nRF52840 Battery Life Estimation

RE: nRF52840 Battery Life Estimation

awneil — Fri, 10 Jul 2020 09:15:05 GMT

RE: nRF52840 Battery Life Estimation

OpenCircuit — Fri, 10 Jul 2020 09:04:03 GMT

Yeah, that's right. I just wondered that if I should consider any BLE events, radio etc. during consumption. Now I learned that the average values are enough for estimation.

RE: nRF52840 Battery Life Estimation

awneil — Fri, 10 Jul 2020 08:45:20 GMT

Also, don't forget that real batteries are not perfect:

they have self-discharge
their actual capacity varies depending on the nature of the load
different manufacturers have differing definitions of when a battery is considered "exhausted".
etc

Again, this is not specific to Nordic - it applies to any battery-powered system

RE: nRF52840 Battery Life Estimation

OpenCircuit — Fri, 10 Jul 2020 08:34:02 GMT

Thanks for the contribution Awneil.

RE: nRF52840 Battery Life Estimation

OpenCircuit — Fri, 10 Jul 2020 08:33:22 GMT

Thanks for the explanation Stian.

If I consider my scenario, there are 4 states : advertising, idle, connection & transmission, idle and their average currents are 1280 uA, 4.3uA, 1110 uA and 4.3 uA respectively. In seconds, their durations are 2 seconds, 600 seconds, 2 seconds and 600 seconds and their activeness ratio of the time are 0.17%, 49.83%, 0.17% and 49.83% respectively.

The total product lifetime = (0.0017*1280)+(49.83*4.3)+(0.0017*1110)+(49.83*4.3) = 8.343 uA. If we have 100 mAh battery, it is 100mAh / 0.0083 mA = 11986 hours and 499 days approximately. In estimation it is really high.

So the time unit doesn't matter if we have ratios of the time.

Also, peripheral device has operation voltage interval (1.7 V - 3.6 V). So I think the device won't be active for 499 days in estimation, will it ?

RE: nRF52840 Battery Life Estimation

awneil — Thu, 09 Jul 2020 12:58:54 GMT

Clearly, average values are only going to give an estimate - only you can decide whether that estimate is good enough for your requirements.

The basic concept is that a battery is a store of energy - so what you really need to know is how your system uses up energy.

Generally, a system will have a number of states; eg, idle, transmitting, processing. SO you will need to work out how much energy it uses in each state, and how long it spends in each state - then use that to work out how long it will take to use up all the energy in the battery.

This is basic physics - not specific to nRF ...

RE: nRF52840 Battery Life Estimation

Stian Røed Hafskjold — Thu, 09 Jul 2020 12:50:01 GMT

Yes, you use the average current to get the battery life.

You divide the battery charge with the average current and you get the battery life time: mAh / mA = h

If you have different "average currents" based on the application state, you need to figure out an estimate on how long your product is in each state. Example: Let's say it's advertising 90% of the time and in connection 10% of the time; and the average current during advertising is 20uA and 500uA during connection, you get 0.9*20+0.1*500 = 68 uA in total average during the product lifetime. Then if you have a 100mAh battery, the battery life time will be 100mAh/0.068mA = 1471 hours.

RE: nRF52840 Battery Life Estimation

OpenCircuit — Wed, 08 Jul 2020 11:49:02 GMT

It shows the average current. My measuements are average and don't have a time unit. Is it correct way to calculate battery life directly with these average current values ?

RE: nRF52840 Battery Life Estimation

awneil — Wed, 08 Jul 2020 11:33:09 GMT

https://devzone.nordicsemi.com/nordic/power/w/opp