So vielen dank, nun gehts.
habs wie folgt gelöst:
Vref intern genommen. 2,56V
Ue = Vref / 1023 * ADC5_wert * ( 1+ R1 / R2 )

Code:
#define R1 12		// 12k
#define R2 10		// 10k
#define Vref 2.56	// Internal 2.56V Voltage Reference

// Ue = Vref / 1023 * ADC5_wert * ( 1+ R1 / R2 )
float BatPower(){ 

	// REFS0=1; REFS1=1; MUX0=1; MUX2=1
	ADMUX |= 0b11000101;
	ADCSRA |= (1 << ADSC);							// Start conversion
	while (!(ADCSRA & (1 << ADIF)));				// wait for conversion complete
	ADCSRA |= (1 << ADIF);							// clear ADCIF

	return( (float)Vref / (float)1023.0 * (float)(ADCL + (ADCH << 8)) * ( 1+ (float)R1 / (float)R2 ) );
}