Batteries, current consumption and battery life time
Teensy 3.6, 96MHz, voltage fed by micro-USB 5V or by connecting a battery holder to Vin & GND
Which battery should I use?
USB Powerbanks seem very well suited: they are simple to use, have large capacity and ready to use plugs. However, they have one large disadvantage:
- they need regular current draw in order to stay alive (they have a built-in shutoff, when current draw is low for a certain time, which differs among different models). Otherwise they switch off and cannot be activated again by the Teensy, but only by plug-off/plug-in or by manually pressing the power bank button. This is relevant whenever the Teensy is in hibernate mode which does not draw enough current to "make the power bank happy" and then is not able to wake up, because the power bank has gone to sleep
The Teensy script has a built-in feature to keep the power bank happy. It can be found in "hibernate_if.h" and is switchable before upload "SHORT_SLEEP". The feature is also adjustable and you can adjust the number of seconds of short sleep. After this time the Teensy wakes up for one second and then goes to sleep again. BEWARE: this is enough for some power banks, but there are others, which need more current draw to wake up. In those cases, the Teensy can not provide the current draw to reactivate the power bank. Experiment with your type of power bank!
Usable from our experience:
- Anker A1210 (26800mAh) --> allows recording for 9 days [24h, 1 min of rec every 10 min]
- Anker PowerCore 13000
NOT usable from our experience:
- Anker PowerCore II 20000
Second possibility to avoid this USB power bank problem is to use a battery holder with three or four cells of the AA or C type and connect the two wires to Vin and GND at the Teensy headers.
NEW: I do not recommend the use of USB power banks any more
- the #define SLEEP_SHORT with the according ShortSleepDuration works for some types of power banks, but some power banks do not wake up, because the current from the Teensy is still too low
- I purchased an ANKER PowerCore 13000 about a year ago, it works very well and wakes up, BUT then I purchased exactly the same model again a few weeks ago, and it looks exactly the same from the outside, but that does not work at all and is not waking up . . . seems they radically changed the battery management system
So, I now soldered battery holders to all my prototypes (3 x AA batteries = 3.6 to 4.5 Volts depending on your battery type, connected to Vin and GND) and I do not use power banks for recording any more. This has two additional advantages:
- much more reliable
- current draw is drastically reduced, because it seems that a significant proportion of the current drawn was consumed by the power bank alone, and not the Teensy
- AA batteries can be purchased everywhere worldwide, even in remote areas in the tropics and can be taken with you and help a lot if you do not have the means to recharge your power bank
- for longer periods, you could even use C or D battery types to allow for several weeks of unattended recording
Current consumption, measured by cutting Vusb & Vin and inserting a quality (micro)-Ampmeter (DVM UT61E)[Frank DD4WH, 2018_03_18]
Hibernate: 0.15mA = 150.3µA with peaks of up to 0.5mA = 500µA
Booting after hibernation: peak currents up to 381mA (for a very short time: fractions of a second)!
No SD card inserted = error: 41mA
Recording and writing to the SD card: about 52mA, fairly variable: with measured minimum peaks of 28mA and maximum peaks up to 112mA (with peak function of DVM)
Audio buffering without writing to SD card: 32mA
What does that tell us:
we can expect a mean current of 60-70mA when recording to SD
hibernation mean current should be < 2mA (when waking up every 60sec)
with a 50% (recording/not recording) time scheduled duty cycle (equivalent to 36mA mean current consumption) we should expect a battery life time of about 360 hours = 15 days with a small USB power bank of 13000mAh --> a conservative prediction
for triggered recording schemes with 50% recording/50% buffering we should expect ((70mA+32mA)/2) = 51mA on average, which means a predicted battery life cycle of 13000mAh/51mA = 255hours, which is 10 days
All values predicted from current measurements in the lab, not yet validated in the field.
Recording was achieved for nine days with a USB powerbank with 13000mAh [temperatures were quite low with minus degrees at least in the night !]. After that the power bank was empty.
Recording scheme was:
Recording windows: 03:00h to 10:00h and 17:00h to 24:00h --> 7h + 7h = 14h window
Recording times inside these windows: 2 x 1min and 3min pause --> 24min recording per hour
--> RECORDING: 9 days * 14h * 24min = 3024min = 50 hours and 24min of recording
--> STANDBY/HIBERNATION: 9 days * 10h * 60min = 90 hours of standby
That would correspond to 90 hours * 2mA = 180mAh for HIBERNATION and 50.4 hours * 80mA = 4000mAh
Comparing with the above measurements, the power bank should allow more than triple the time.
There are two possibilities to explain that:
the average power consumption with that recording scheme is considerably higher than expected
the USB power bank has a considerably lower capacity than 13000mAh [considering the low temperatures near 0 degrees at night during the field test period this is quite normal, I would think!?]