calculate current

[Kristian Hansen]

Hi,

I'm trying to do a rough calculation of current consumption for the nRF52840 chip.

The unit should sample 6 times an hour by using I2C to acquire samples from a gas sensor (with a microprocessor) for at least 30 days. I'm thinking I will use the RTC to keep track of date and time, storing 16 bits for date and time 6 times an hour.

I might also write to flash each time it samples (3 x 16 bits, 6 times an hour). I've also given ~100kB to work with at all times in RAM.

The nRF should be constantly ready to upload to the mobile, so the user can check in for new measurements at any time, and/or send notification when a certain level is hit. I used the online power profiler to get an estimate of the current consumption.

Here is what i have come up with so far:

I would appreciate it if anyone could point me in the right direction, thanks!

[Bjørn Kvaale]

Hi Kristian, I am a bit uncertain what your question is. I guess you are wondering if a coin cell battery would be enough to power the nrf52840 for >= 30 days? Is that correct? Considering coin cell batteries can have a capacity of roughly 225 mAh (see this great blogpost), one coin cell battery would last roughly 3214 hours before it would need to be switched out (i.e. 225 mAh/0.070 mA avg total consumption). That is more than the 720 hours in one month. So basically, if your consumption estimates are accurate, you should easily be able to use the nrf52840 for >= 30 days! Worst case, you could consider attaching a Li-Po battery to the nrf52840 (see infocenter).

[Bjørn Kvaale]

But given you're usecase, I'm pretty sure a coin cell battery should work. This is again assuming the current estimates are quite correct. You could also consider using a multimeter or buying a power profiling kit to more accurately measure the current.

[Kristian Hansen]

Hi, thank you for your reply. Yes, I think a coin cell should be enough for the nRF. I still haven't received my devkit so I can't do any measurements yet, therefore am I trying to do a calculation I can work with for now. I'm asking mainly if there is any big calculations I might have missed that could change the consumption by a lot.

[Bjørn Kvaale]

Communication over I2C requires CPU or DMA and the HFCLK (high frequency clock) in order to operate. So you will need to add CPU or DMA current calculations + the HFCLK current consumption. In practice, the average run current for I2C communication is around 1.2 mA. This would lead to around 864 mAh, which is unfortunately more than a coin cell capacity. I would double check this once you get your PPK, but you might unfortunately need to buy a Li-Po battery (or another higher capacity battery) to let it run for >= 30 days straight. This blogpost might help to lower current consumption on I2C bus.

I would also take a look at these two cases if I were you: case 1 & case 2. Case #1 is slightly similar to your query & in case #2, endnode says that from his experience, theoretical calculations can often vary quite drastically with the actual values.

[Bjørn Kvaale]

These two blogposts (1 & 2) might also help a bit.