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I am making my first battery powered circuit. I am confused as to what happens when a battery is getting close to empty. Here is a quick example of what I am confused about:

From my understanding a 12 V battery in a car does not go to 0 Volts when it is empty. A car battery will go between say 12.6 V and 12 V. When it is at 12.6 V it is fully charged and when at 12 V it is considered empty.

Let's say the battery is at 12 V and it is considered empty. Am I still able to reliably use this 12 V in a circuit, but the battery will not be able to supply any current, or am I unable to use this 12V once the battery is empty?

Basically what I am trying to figure out is how to turn off this circuit with a switch. My real application is using a 36 V battery and in normal operating conditions will use 15 A of current. This means I can't really use a normal switch because the voltage and current ratings are too high and it will burn it up.

Instead I was thinking I could use a P channel MOSFET on the positive side of the battery with a switch and a voltage divider shown below:

enter image description here

The circuit will be on when SW1 is not connected to 36 V and it will be off when SW1 is connected to 36 V.

The thing I am unsure of is if I can expect this behavior if the battery is empty? Can I reliably use the voltage of this empty battery, or would it give me unexpected behaviors that turn on the MOSFET when it should be off? Is there a better way to do turn off a circuit rather then using a P-CH MOSFET?

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  • \$\begingroup\$ Your MOSFET is the wrong way round. The body diode will mean that your load is always on. \$\endgroup\$
    – Finbarr
    Aug 23, 2023 at 12:23
  • \$\begingroup\$ And "dead" is not a particularly useful description. I'd consider a car battery dead when it no longer has enough energy left to start the car, although it may have still have quite a lot actually left. \$\endgroup\$
    – Finbarr
    Aug 23, 2023 at 12:26
  • \$\begingroup\$ a 12 V battery at 12 V can still supply lots of current. It's just that you don't want to draw lots of current as it will rapidly drop to a damaging low level of voltage. However, you can safely draw the tiny amounts of current needed to operate comparators and similar electronics needed to control your FETs, without the voltage dropping significantly. \$\endgroup\$
    – Neil_UK
    Aug 23, 2023 at 12:48
  • \$\begingroup\$ The battery is empty, not "dead". dead means that it's damaged, unusable, not rechargeable. Empty means that it's OK but its charge is depleted: it can be recharged. \$\endgroup\$ Aug 23, 2023 at 13:01
  • \$\begingroup\$ "Basically what I am trying to figure out is how to turn off this circuit with a switch" That is a different question from the one in your title: "Can you reliably use the voltage of an empty battery in a circuit?". I suggest you start a new submission and ask that question instead. Someone may close this question for "lack of focus" because you asked two separate questions.. \$\endgroup\$ Aug 23, 2023 at 13:07

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Maybe a useful analogy for you to think of empty batteries is that its voltage source stays constant but its series resistance keeps increasing, so its load-driving capability diminishes.

If your circuit has a huge input impedance, the increased series battery resistance is negligible and there'll be hardly a difference between a fresh or empty battery.

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Can you reliably use the voltage of an empty battery in a circuit?

The voltage? Yes, but at very little current (such as 1 mA), for example just to keep a controller alive, and only for a limited time.

The current? No, not a high current. Doing so will degrade or damage the battery. Specifically, the cells with the least charge could get reversed (the voltage across them becomes negative) permanently damaging them.

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