Not Enough Cold Power

AMOS the backyard datalogger was particularly challenged this past week by very cold, windy weather. Although its 10 amp-hour lithium ion battery is rated for operation at -20 degrees C, it still tended to lose charge rather quickly on nights when the temperature dropped below -15 degrees C. On nights when AMOS was outside, I would usually unplug it before going to bed in order to prevent the battery from getting totally depleted (which would engage its internal battery management system to shut itself off). The Raspberry Pi is relatively power hungry, requiring on average about 5 W of power to run. Just running the Pi, the 10 amp-hour, 12 V battery would not last more than 20 hours.

Running just the RFU220 (wireless) module, the power required is only about 0.5 W, so I decided to try shutting down the Pi for the better part of an hour, waking it up at the top of each hour to boot up, synchronize with a timer server on the Internet to get the correct time (no real-time clock yet), and then log a bit of data, before the RFU220 shuts it back down. The whole process typically required a few minutes to complete. Indoors, it worked fine, AMOS still had lots of charge after operating more than 20 hours in this fashion. Outdoors was a completely different story though. Once the sun went down, most of the battery's life was gone (maybe 20% left?) by about 11 pm. Also, I noticed when the temperature dropped below -10 degrees C, that the speed controller would not function correctly, and the air propeller would beep rapidly and shake back and forth in protest. Turning off the +12 V power supply stopped the shaking and beeping.

So bottom line I guess, is that AMOS won't be going on a polar expedition any time soon.

The ACS712-based current sensor that was described last week was tested out quite a bit this week. Turns out that it is not particularly accurate for low current levels (less than one amp) and seems to have a strong temperature dependence. However, it is useful as a rough gauge of current consumed when the propeller is running, and is also useful in seeing how quickly charge is being applied (when the sun is out and the measured current goes negative).

(The ACS712-based current sensor. The hand model used here should probably use more moisturizer.)


Recently I have been working on the wireless serial communications software for AMOS, improving various things to make it more reliable, especially when signal quality becomes poor.