Battery estimate calculation, nRF9160. Community discussion welcomed!

[Torje]

Hello everyone,

I'm trying to do some battery calculations for our coming asset tracker based on the nRF9160. I would very much like some discussion on the topic, as a sanity check!
As a background, all the app is doing is waking up, getting a fix, sending it, going to sleep. Once per day.

I've used the Power Profiler kit 2 (PPK2) to measure:
- Current floor (PSM)
- Stuff happening before GPS search
- GPS search
- Stuff happening after GPS fix

I'm trying to make a worst case calculation, using 300 seconds of GPS searching. Connecting to LTE and cloud and getting AGPS before, sending data and disconnecting from cloud after. I'm basing my calculations on a 5000mAH battery, and doing the calculations both in AH and Coulomb.

The calculations now looks like this:

Simplifications: I am allowing overlap with current floor time as I consider it negligible. "Per min-AH" cells are in ampere minutes.

If I assume 30 seconds per fix the device lasts for 21 years. This seems too good to be true, so I think I've missed something.

My main concerns are: Am I way off, or onto something? Is there something I haven't taken into consideration? Are the estimates "valid"? Does anyone have any experience or knowledge to share? Based on this, it seems communication protocols etc. matters very little compared to actually getting the fix faster

Please fire any questions at me, and look at this post with the most critical eyes. It would be a great help.

Thank you all!

Kind regards,

Torje

[Øyvind]

Hello Torje, 

As an initial and general comment to your ticket regarding the power consumption, please make sure to read through the white paper nWP044 - Best practices for cellular IoT development and the GSMA Improving Energy Efficiency for Mobile IoT. How are the numbers compared with the Online Power Profiler for LTE? Make sure to read the Online power profiler user guide for LTE.

Where have you done your tests? What current consumption do you get in an area with bad/limitied coverage? 

Kind regards,
Øyvind

[Achim Kraus]

If you calculate with a 4.66µA "current floor", I guess you should also consider the self-discharging of the battery. Especially, if you run over a couple of years, even 2-3% per year maybe more than nothing. (Just to mention: using a LiPo Accu comes with 2-3% per month and so runtime longer than a year discharge the battery easily by a third.)

Your calculation didn't consider "LTE radio times" (NB-IoT or LTE-M). Once you consider that as well, you need also to take "network searches" into your account. E.g. in my experience, an message exchange may take about 100mC, but a network search easy up to 1800mC (depends on a lot of parameter). If your device is moving, I guess you need to consider such network searches.

As longer your device is in the field, it may requires to consider FOTAs, that also drains the battery of such a device.

Anyway, if your device is frequently exposes some light, there are also ideas to use a solar panel. I don't have experience with that and the long-term usability.

[Torje]

Hello Øyvind,

Thanks for replying, and thank you for all the resources. When measuring the GPS Searching, I made sure to measure where there wasn't good coverage. As I am making a "worst case" estimate. Using the online profiler, I get much better values for avg current and total charge of getting a fix. But I consider the actual measured data more valid than the ones from the online tool. As a side note, these tests are done on a Thingy. Using the values from the online tool, I would get fifty years on a 5Ah battery. I am not seeking the "best possible theoretical result" - but the "worst actual result".

But for the sake of a great argument point, lets say I'm using data from the online tool and calculate 50 years battery life. Then there has to be some errors in my calculations, some important points I'm not taking into consideration. 

[Torje]

Hello Achim,

Thank you for some important considerations I had not done. I will include estimates for discharge as well.

The radio times are included in the "pre" and "posts". Its until psm floor is reached again, so this should be taken into account.

My main concern is perhaps that my estimates are way too optimistic. I'm getting tens of years of battery life, and it doesn't seem realistic. I will also include measurements for FOTAs, thanks for the tip.

[Achim Kraus]

> The radio times are included in the "pre" and "posts". 

Unfortunately, I guess, this still doesn't consider "network searches", especially, if the device is moving.

> I'm getting tens of years of battery life, and it doesn't seem realistic.

There are a lot of calculations, which also results in years with one wakeup a day. Until you run the device for a couple of weeks, I would not relay too much on the results.