Will a WiFi battery monitor slowly drain my battery if the truck sits all winter?

The short answer is that modern ESP chip implementations, like those using ESPHome, aren't necessarily drawing a constant, high current. When you use deep sleep modes correctly, the ESP can wake up, take a measurement, transmit the data, and then go back to sleep. It's not continuously broadcasting on WiFi while waiting for a trigger; it's waking up on a schedule and then powering down completely. The small residual draw you're worried about is often just the power supply's quiescent current, not the ESP itself. To address the power supply worry, low quiescent current supplies are designed to have an idle draw measured in microamps, whereas a typical cheap wall adapter might draw milliamps. This keeps the parasitic draw from the electronics to an absolute minimum. Even if the chip is technically 'awake' while connecting to the access point to initiate a WiFi link, that window is typically milliseconds, not minutes. The ESP spends the vast majority of its time in a power state that draws near zero.

Regarding the low battery cut-off, the ESP shold cut out well before the battery physically dies, provided the code is written right. You absolutely should configure the monitor to read voltage and trigger the notification or a local alarm while the voltage is still high enough for the ESP to operate. usally, if the voltage is below the operating threshold (like 10V or 9V), the ESP just stops responding. The goal is to trigger the alert when the voltage is still in a 'danger zone' (say, 11.5V) but before the battery hits a point where it can't sustain even a brief wake cycle. I've run these on vehicles parked for weeks, and the only time I've seen issues was with grossly undersized power supplies or if the device wasn't configured to enter a deep sleep state between its scheduled readings. The built-in features often include sensors that can detect temperature or even low voltage and handle the alerts independently of the cloud connection, which is always safer for somethig in a garage or under a car.

The part about the device having to see the voltage drop while it is awake makes sense. If the ESP is sleeping, it cannot see the battery failing, right? So the notification has to happen during one of those quick wake-up cycles. It sounds like the whole point is making sure the reading happens at a time when the voltage is low enough to matter for a user, but high enough for the chip to actually run its little program. Should I set up the thing to check the battery reading once an hour, or would checking every few minutes be safer for the alert to trigger?

I went this route a while back for a project car that sat for months. I ended up using a small dedicated shunt monitor instead of trying to pull the voltage right off the main battery terminals. It gave me much cleaner data without worrying about parasitic load on the main circuit. It made the setup much tidier, too. My biggest hurdle was making sure the signal didn't get noisy when the engine finally cranked up and everything started drawing heavy power. I had to make sure my sensing line was shielded, or I was gonna get a bunch of garbage readings. How did everyone else handle the noise issues once the monitoring acutally started working?