Why rechargeable batteries use 1.2V

Question

I have noticed that AA (or AAA in that case) rechargeable batteries are always of nominal 1.2V while non-rechargeable have nominal voltage of 1.5V. This works just fine in some cases, e.g. when the voltage requirement refers to just one battery and to a not really demanding load such as a tungsten lamp. In the latter case only the lumens would be affected and since this amount of voltage would normally be used as an indication lamp no harm is made.

However, in the case where more demanding loads exist, e.g. when added 3 or more batteries for an electronic device then this might just lead to stop functioning. 3*1.5 gives nominal 4.5V while 3*1.2V gives 3.6V so if the threshold is at 4V then even really fully charged batteries are inadequate to use.

I know that the nominal voltage is just an indication and a typical battery has a declining voltage, e.g. for 1.5V this means 1.6 or 1.65V fully charged, but the question is still valid.

Answer 1

Not all rechargeable batteries have a nominal voltage of 1.2V; it is specific to a few chemistries that happen to be popular. According to Wikipedia, the following rechargeable battery chemistries have cell voltages of 1.2V:

• Nickel-iron
• Nickel-cadmium
• Nickel-metal hydride

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:

The open-circuit voltage is 1.4 volts, dropping to 1.2 volts during discharge.

Nickel-cadmium:

Ni–Cd cells have a nominal cell potential of 1.2 volts (V). ...a Ni–Cd cell's terminal voltage only changes a little as it discharges.

Nickel-metal hydride:

A fully charged cell supplies an average 1.25 V/cell during discharge, declining to about 1.0–1.1 V/cell...the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts.

In conclusion, the 1.2V is only nominal voltage. 1.2V was a round number close enough to the typical voltage of those three chemistries and thus became the voltage on the label. However, every battery chemistry has different characteristics including voltage across a discharge cycle and open-circuit voltage.

Answer 2

Each type of battery chemistry yields quite different characteristics, including terminal voltage.

At the opposite extreme are Lithium cells in AA & AAA format, but Lithium-based batteries have a 3.x Volt terminal voltage - how do the manufacturers get a 3.x V lithium cell to output 1.5V? They put a tiny switch-mode voltage regulator in the cap!

Well designed electronics powered by AAA/AA/C/D cells should be designed to work down to well under 1.0 Volts, ideally down to about 0.8V to extract the most energy form the cell - any cell chemistry type. When this is the case, it doesn't matter that NiCad & NiMH rechargeable cells have that lower terminal voltage, they'll still deliver all their energy. Back in the old days, though, when a lot of electronics gadgets weren't designed to operate to < 1.0 V, the gadget fitted with NiCad rechargeable batteries would malfunction &/or shut off prematurely, and gadget manufacturers would warn customers not to use them, and to use non-rechargeable carbon/alkaline cells.

Of course the conspiracy theorists said the non-rechargeable battery companies were paying the gadget makers to do this deliberately, but the reality is that it's simply been the growing prevalence of low-dropout linear, and switch-mode, power supplies that allows gadgets to work down to much lower cell voltages.

Answer 3

Because rechargeable batteries use different chemistry than disposable batteries, and voltage depends on the chemistry being used. To replace three 1.5 V batteries, just use not three but four rechargeable ones. Mount the fourth one in series with the remaining three, 4×1.2 V is 4.8 V, which is close enough to 4.5 V.