My mailbox notifier stopped transmitting the other day. It lasted almost 2 years since the last charge, with last reading at 3.56V. This was based on an old MotionMote R1 and Moteino R4, plus a chinese 2000mAh Lithium Polymer battery. This device reports MOTION when mailbox is opened, along with a calculated LastOpened-ago metric and battery readings. Makes my life easy knowing exactly when there is mail or someone just opens it. And I can get IPCam snapshots if I need to when motion is reported. All very convenient.
Note: Older shapshot above just to show the metrics. Also the START metrics and Voltage logging were added at some point in time so it won’t reflect the real start date.
Based on my previous LiPo freezing to death issues, I didn’t expect this to last thorugh a winter, especially being chinese, but it has gone through two Michigan winters and done the job well. The voltage was pretty linear decline as expected. I could optimize sleep current more and drop MCU sleep current to 35nA (not the PIR which needs to be active) with a TPL5110 timer, but the RFM69_ATC does a great job keeping TX power at a minimum while maintaining strong enough signal, this code upgrade alone has probably doubled battery life. Simply using watchdog sleeping is easy and decent enough for a ~2yr recharge cycle.
So it was time to service the unit, charge the battery, replace the antenna which being a simple insulated wire takes quite the abuse over time. And the unit being in an acrylic case without more than clear tape at the seams, there was a bunch of corrosion to clean up, and power switch to replace. I’m pretty happy these 2014 boards do well after so long, I don’t need to upgrade them to the newer MotionMote R2 and Moteino R6 boards, it just works.
Then I realized this used an old inefficient diode which dropped more voltage and wasted more reverse current, so I replaced that as well. The PIR is the chinese HC-SR501, uses around 55uA when idle, around 250uA active, I could do much better with the Panasonics 2uA PIRs which are available in the current MotionMote kit but I’m not sure it’s worth the investment for such a simple device that just reports when the mailbox is opened.
Having said all this, I believe the LiPo gets pretty worn by the extreme temperatures (both winter and summer) which LiPos don’t like at all. I am looking forward to see how this will do in the next 2 years. It may fail completely and I wouldn’t be surprised. If that happens, I may try a LiFePo4 solution next time.