This issue may be common knowledge but it was news to me and caught me by surprise last spring.

The prior fall I tucked my Nano3 away for its long Chicago winter sleep. The battery was found dead in the spring. After I charged it back up I found that the recorder "seal" was broken (“DIGITAL SIGNATURE FAILED”). I had to send it to Cumulus-Soaring to get it "re-sealed".

I was then told that you need to keep the Nano3 battery charged up or the unit will loose its seal. Unsure about the other Nano models. None of my previous, or current, recorders have this requirement (but none have an internal battery either). Maybe I missed it but the manual doesn't seem to mention this potential for low battery seal failure.

I guess I'm surprised that the state of the seal isn't kept in non-volatile memory. Lesson learned.

- John OHM Ω

PS - The Nano3 manual does mention this about battery storage...

4.11 Storing Nano - The NANO3 must be stored in a dry environment with temperature lower than 25°C (77° F). If the NANO3 will not be used for an extended time, it is recommended that you charge the battery to 50% beforehand. The NANO3 and the BATTERY have protection circuits which protect the battery from being overcharged or overly discharged. If the NANO3 is stored with an empty or near empty battery, eventually the battery will be discharged so far that it will not be possible to charge it again.

5.7.13 Password
Following passwords are available for users:
11111 - Prepare battery for storage (discharge to 50%)
[other passwords omitted - see manual]