. 95%
4. 80%
5. 100%

We observed that +Vref voltage itself is fluctuating for each uplink when powered with batteries. So we are suspecting variation in Vref might be the reason. If so how to resolve this issue or is there any other reason for this??

[Andrew Neil]

@Amotu.1​ "as battery started draining percentage should decrease linearly"

Should it?

If you're just measuring the terminal voltage, then that will tend to "recover" if the battery is given a time to "rest" - so it's quite possible (likely, even) that the voltage will vary depending on the duty cycle of the system.

"We observed that +Vref voltage itself is fluctuating for each uplink when powered with batteries"

Have you also measured the battery voltage?

An uplink means that the radio transmitter has been active - which has a very high current drain relative to the average. Therefore it is likely that the battery voltage will drop during & immediately after an uplink.

[Amotu.1]

@Andrew Neil​ I agree with you that radio transmitter has a very high current drain. So how can we calculate accurate battery percentage?

Because at least +Vref level should be constant to calculate uniform step size voltage if we use ADC.

[Andrew Neil]

@Amotu.1​ - "how can we calculate accurate battery percentage?"

You'd need to use a proper "coulomb counter" or "gas gauge" chip.

Measuring terminal voltage will only ever give you a rough estimate - for things like lithium batteries, it's particularly poor.

Also, if you want consistent results, you'll need to ensure that you measure under consistent conditions.

"at least +Vref level should be constant"

It's up to your design to ensure that it is.

You said that you've observed that it's fluctuating - so you need to investigate that, find the cause, and fix it.

Remember that this is just a simple demo - not necessarily a fully-finished, shippable product ...