In a previous project log I missed the mark substantially on battery life estimations. While I still don't have a conclusion, I wanted to confirm the tracker's power profile. My measurements so far were done with a cheapish multimeter, and they were:

• Sleep: 9uA
• Radio tx: 33mA
• Radio rx: 14mA
• GPS on: 35mA

Here are updated values, taken with a DMM6500.

Taking a closer look at the sleep current reveals the way the PIC18F sleeps for a long time: it has to wake up every 2 seconds to service its 16bit timer fed by a watch crystal.

Averaging the currents drawn in different modes comes down to:

• Sleep: 8.85uA
• Radio tx: 28.9mA
• Radio Rx: 14.2mA
• GPS on: 26.5mA

So this is actually lower than the initial measurements. I'm not bothering with the 3mA drawn in between Rx and Tx when the MCU is awake, as that is negligible compared to the time the GPS is on.

So my wrong estimates on the battery capacity may still come from:

• Lower than advertised battery capacity at 0.1C discharge rate
• I'm measuring battery voltage wrong
• I'm using a voltage-to-charge LUT that's not appropriate for the battery