circuit with the idea to use batteries without a voltage regulator,
because they use current every time. The sensor board should stay in
low power mode most of the time, so a voltage regulator (even the
ultra low power
ones) could be responsible to a significant amount of power drain.
long ago, Espressif updated their datasheet of the ESP8266. Now they
state the operating voltage
is between 2.5 V
and 3.6 V (in
the past, they only guaranteed a minimum voltage of 3.0 V).
my own experiments, I found a working minimum voltage of 2.3 V using
different modules. This allows to use different battery technologies
the wide known LiPo batteries are no candidates, because their
voltage ranges from 3.0 V (drained, better go not lower than that)
to 4.2 V fully charged.
microcontroller circuits there is another interesting alternative:
nominal voltage of one cell is
3.2 V, which is near the 3.3 V of modern electronics. They are fully
charged at 3.6 V (perfect) and can be discharged to
2.5 V – 2.0 V (depends on the manufacturer). I managed to get cells
in the AA-battery size with 700 mAh capacity and used two in
next possibility are NiMH-cells. You have
to use two cells in series because their nominal voltage is 1.2 V per
cell. Fully charged they are near 3 V, then
they have a long plateau around their
nominal voltage and at 2.3 V they are over 90 % drained. Sounds good.
I used Eneloops which have a low self-discharge rate. Their capacity
is araound 2000 mAh.
alternative you could also use alkaline batteries. If they last long
enough, they could be an option because they are way cheaper than the
rechargable NiMH-cells. 1.5 V is their nominal voltage, which is
slightly higher if they are new. They can be drained until no voltage
remaines, but they are quite dead at 2.3 V
(or 1.15 V per cell). The capacitiy is a bit higher than the Eneloops
at around 2500 mAh.
programmed a test procedure to get battery life results in
time-lapse: The sensor board wakes up, connects to my Wifi, reads the
sensors, reports the states to my MQTT broker, sleeps for 5 seconds
an repeats everything. At every wakeup it increments the reboot
number and stores it in the RTC memory to last between the wakeups.
got the following results:
|Battery|| Voltage Range|| Reboots||Lifetime (10 reboots/h)|
|LiFePO4 (2P)||3.5 V – 2,26 V||53706||223 days|
|Eneloop (2S)||2.9 V – 2.27 V||82883||345 days|
|Alkaline (2S)||3.3 V – 2.26 V||91652||381 days|
a wakeup rate of 10 wakes per hour or every 6 minutes, every battery
technology can achieve the desired goal of 6 months of operation. Of
course the lifetime will be lower in the real usecase, because the
test didn’t acount for the standby currents. The winners are
clearly the alkaline cells, followed by the Eneloops. For
environemntal reasons, I will choose the NiMH, because they are