Stop cylindrical lithium batteries from draining

[JonasBchrt]

If no external power is available to the system (from batteries, solar...), then the Witty Pi draws power from the small cylindrical lithium battery and keeps the Raspberry Pi running. The Witty Pi is configured to shut down the Raspberry Pi as soon as the lithium battery voltage falls below 4 V. So, in theory, the battery voltage should never drop much below 4 V. However, we repeatedly found lithium batteries drained to around 3 V after days of power loss despite the 4 V threshold being enabled in the Witty Pi's configuration.