I'm designing a circuit to monitor the voltage of a Li-Po battery (single cell 3.7v, 2500mAh, 25A) and switch off the battery when the voltage drops to a certain threshold. The Circuit will also pulse an LED to indicate the Battery needs to be recharged.

Battery Monitor / Protection Circuit

The ATtiny85 will be programmed to monitor it's internal Vcc voltage and turn on the MOSFET if the battery is above 3V. When the battery is below 3V it will turn off the MOSFET and pulse the LED every couple of seconds. The micro will use sleep mode to reduce the battery drain of the monitor circuit.

Does this seem like a reasonable circuit to accomplish this task? Is the value of R2 reasonable? (not sure how to calculate this value) Can the ATtiny85 maintain the output on PB1 when it's in sleep mode? What kind of parasitic losses on the battery does the use of the MOSFET introduce?

Also, as the battery voltage decreases the voltage driving the gate (PB1) will decrease as well. I don't think this is a problem but not sure.


1 Answer 1


Your battery is drawn upside down in the schematic, so that's a problem.

The 200\$\Omega\$ resistor in series with VCC to the ATtiny will cause a large drop in the VCC voltage when the LED is illuminated...remember that the LED current must go through R1 as well.

I'm assuming that the LED is a small red LED requiring about 10mA. If that's not the case provide a link to the data sheet. I'm also assuming that the load is resistive and does not require much current...provide more information if that's not the case.

  • \$\begingroup\$ Is R1 even needed? Was attempting to give the micro a bit of surge protection. \$\endgroup\$ Sep 19, 2018 at 14:25
  • \$\begingroup\$ Are you worried about voltage surges? Why do you think those might occur? \$\endgroup\$ Sep 19, 2018 at 14:31
  • \$\begingroup\$ Yes, the load is resistive and the current should be less than 2A. The battery is rated for up to 25A continuous draw. The load is basically another microcontroller with several peripherals so the current demand will vary. \$\endgroup\$ Sep 19, 2018 at 14:36
  • 1
    \$\begingroup\$ So you are worried about voltage drops when the current surges? Replace R1 with a Schottky diode and increase the size of C1. Also, you should include this information in the question, not in a comment to an answer. \$\endgroup\$ Sep 19, 2018 at 14:39
  • \$\begingroup\$ I was worried about surges when the battery is removed / reinserted. \$\endgroup\$ Sep 19, 2018 at 14:39

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