Ni-Cad battery voltage

Anyone know why Ni-Cad rechargeable batteries are always rated 1.2 volts instead of the standard 1.5 Volts? Can’t they make a 1.5 V rechargeable battery?

I believe it is very much limited by the battery technology; alkaline and zinc/carbon technology is limited to a single voltage too; ‘9 volt’ batteries (which are true batteries; most of the things that we commonly refer to as batteries are in fact cells) actually contain 6 x 1.5v cells(AAAA size) connected in series.

The voltage in a battery cell is determined solely by the chemistry. NiCd’s are 1.2V, NiMH’s are 1.25V, alkalines are 1.5V, and lead-acid cells are 2.0V. It has to do with molecular bonding energy, without getting too technical.

The voltage is determined by the type of chemical reaction. As long as you are using nickelous hydroxide and cadmium hydroxide, you get 1.2 V.

However, voltage is not constant. Alkaline batteries are 1.5 V when fresh and not in use. But they have high internal resistance so if you connect it to a load, the voltage will drop. Also the voltage decreases steadily as the battery is used up, so near the end of its life it’ll have less than 1.2V. Ni-Cd batteries have very small internal resistance and a very flat discharge curve, so even under load the voltage remains close to 1.2V until it’s almost used up.

Welcome to the board, by the way.

The implication then is that most devices must be current driven, since they seem to function with 1.2V, 1.25V etc. And yet I have a battery driven tape recorder that seems to slow down as the battery weakens.

So when a battery is “dying” is its voltage dropping or is the current dropping, or both?

Both. Actually, technically the current is dropping, since the voltage is determined by chemistry. As the battery drains, its internal resistance decreases, and per Ohm’s Law, the current decreases too.

It depends on the device. Flashlights get dimmer as the battery is used up. Electronic devices often contain voltage regulators - these accept a wide range of voltages and output a constant voltage. But regulators can’t increase voltage, so when the input voltage drops below a certain limit the output voltage starts to drop as well, which is probably why the tape recorder suddenly slows down.

When a battery is “dying,” the voltage drops. What happens to the current depends on the device. If it’s something simple like a light bulb or resistor, a voltage drop results in a current drop. But some electronic components draw constant current, or even increased current when voltage drops. PWM (pulse width modulation) voltage regulators behave that way - it draws constant power, so when voltage drops it draws more current to compensate. (power = voltage * current)