I want to make a portable data logger. So it should be a battery-powered device. One of tasks that I should do is estimating battery voltage and displaying it but because of the power problem I don't want to use voltage divider and ADC. I'm exploring circuit of a similar device in market. I'm sure that there is no voltage divider with resistors and there is no additional IC in circuit because I explore completely the circuit and I know role of each component in circuit. Microcontroller is stm32f103cBt6.
I can't understand how the device estimate battery voltage without any additional circuitry. How they do that in your opinion?