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I want to turn off my micro-controller (Arduino) as soon as the battery reaches a specific low voltage. So, for example, if I have Zener diode with 9.1V Zener voltage, does that mean than as soon as it reaches it, it'll shut down until the voltage rises (battery is charged?) As I can understand from datasheet, it will actually fluctuate and wont be stable. Is there another way to cut-off voltage to a micro-controller to prevent battery degradation?

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    \$\begingroup\$ Fluctuate? No, but there is a a resistive slope and not a hard on-off threshold voltage. Have you tried to simulate it with a real Zener or checked the datasheet for a real Zener diode? Also, please post a schematic of what you are thinking about since the MCU+Zener isn’t entirely clear. \$\endgroup\$
    – winny
    Feb 11, 2022 at 9:29
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    \$\begingroup\$ Battery monitor using a low power comparator that can be used to turn off a load. \$\endgroup\$
    – Andy aka
    Feb 11, 2022 at 9:30

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This is a general purpose battery switch sensitive to battery voltage, and is much more accurate and stable than a zener diode. It uses little power when on, and no power when off.

schematic

simulate this circuit – Schematic created using CircuitLab

IC1 switches at 1.25 V on the control terminal, think of it as a high gain transistor with a very well defined VBE of 1.25 V. Adjust the battery threshold voltage with the R11 and R9 ratio.

Once IC1 is on, it turns the FET on, which keeps IC1 on. If it's off, the FET is off, and it stays off. This sort of latching arrangement is often called a thyristor latch.

Once the battery voltage has dropped below the threshold, it turns off and stays off. I've taken advantage of this latching action by adding push switches for on and off, to make it double as a switch.

D2/3 allow it to not turn on at all if the battery is below the low threshold initially. Omit D3 and replace D2 with a short if you want it to start up regardless if the on switch is operated.

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