According to Wikipedia, the following rechargeable battery chemistries have cell voltages of 1.2V: At a glance, it would appear that nickel is the common denominator, but this is not the case, as nickel-hydrogen and nickel-zinc have voltages of 1.5V and 1.7V, respectively. So, excerpting the relevant sections of Wikipedia: Nickel-iron:
Multiply that number by six cells and you arrive at 12.6 volts. Battery chemistry can vary from one battery to the next, so we say "about 12.6 volts," because a battery could be fully-charged at only 12.5 volts or 12.8 volts and some batteries may be fully-charged at even higher rates.
Voltage is then defined as the pressure that pushes electrons (current) between two points to enable them to power something. Battery voltage refers to the difference in charge due to the difference in the number of electrons between the negative and positive terminals of the battery. This is also known as “electrical potential.”
One common concern is whether devices designed for 1.5-volt batteries can effectively run on 1.2-volt rechargeables. The answer is a resounding yes. In most cases, devices that accept 1.5-volt batteries can seamlessly transition to 1.2-volt rechargeables.
Understanding the voltage rating of a battery is crucial for selecting the right power source for your devices. While disposable batteries often boast 1.5 volts, rechargeables operate at 1.2 volts. This lower voltage doesn’t mean they’re less powerful; rather, it’s a testament to their unique chemistry.
In addition to the chemical reaction, higher-voltage batteries like a 12V battery have multiple cells in series to increase the voltage. A single AAA battery is only one cell, whereas an RV battery has 4 to 6 cells. This is why the average, fully charged car battery will measure around 12.6 volts (also known as the resting voltage).