Check the battery status with NodeMCU?

With a recent firmware you can use adc.readvdd33(). That should be enough for your case

I read somewhere that adc.readvdd33() was deprecated? Effectively it is for many of the ESP8266 modules available, the docs say, "If the ESP8266 has been configured to use the ADC for sampling the external pin, this function will always return 65535". So that means that any ESP8266 that has an ADC pin (like ESP8266-07 or -12, etc.) has this shunted in firmware.

But by adding a couple of resistors to make a voltage divider, you can still use the ADC pin for this.

[![schematics][1]][1] [1]: http://i.stack.imgur.com/FEILF.png Those resistor values will allow it to read 0-12V, as a value between 0-1024. (The voltage at the ADC pin must be less than 1V.)

val = adc.read(0)

Addendum: Adding this to your circuit incurs a power draw of approx. 0.01 milliamps, small but more than nothing. Multiply the values by 1000 to reduce it to infinitesimal. Or use 18 megaohm for r1 and 2 megaohm for r2, which divides the voltage by 10, and (wild guess) drains less current than most if not all batteries will attenuate when disconnected.