Simple car battery monitoring circuit

For a project running off a car battery, I need to know if the battery has a high enough voltage for me to start my electronics. There are tons of Battery gauge chips out there, but they're all very advanced and quite expensive. All I need to know is that the voltage is above 12V or I'll put my microcontroller to sleep and not turn on any peripherals.

If your circuit is battery operated, you'll want to make sure it never drains the battery beyond it's lower limit. For simple use cases, you can just use a Supervisor Chip with a preset cutoff voltage. Many LiPo batteries will have this built in, but in my case I'm running off an AGM car battery connected to a Solar charger. If the battery is close to it's limit, I want my electronics to only wake up once a day to report the battery state. This does not need to be precise, but I need it to be controllable.

This simple Voltage Monitor is using a complementary Mosfet pair to measure a voltage source (such as a 12V car battery) without continously draining it. If you toggle Enable, you will see the scaled voltage output at VM1. If not, your circuit isn't consuming any energy at all.

This is just the bare circuit, but in my actual circuit I'll use a DMC2400UV or DMC2053UVT (adds ESD protection) since these are easy to use and delivered in a tiny, convenient package. Instead of SW1, I'll use a 3.3V microcontroller output and then I'll measure at VM1 using an analog input. The values of R3/R4 can be adjusted to scale to any sensible voltage range. Based on the reading, one can easily estimate the remaining battery. For a 12V battery, a reading of 12.7V is a fully charged battery and 11.9V is a fully discharged battery. By enabling the circuit above and reading the voltage at VM1, I can reliably measure this using just the ADC of my microcontroller.

You can test the circuit here and play around with the parameters. Change the battery voltage by editing V1. Toggle on/off by selecting the Enable button and press Space. The circuit is based off the the idea that Nordic Thingy used to do it's voltage detection.