This blog post also had contributions from Larry Wenstein and Daniel Lowy
As you evaluate different battery technologies, you will come across common terms. Understanding what these various terms mean will allow you to compare your options more effectively, and will help ensure that you select the solution that best matches your application needs.
Battery Terms and Definitions
Some of the most common battery terms that you should know include:
1. Primary and secondary – These two terms are essentially interchangeable with disposable and rechargeable, respectively. So, if secondary batteries are available, why do we still use primary batteries? In general, a primary cell can maintain its charge for years on end, while a secondary cell must be recharged in a matter of months. All else equal, primary batteries generally have higher capacities than the secondary batteries.
2. Chemistry – A “battery chemistry” refers to the materials system that creates power in a given battery. A battery chemistry include the electrode materials and electrolyte. A cathode refers to the electrode to which electrons flow in from the external circuit- or where reduction takes place during discharge. An anode is the electrode from where electrons flow out and into the external circuit, or where oxidation takes place. (Note that this is true for primary cells. For secondary cells this convention holds true under discharge, while the opposite applies during charge). A number of primary and secondary battery chemistries are available, with their own characteristics in terms of safety, cost, biocompatibility, and so on.
Common primary battery chemistries include zinc-MnO2 and various lithium chemistries. Zinc-MnO2 systems have a zinc anode, a manganese dioxide based cathode, and various electrolytes (caustic KOH solution for alkaline batteries, and ZnCl2 and/or NH4Cl based electrolytes for so-called “carbon zinc” batteries); alkaline (KOH electrolyte) batteries with zinc anodes and silver oxide cathodes are also available, as well as batteries which involve air in the cathode reaction (zinc-air batteries).. Lithium batteries, as the name implies, use lithium as the anode, and have organic electrolytes, which are compatible with metallic lithium. Lithium batteries use various cathodes, including but not limited to MnO2, SOCl2, I2, FeS2, and Cfx.
Secondary battery chemistries include lead-acid, nickel-based alkaline cells, and lithium-ion. Lead-acid batteries use a sulfuric acid electrolyte; the anode and cathode are lead and lead oxide, respectively, when charged, and lead sulfate when discharged. Nickel-based alkaline cells use a nickel hydroxide/oxyhydroxide cathode in a caustic KOH electrolyte; this cathode can be paired with rare earth metal hydrides (Ni-Mh batteries), cadmium (Ni-Cd batteries, largely obsolete), or even iron (Ni-Fe batteries largely obsolete). Today, much effort focuses on applications of lithium-ion batteries. Lithium-ion batteries are batteries in which lithium ions shuttle between the anode and cathode during charge and discharge; there are a variety of materials used for both electrodes, and various solid and organic electrolytes are used i.e. lithium-ion is NOT one chemistry! The most common anode material for lithium-ion batteries is graphite, while much research has focused on bringing silicon anodes to life. Various metal oxides, such as LiCoO2, LiFePO4, LiMn2O4, and LiNiMnCoO2, are used as the cathode, while the electrolyte normally has a lithium salt dissolved in an organic solvent.
3. Voltage – The term voltage is used in many ways – theoretical voltage, open circuit voltage, closed circuit voltage, and nominal voltage. What do they all mean and what factors influence them? Read on to learn more. If equations intimidate you, hang on for just a minute. There is a lot to be gained from understanding the theoretical basis, including how environmental, state of discharge and concentrations affect cell voltage.
The theoretical voltage of a cell is the difference in potential between the two electrodes of the cell, based on thermodynamics. Because the absolute potential of an electrode cannot be measured, electrode potentials are commonly measured against a reference electrode at standard conditions. The standard cell voltage is then simply the difference of the standard electrode potentials of the two electrodes:
The standard electrode potentials are listed in tables, and can be retrieved from many resources.
What happens if you need to account for different temperatures and concentrations? The Nernst equation can be used to calculate the cell voltage under such conditions.
The cell voltage from the Nernst equation is:
R: the Universal Gas Constant (8.3145 J mol-1 K-1)
F: Faraday’s constant ( 96,484.6 C mol-1)
n: The number of moles of electrons exchanged between the redox and oxidation reaction.
Q’: The reaction coefficient
Further it can be used to understand how the cell voltage drops as discharge progresses and the concentration of reactants decreases and discharge product builds up (Watch this Khan Academy video for a demo of how the Nernst equation can be used to predict theoretical voltage as cell discharge progresses).
Theoretical cell voltage therefore depends on the cell chemistry, concentrations of the species, and temperature. Open circuit voltage (OCV) which represents an approximation of the theoretical voltage, is the voltage measured when the cell is not drawing current i.e., in a no-load condition. Closed circuit voltage (CCV) is the voltage when the cell draws current. This is lower than the OCV due to internal resistance and other losses. Nominal voltage is the typical operating voltage for a particular battery system. Cutoff voltage is the low voltage limit till which useful capacity can be obtained. This is often dictated by the application, although in the case of lithium-ion chemistries there is generally a low-voltage cutoff below which the battery suffers permanent damage.
4. Polarization The voltage drop between open-circuit and closed-circuit voltage is generically known as polarization. Polarization may be classified as ohmic, activation and concentration polarization. Concentration polarization arises from depletion of active materials in the electrode regions, due to slow diffusion of reactants from the bulk.
Activation polarization is the result of resistance where the electrode meets the electrolyte and represents a kinetic limitation to the charge transfer process at the electrode-electrolyte interface. Ohmic resistance reflects how much a given material system opposes the movement of electrons.
In general, activation polarization decreases with increasing current; concentration polarization will increase with increasing current, and generally is the factor limiting high rate pulse capability. Note, however, that concentration polarization can build up and reverse slowly during a pulse protocol (where one applies a pulse periodically) This effect is difficult to model, and must be obtained empirically.
5. Internal Resistance –The voltage drop under load contains both ohmic (roughly proportional to current applied), and non-ohmic components. The ohmic component is is called the ohmic drop or ohmic polarization IR, where I is the magnitude of current drawn, and R is the ohmic resistance. The ohmic resistance of a cell is the sum of a) the electronic resistances of the electrodes, current collectors and contact tabs b) the ionic resistance of electrolyte within the ionic solution or gel, the separator, and the porous electrodes, and c) the contact resistance between the active material and current collectors. Internal resistance is dependent on the battery chemistry used, cell construction, battery age, state of discharge, and temperature.
There are a few different ways to measure battery internal resistance; these techniques will give different values for the same battery. Very often, data sheets will give the 1 kHz resistance, which is obtained by applying a small voltage at 1 kHz to the battery and measuring the resistance. This is very frequently the measurement that battery manufacturers use for internal quality assurance purposes. Another measurement (which obtains so-called pulse resistance) involves applying a short (milliseconds to 10’s of milliseconds) discharge pulse, and measuring the voltage drop. Finally, to account for polarization effects which occur over time, sometimes a test will apply a steady-state current for a period of time, then apply a pulse on the order of 100’s of milliseconds to seconds; in this case, the rise in voltage and fall in voltage are used to calculate the internal resistance.
6. Capacity – Charge capacity or capacity is the amount of charge (usually expressed in Amp-hours, A-h, , that can be withdrawn from a battery under specified conditions. Theoretical charge capacity Q may be calculated using the expression Q (charge in Couloumbs or A.s ) = nF, where n is the number of moles of the active material, and F is Faraday’s constant which is 96,500 C mol-1. For a given chemistry, theoretical capacity depends only on the mass of active materials used; in general one electrode is the limiting electrode. In practice,cell capacity is only a fraction of the theoretical value. Actual battery capacities are dependent on the discharge rate, cutoff voltage and the operating conditions, such as temperature. A battery with a higher capacity will have a longer runtime, all else equal.
7. C – Rate – C – rate is an indicator of a battery’s current handling capability. It is a useful tool to compare batteries with different sizes. To determine the C rate, use the expression C Rate = Current (A)/ Rated Capacity (Ah), i.e. normalize the current draw to the battery’s capacity.. In other words, a 2C discharge takes place at twice the battery’s rated capacity, while a 0.5 C discharge takes place at half the battery’s rated capacity.A higher C rate indicates a higher current draw, or a more demanding discharge As a general rule, capacity will decrease beyond a certain point with increasing C-rate. Normally, battery manufacturers will provide discharge curves at various C-rates, and comparing these curves will help provide a rough idea of how “fast” a given battery is.
8. Energy density and power density – Although these are two separate terms, the concepts are connected and it is easier to define them together. The energy density of a battery tells you how much energy a battery can hold in a given volume (expressed in Watt hours/liter); the larger the number, the longer the runtime. The power density of a battery indicates how much power it can deliver on demand (expressed in Watts/liter). A water bottle is a common analogy used to describe these two concepts. A gallon jug with a small opening will have a high energy density because it can store a lot of liquid and a low power density because the opening limits how fast the liquid can be poured. On the other hand, a pint-sized wide-mouth jar will have a high power density because it can release all of its contents quickly, but a low energy density because it cannot store as much liquid. The latter case will yield lower capacities.
Theoretical energy densities are dependent on the battery chemistry only, and are never realized in practice. Practical energy densities are lower due to several factors: the need to account for non-reactive components such as separator, cell containers etc., the fact that real batteries never discharge at the theoretical voltage, and that batteries are not discharged completely i.e. to 0 V. Practical battery energy densities therefore depend on the discharge rate with higher discharge rates yielding lower capacities and therefore energy densities, the specified cut-off voltage, and the operating conditions such as temperature.
These are just a few examples of common battery terms that you should know. If you would like to dig deeper, we have compiled some educational websites to help guide you.