I would like to power an ESP32 devkit board with four NiMH AA batteries.

The ESP32 board has an AMS1117 voltage regulator. It has a dropout voltage of 1V. The output voltage is 3.3 Volts so my understanding is that the minimum supply voltage is 4.3 Volts.

For 4 NiMH AA battries:

  • Full: 4 x 1.4 = 5.6 Volts
  • Nominal: 4 x 1.2 = 4.8 Volts
  • Empty: 4 x 0.9 = 3.6 Volts

This means that the voltage regulator should stop working when each cell reaches 1.075 Volts (4.3 / 4). This voltage should be safe for the NiMH batteries, my goal is to not damage the batteries by discharging them too much, is this "protection" enough to avoid deep discharge ?

External monitoring (absence of communication) will warn the user about the low ('dead') battery so the ESP32 should not stay in this condition for long.

  • \$\begingroup\$ A 1V dropout isn't low dropout. BTW. That's like me selling you a lightweight 10kg laptop. \$\endgroup\$ – TimWescott Aug 26 '20 at 22:05
  • \$\begingroup\$ There are other regulators that really are low dropout and have that same pinout, BTW. \$\endgroup\$ – TimWescott Aug 26 '20 at 22:05

The dropout voltage of 1V is arbitrary. The datasheet says it is 1.1 to 1.3V at 0.8A load, and the dropout is measured when Vout has already dropped by 1%. Dropout voltage is larger if current is larger and smaller if current is smaller.

The voltage regulator does not stop operation at the dropout point, it just can't keep the output at the rated voltage when there is not enough input voltage.

So no, it is not enough to prevent batteries from draining completely. That regulator has no undervoltage lockout.


my goal is to not damage the batteries by discharging them too much, is this "protection" enough to avoid deep discharge?

No, because as @Justme pointed out, the regulator doesn't stop delivering power, it just stops regulating.

You need to monitor the battery voltage and turn the thing off.

As an added wrinkle, NiMH cells (and most other cell chemistries) will bounce back to almost their fully charged voltage over time until they have to deliver current, at which point they'll quickly drop in voltage again. So if you just use some battery voltage monitor that forces the thing off, it'll turn off, recover, turn on for a bit, turn off, etc., until the batteries are really depleted.

You want to turn the thing off, and have it remain off either until it's put on charge, or until the user turns it on again.

Note, too, that NiMH cells (and, incidentally, dry cells) will usually deliver useful current without damaging the cell down to 1.0 or 0.9V per cell -- so you probably want to use a real low-dropout regulator so that your batteries can run down to 4V or 3.6V before you shut down.


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