Quiz: calculate battery life

1

Hi, i have this problem:

A bike tracker uses GPS to sends its location over LoRa. If a bike is moving (this is detected with an onboard accelerometer) the device sends the GPS coordinates every 2 minutes. If the bike is not moving, the GPS coordinates are sent every hour. Before the device knows the accurate location, the GPS module needs to acquire signals from at least 4 satellites to calculate the location. In this example, this takes 15 seconds. Sending 12 bytes using SF10 takes up 0.4 seconds per packet.

Battery consumption

  • 0,08 mA - Deep sleep mode
  • 50 mA - Active GPS
  • 50 mA - Sending data over LoRa
  • 12 mA - Active LoRaWAN Module for downlink message

How many days will a battery of 600mAh last before it runs our of power if the bike is used 60 minutes a day?

-> I made my calculation and i got 92 days. However the result is wrong. I have also assumed that the receiving window is 41ms@SF10. Does anyone of you would have tried and got different result?

thanks.

2
  • So what happends if you wait for a traffic light for 30 seconds and then start moving again ?
  • Why not power it with a 18650 (3100 mA)
3

If you have a decent GPS, with backup circuit, then you can turn it off and it could be expected to hotfix in 5 seconds or less.

Very few (no ?) GPS will aquire a new fix in 15 seconds from cold.

Deep sleep of 80uA is very high.

4

this is the quizz (not written by me :slight_smile: ). But yes you are right, therefore i made the assumption the bike is always moving.

5

can you please elaborate more on

If you have a decent GPS, with backup circuit, then you can turn it off and it could be expected to hotfix in 5 seconds or less.

thanks.

6

A GPS from cold need to download information on the satellites on their positions etc, this can take 45 seconds or longer. Once it has downloaded this information a lot of GPSs can store it. Remove the main power from the GPS and when it starts up again, as long there is no long off period, the GPS will use the backup satellite information and aquire a new fix very quickly.

Google ‘GPS Hotfix’ for more details.

The speed of hot fix is almost directly related to how good the GPS antenna is, read here for more information;

GPS Antennas and their effect on LoRa Tracker power consumption V2

By the way, assuming this is LoRaWAN have you checked how long the GPS location packet actually is, 12 bytes seems a little short ?

2 Likes
7

you can see each of the coordinates as a float. A float can be packed into 4 bytes, so a long/lat combination only needs 8 bytes.

8

Yes I realise that.

So you sending are these co-ordianates via LoRaWAN\TTN ?

9

yes. I’m using a lopy (a micro python thingy) to pack the coordinates into bytes and send it via lora to TTN. Decode it back into coordinates using the ttn payload Formats Decoder/Converter