Power consumption in Ampere Hour for nRF52840-DK

[JasonGeo]

Hi,

I am trying to measure power consumption on a nrf52840-DK dev board running custom firmware.

I am using PPK-2 and nRF Connect Power Profiler app v3.1.2 for the power measurement.

During operation, I am seeing 832uA on the average window and 14.1mA on the max window (Tx pwr set to 8 dBm) over a 10 seconds "span". I am not really a hardware engineer hence I struggle to debunk the numbers.

Does 832uA means the board is drawing on average 832uA of current per second?

What would this value equate to in terms of ampere hour? Reason for asking is I have a battery rated for 120Ah. I want to calculate how long I can run the DK board on this battery. Can you provide a formula for calculation?

There is a big difference between average and peak current, is that normal? Should I use the average or peak current for proper power consumption?

Thank you.

[Ivan Herrera]

Hi Jason!

It might be worth skimming through a basic physics course as this will clear up a lot. 1 Ampere Hour is the equivalent of the amount of charge that 1 Amp of constant current would have drawn for 1 hour straight. The formula is: current x time = ampere hours. So if you want to solve for time, you divide ampere hours by your current.

It's normal for the average and peak current to differ a lot. Use the average current you expect your application to use. 

[Kazi Afroza Sultana]

Hello JasonGeo,

You can run the experiment for a bit longer time (for example: around one hour or less) to get a bit more accurate value of average current and maximum current. It is not uncommon when these two have difference. For example: there is different in current consumption for idle period and advertising period. 

 ''I want to calculate how long I can run the DK board on this battery. Can you provide a formula for calculation?''

battery life calculation formula is -

Battery Life (hours) = capacity (mAh) / Average current (mA)

There is a few online calculator Battery Life Calculator | DigiKey Electronics to do this. You can calculate on there as well. 

Thanks.

Best Regards,

Kazi Afroza Sultana

[JasonGeo]

Thanks all for the quick response.

Using the suggested formula:

capacity (120000 mAh) / Average current (1 mA) gives 120000 hrs which equates to 13.7 years!

My team finds it hard to accept that figure. In the field, we are seeing an operational life of 10-12 months.

Where is the missing puzzle?

[PaKa]

What is the battery self discharge rate? This is very often the limiting factor for battery applications. Remember that this is also temperature dependent.

[Kazi Afroza Sultana]

Hello Jason,

You were asking for formula, so that I mentioned in my reply. 

The original life time is not easy to presume. Because the life cycle of battery depends not only on its charge levels, but also the materials (chemical), environmental factors (for example: temperature, ageing), the way of using ( charging after 90% or fully drained out). So, there are several issues that could fail the battery. You may check the datasheet of the battery manufacturer to know the practical life cycle. If you can manage to charge and discharge the battery according to the limit at a specific temperature you could probably get the life cycle close to the specification.  

There is a lot of academic papers which describe the estimation of the life cycle of a battery as a statistical approach and calculated on historical data. A battery of a specific chemistry ( lithium, alkaline, carbon etc) will fail on average according to particular statistical distribution. Therefore, mean term between failure (MTBF) parameter is used to measure the life time of battery by battery manufactures. 

Thanks.

Best Regards,

Kazi Afroza Sultana