A battery consists of one or more voltaic cells. Each voltaic cell consists of two half cells. Negatively charged anions migrate to the anode (negative electrode) in one half-cell, while positively charged cations migrate to the cathode (positive electrode) in the other. The electrodes are separated by an electrolyte that is ionized to create the anions and cations and permits movement of those ions.
Sometimes, the half-cells have different electrolytes. In that case, a separator prevents mixing, but the ions can squeeze through. In many cells, from carbon zinc through nickel cadmium (NiCd) and lithium ion (Li-ion), the electrolyte is merely a buffer for ion flow between the electrodes. In other cases, most familiarly the lead-acid cells in an automotive battery, it’s part of the electrochemical reaction.
The open-circuit voltage (OCV) of a charged battery is the electromotive force (EMF) of a cell, which comes from the difference between the reduction potentials of the reactions in the half-cells. During discharge, the battery converts the heat that would be released in the chemical reaction between the anode and the cathode into electrical energy.
Real-life batteries exhibit an equivalent series resistance (ESR) internally, which drops some of the OCV when the battery is used in a circuit. ESR increases as a battery discharges, so the actual terminal voltage droops with use. Batteries also tend to selfdischarge— some kinds more than others. NiCd and nickel-metalhydride (NiMH) batteries will self-discharge at a rate of approximately 20% per month; Li-ions 5% to 10%; lead-acids 3% to 4%; and alkalines less than 0.3%.
Most batteries are inherently rechargeable by reversing the chemical reactions that took place during discharge. They vary, though, in how fully and how many times they can be recharged. Charging is a classic redox chemical process. The negative material is reduced, consuming electrons, and the positive material is oxidized, producing electrons.
To illustrate the chemistry of charge and discharge, Oak Ridge Micro Energy offers the example of a battery with a metallic lithium anode and a lithium-cobalt-oxide (LiCoO2) cathode. When formed, the battery is in the discharged state, so the first step is to charge it to the lithium battery’s nominal 4.2-V nominal OCV. This results in the extraction of half of the lithium from the LiCoO2 cathode. The battery can then be discharged down to 3.0 V. The chemical reactions of the charge and discharge steps are:
LiCoO2 = 0.5 Li + Li0.5CoO2
0.5 Li + Li0.5CoO2 = LiCoO2
Separating the discharge process at anode and cathode:
0.5 Li = 0.5 Li+ + 0.5e–
0.5 Li+ + 0.5e– + Li0.5CoO2 = LiCoO2
The lithium ions move through the electrolyte. The electrons flow through the external circuit. The Li+ ions fill the vacant spaces that were created upon extraction of the lithium ions from the cathode during charging.
In terms of battery characteristics, capacity is the quantity of charge that can be supplied. It’s usually expressed as current times time, i.e., ampere hours (Ah). How is that charge? Consider that if C is the charge in Coulombs, 1 A = 1C/s. Therefore, 1 Ah is essentially 3600C.
The power delivered by a battery is the energy supplied per unit time usually expressed in units of watts, milliwatts, or microwatts, where 1 W = 1 J/s. Battery energy, usually expressed in watt hours (Wh), is the operating voltage times the charge supplied. The energy could also be expressed in joules, where 1 Wh = 3600 J.
Conversely, power in watts is energy per unit time, that is, J/s. Unlike the battery potential, energy depends on the size of the battery, i.e., the amount of charge that can be supplied is proportional to the mass of the cathode.
In comparing batteries or battery types, the key characteristics are energy and power density—that is, energy and power per unit volume and mass.