I brought 6F22 9V battery. One's output voltage became nearly 6 volt after some use.But when I connect it with a MCU ckt , It gives only 3.37V as output ( voltage tested directly from the battery's nodes ). Then I connect a new battery which gives 9V as output when it is not connected with circuit. But when I connected it with the ckt, it gives around 6.7 V .
=> Why this output voltage of battery changes after connected with the circuit? As I know Battery is a constant voltage source.