Another NB-IoT number floating around is that very power efficient NB-IoT devices should be capable to be driven from a small battery for more than 10 years without needing a recharge. Again, I wondered where this number comes from and which assumptions were made to declare that this is possible.
Again, the calculation can be found in 3GPP TR 45.820, for NB-IoT in Chapter 220.127.116.11 on ‘Energy consumption evaluation’.
The battery capacity used for the evaluation was 5 Wh. That’s about half or even only a third of the battery capacity that is in a smartphone today. So yes, that is quite a small battery indeed. The chapter also contains an assumption on how much power the device draws in different states. In the ‘idle’ state the device is in most often, power consumption is assumed to be 0.015 mW.
How long would the battery be able to power the device if it were always in the idle state? The calculation is easy and you end up with 38 years. That doesn’t include battery self-discharge and I wondered how much that would be over 10 years. According to the Varta handbook of primary lithium cells, self-discharge of a non-rechargable lithium battery is less than 1% per year. So subtract roughly 4 years from that number.
Obviously, the device is not always in idle and when transmitting the device is assumed to use 500 mW of power. Yes, with this power consumption, the battery would not last 34 years but less than 10 hours. But we are talking about NB-IoT so the device doesn’t transmit for most of the time. The study looked at different transmission patterns. If 200 bytes are sent once every 2 hours, the device would run on that 5 Wh battery for 1.7 years. If the device only transmits 50 bytes once a day the battery would last 18.1 years.
So yes, the 10 years are quite feasible for devices that collect very little data and only transmit them once or twice a day.