Basic power consumption evaluation

[scottjenson]

Of course accurate power consumption isn't possible to predict. However, it would be helpful to have a couple of basic numbers so we could do some back of envelope power calculations before we put too much time into this DK:

1. Can you tell us the power draw when the device is  in deep sleep? (assuming you've locked down all GPIO pins, etc)

2. Assuming a strong LTE signal, how much much power is required per 1kb to upload?
Yes, this is a highly variable number, which is why I said strong signal and assume a 1kb packet. Again, just a ball park here. The reason I want to use LTE-M over NB-IoT is that I have a fairly large amount of data I need to send quickly. I'm trying to understand, VERY roughly, what it would take to send these packets.

3. I assume "Deep Sleep' is very tricky as if you turn off the LTE-M radio, you'll need ~30 seconds to require a tower. Is there a low power mode that keeps a the LTE-M radio ready (for lack of a better word) so when I do wake up, I can send a packet right away?

I totally understand that hard answers are impossible. I'm just trying to figure out if my "10 x 2kb packets/day" application is even possible using this type of device. My worry is that even a back of the envelope calculation will show that this use case will burn through a battery (1000mah) in hours and not the weeks I'm hoping for.

[Håkon Alseth]

Hello,

 

 

1. Can you tell us the power draw when the device is  in deep sleep? (assuming you've locked down all GPIO pins, etc)

 The floor current, with LTE Power Saving Mode (PSM), at this moment is 7 uA (but we're aiming for 3 uA when production silicon is ready). More details are available here under the section "PSM":

https://devzone.nordicsemi.com/b/nrf9160-development-status/posts/nrf9160-development-status

2. Assuming a strong LTE signal, how much much power is required per 1kb to upload?
Yes, this is a highly variable number, which is why I said strong signal and assume a 1kb packet. Again, just a ball park here. The reason I want to use LTE-M over NB-IoT is that I have a fairly large amount of data I need to send quickly. I'm trying to understand, VERY roughly, what it would take to send these packets.

 Unfortunately, this is the section that we do not have much specific data on yet. We have the peak and active currents at different output power settings in the OPS:

https://www.nordicsemi.com/DocLib/Content/Product_Spec/nRF9160/latest/_tmp/alta-nRF9160/autodita/CURRENT/parameters-id_current_modem

The top section "Bx" is the band 'x' which is used for that specific measurement.

 

We also have some numbers for different PSM/eDRX scenarios in the above linked devzone development status, which can help on giving a indication on what to expect in the final product.

3. I assume "Deep Sleep' is very tricky as if you turn off the LTE-M radio, you'll need ~30 seconds to require a tower. Is there a low power mode that keeps a the LTE-M radio ready (for lack of a better word) so when I do wake up, I can send a packet right away?

 Yes, this is a feature I briefly mentioned, called Power Saving Mode or PSM for short.

With PSM, you can set a timer for when you're going to back "online" again. This is beneficial for applications that send data not very often (> 10 minutes) and does not require high latency.

The minimal timer value set here is 10 minutes, so if you plan to send data only once an hour and do not have higher latency requirements, PSM would fit your application.

When you wake up from PSM, you will be in connected mode (uses around ~1 to 2 mA on avg, per the development status page) for some time before going back to PSM.

If you plan to send data in a shorter interval, eDRX (extended discontinuous reception) mode can be used. This is from 5.12s intervals, and upwards, for Cat M1. This would be a more power hungry solution than PSM, but you'll gain in latency and overall throughput.

I totally understand that hard answers are impossible. I'm just trying to figure out if my "10 x 2kb packets/day" application is even possible using this type of device. My worry is that even a back of the envelope calculation will show that this use case will burn through a battery (1000mah) in hours and not the weeks I'm hoping for.

Even if you choose a lower latency, like normal DRX at 2.56 sec, the average should be in the range of 250 to 300 uA (of course depending on many factors like how much uplink, how much downlink at this interval, what the input voltage is), and with a 1000 mAh battery, you should last around (1000 mAh / 300 uA) = 3333 h, which is just shy of 139 days.

Most applications have added sensors, processing, LEDs that blink every now-and-then, which needs to be accounted for in addition to the modem itself.

 

Kind regards,

Håkon