Attiny 85 Low voltage cutoff

main.c

#include <avr/io.h>
#include <util/delay.h>
#define MOSFET 4
#define LED 1

/* Returns the voltage of a battery in mV */
uint16_t measure_voltage() {
	ADCSRA |= (1 << ADSC); //Start conversion
	
	while ((ADCSRA & (1 << ADSC)) != 0) 
	    ;
	
	uint16_t value = ADCL | (ADCH << 8); // Read value
	
	// Value from 0 - 1024 -> 0 - 25.2v, 0.025v/digit -> 25,200 max value, -> 25/digit
	return value * 25;
}

int main(void) {
	//Configure Mosfet to output and turn on
	DDRB |= (1 << LED)| (1 << MOSFET);
	PORTB |= (1 << MOSFET);
	
	// Configure ADC
	ADMUX = (1 << REFS1) | (1 << REFS2) | (1 << 1) | (1); // 2.56v Vref
	ADCSRA |= (1 << ADEN);	
			
	// Measure voltage at start up to estimate battery size
	uint16_t batt_ref = measure_voltage();
	uint8_t cell_count = batt_ref / 3400; // Estimate cell >= 3.4v
	uint16_t min_voltage = cell_count * 3200; //Cutoff voltage
  
	uint16_t meas;
        while (meas = measure_voltage(), meas >= min_voltage) {
      	    for (uint8_t i = 0; i < cell_count; i++) {
		PORTB |= (1 << LED);
		_delay_ms(150);
		PORTB &= ~(1 << LED);
		_delay_ms(100);
            }
      	    _delay_ms(2000);
	}
	
	// turn off mosfet, adc and sleep
	PORTB &= ~(1 << MOSFET);
	ADCSRA &= ~(1 << ADEN);
	MCUCR |= (1 << SE) | (1 << SM1);
}