If you have 2 (e.g. 1.5V) batteries in series and one is dead/empty, what is the reason why a device is not working? Lack of voltage (1.5 + 0 is simply not enough for the device to operate)? From my understanding, for batteries in series, the current is the same as for one of them. So, a common sense says to me that it must somehow connected with voltage, not current. Is it true?
A battery is considered dead when: 1.5 volt battery is .8 volts and below, a 9 volt battery is 5.4 volts and below.
So if you have 1 good battery and 1 dead battery, you will have 2.3 volts instead of 3 volts.
You will lack the voltage required to run a circuit. (The amperage available stays the same).
This is where you have to understand the difference between amperage available in the battery and amperage in the circuit.
A battery works by harnessing an electric potential to drive electrons through a conductor. When two batteries are in series, there's a sandwich of different potential regions in the pattern negative-positive-negative-positive. As electrons leave the negative end of this sandwich and through a conductor, electrons are also simultaneously filling and neutralizing the positive end too. Now the positive end is becoming more and more negative, and this causes electrostatic repulsion in the negative region next to the positive end. The repulsion pushes electrons into the next positive region to satisfy the fact that the potential is being depleted and put to use. Now let assume the pattern becomes negative-positive-neutral-neutral, you can see already that it doesn't add up. There isn't a way the electrons leaving the negative end can make it to the positive region needed to be neutralize to have electrostatic neutrality.