Is your feature request related to a problem? Please describe.
I would like to be able to use 12v lead acid batteries to directly power the Pinecil v2 without worrying about over-discharging and possibly damaging the battery.

Describe the solution you'd like
In the settings menu, under Power Settings > Power Source, I believe it would be a useful addition to the firmware to add a Low Voltage Cutoff setting specifically for use with 12v sealed lead acid batteries.
The accepted maximum discharge cutoff voltage for all 12v lead acid batteries is 11.6v.
Discharging any lead acid battery below 11.6v can damage the battery and reduce it's life-span.

Describe alternatives you've considered
Of course LiPo batteries have a much higher power density to weight/size ratio and can handle a bit more amperage draw, but SLA batteries are very common and fairly inexpensive and even used ones can still serve as a viable power source for most soldering jobs as long as the battery is still capable of holding a full charge around 13v.

Additional context
After some testing I have concluded that deep-cycle (20hr rated) 12v sealed lead acid batteries that have a capacity of 4 Ah and higher are suitable for use as 'portable/off-grid' power supplies for the Pinecil V2.
I tested this using an old 4.5 Ah SLA battery salvaged from a UPS. Even with it's age and being at the end of it's useful life, I was still able to get about 2-3 hours of intermittent soldering time from a full 13v charge. (My tests were actually DE-soldering time, using braided copper solder wick, so this actually requires considerably more heat and prolonged heating times than assembly soldering.)

While the iron was first heating up (peak wattage), there was a voltage drop of about 0.75 volt across the battery.
This voltage drop reduces to about 0.50 volt after the iron reaches the set operating temperature and the temp-control PID takes over.

This voltage drop should be less with larger capacity batteries, but given the intermittent nature of most soldering jobs, it shouldn't harm smaller batteries.
The maximum amperage draw on the battery is:
about 2 amps @ 12v (24 watts) when using the 6 ohm tips...
and about 1.5 amps @ 12v (18 watts) when using the 8 ohm tips.
For smaller capacity batteries, this voltage drop can also be reduced a bit by setting the Advanced Settings > Power Limit to between 18-20 watts.... but it's not necessary if using a battery in new or good condition.

Thank you for your consideration.
And thank you for such a wonderfully useful and feature-rich firmware for smart soldering irons! :-)