Issues with USB charging of Airnote's battery

Hi to the experts,

I have observed an unusual behavior when charging my operational Airnote through its USB port, as shown in the attached image taken form the dashboard. I need again some help, please…

The charger is a switching regulator with an LC low-pass filtered output that gives 4.9 V with a current up to 1 A. It is the same one that I used for powering Safecast’s air quality monitoring device built four years ago.

The displayed battery voltage was 3.5 V when the USB charger was first connected to the Airnote. I left it there for about 35 hours, a time that I suppose should be enough to fill up completely the 3.7 V Li-Po cell while also powering the Airnote’s circuitry.

Referring to the dashboard panels, this is what happened instead:

  1. At the beginning the battery voltage went up to about 4.32 V, but then it continued rising and after about 4 hours it reached a stable value of ≈4.89 V, very dangerous for a Li-Po cell!

  2. At the end of the 35 hours period the charger was disconnected: in this moment the battery voltage dropped down to a safe level of about 4.22 V.

  3. But less than 12 h after the charger was disconnected, the battery voltage went down again to the discharged value of 3.5 V: a very fast depletion process. Maybe the Li-Po cell is damaged?

  4. During the second half of the 35 hour charging period, the battery voltage showed a few sudden drops (highlighted by a black rectangle in the attached image), going from 4.9 to 4.67 V and then up again. During this period the displayed temperatures were too high by about 5 to 10 °C (verified with an external thermometer).In the same time also the PM readings dropped. Some abnormal overheating in the circuitry?

Thank you very much for your patience, and best regards,


Last hour update.
Two days ago I have repeated the charging cycle as shown in the following dashboard panels:

The Airnote stopped sending data after about 24 h from the beginning of this cycle. I have disconnected the external power supply, but the Airnote remains silent. The display of the Airnote is still showing the PM data.

Second update:

I have switched off and on then on again the Airnote and checked the display. At the beginning everything flows more or less as expected:

  • three dots,
  • display segments check (OK),
  • battery voltage (3.95 V, OK),
  • temperature (NOTE: it is too high, indicating 27.1 °C when it should be 23 °C,
  • humidity (OK)
  • pressure (OK).

Now the countdown from 5 to 1 should begin, but the display remains fixed to 5 for a while.

Then an up-count from 1 to 30 starts, and at the end the display swithces to “-100”, I suppose an error code.

After about 1 minute this code changes into “CL”; a few seconds later also a blinking “+” appears on the left.

After about 30 seconds the display changes from “+ CL” into “+ 4.3”, still with a blinking “+”.

About 30 seconds later the blinking “+” disappears and the display shows a stable “4.3”.

I hope this may help understanding the problem.

Thank you very much and regards,



I forgot to mention that the fan of the PM sensor does not run.

Hi to the experts,

Many days have passed and my Airnote is still out of use, as described above.

Could anybody help me, please?

Thanks and best regards,


Hi @AlberTO,

It sounds like we need to swap out your unit for a new one (and inspect yours to see what may be wrong with it). Can you please DM me with your email address and we can work out the exchange?


Thank you Rob!

OK if I write you at this address?