AA and AAA batteries are different in size, but both have a voltage of 1.5 V. Nickel–metal hydride (NiMH) batteries are rechargeable and come in the same sizes as AA and AAA, but their voltage is 1.2 V. Mobile phone lithium batteries are often rated at 3.7 V.
Have you ever noticed these strange numbers, and wondered why the voltages of these batteries are such fixed values? Today, let’s get to the bottom of it.
Back in middle school chemistry, we all learned about galvanic cells (voltaic cells). If you don’t quite remember, here’s a quick review:
A galvanic cell is essentially a redox reaction: the anode (negative electrode) loses electrons, and the cathode (positive electrode) gains electrons.
In this process, the tendency of the anode and cathode materials to lose or gain electrons determines the potential difference, so the voltage is, in fact, determined by the materials used for the electrodes. Because there is an equilibrium between the forward and reverse reactions, the electromotive force (EMF) of a battery can actually be calculated by certain methods.
Take the common dry cell as an example. Zinc is used as the negative electrode, and its standard electrode potential (under standard conditions) is −0.76 V (oxidation from 0 to +2). Manganese dioxide is used as the positive electrode, with a standard potential of +0.95 V (reduction from +4 to +3). So, in theory, the voltage of a dry cell is 1.7 V. Taking into account that actual battery materials deviate from standard conditions, the open-circuit voltage of a dry cell is about 1.6 V. Due to reaction equilibrium and the effect of load, the working voltage under load is roughly 1.5 V.
There are many types of lithium batteries, so their voltages vary quite a bit. Here are some common cathode materials and their typical nominal voltages for reference:
- Manganese dioxide: 3 V (coin cells)
- Iron disulfide: 1.5 V (lithium iron batteries)
- Lithium cobalt oxide (LiCoO₂): 3.7 V
- Lithium manganese oxide (LiMn₂O₄): 3.7 V
- Lithium iron phosphate (LiFePO₄): 3.2 V
The last three types are generally used in various mobile devices.
You might be wondering: since single-cell battery voltages are all so low, where does the higher voltage required by laptops come from?
In practice, to balance capacity and voltage, multiple cells are usually connected in series and/or in parallel. For example, the common 7.4 V pack in digital cameras is simply two 3.7 V cells in series. Another example: a “11.1 V 5200 mAh” laptop battery pack is actually made up of six 3.7 V, 2600 mAh cells connected as 3 in series and 2 in parallel (a 3S2P configuration). Similarly, the occasionally seen 9 V battery is in fact a stack of six miniature flat cells inside.
Finally, let’s go over some common battery sizes :)
- The No. 5 battery, also known as the AA battery, is designated as R6 in IEC (International Electrotechnical Commission) standards (R indicates a round cell). If you add the material prefix, common variants include LR6 (alkaline) and HR6 (NiMH).
- The No. 7 battery, also known as the AAA battery, is designated R03 by the IEC.
- The No. 2 battery, also called a C cell, is designated R14.
- The No. 1 battery, also called a D cell, is designated R20.
- CR2 and CR-V3 are two types of batteries commonly used in cameras; C indicates a lithium battery with manganese dioxide as the cathode.
- CR2032 is the most common coin cell, widely used for CMOS backup power and in various remote controls.
Title image：4.5-volt, D, C, AA, AAA, AAAA, A23, 9-volt, CR2032 and LR44 batteries
Author：Lead Holder
