RTC battery deep discharge protection
There are tons of ways to do it. I like the idea of using an transistor (MOS, BJT, doesn't matter) and a zener. The zener will conduct as long as the battery is greater than the zener voltage. Remember that the zener in this must be small enough to turn on the transistor. When the voltage in the circuit below gets below 2V, the zener won't conduct at all. This is just off the top of my head, there may be better options.







simulate this circuit &ndash; Schematic created using CircuitLab