An electric battery is essentially a source of DC electrical energy. It converts stored chemical energy into electrical energy through an electrochemical process. This then provides a source of electromotive force to enable currents to flow in electric and electronic circuits. A typical battery consists of one or more voltaic cells.
The fundamental principle in an electrochemical cell is spontaneous redox reactions in two electrodes separated by an electrolyte, which is a substance that is ionic conductive and electrically insulated. The voltage of electric batteries is created by the potential difference of the materials that compose the positive and negative electrode in the electrochemical reaction. The voltage produced by each lithium-ion cell is about 3.6 volts.
Cut-off Voltage
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery. In most batteries, deep battery discharge causes irreversible degradation of the materials. The coulometric capacity, the total Amp-hours available when the battery is discharged at a certain discharge current from 100% SOC to the cut-off voltage. Some electronic devices, such as cell phones, will automatically shut down when the cut-off voltage has been reached.
When testing the capacity of a NiMH or NiCd battery a cut-off voltage of 1.0 V per cell is normally used, whereas 0.9 V is normally used as the cut-off voltage of an alkaline cell. Devices that have too high cut-off voltages may stop operating while the battery still has significant capacity remaining.
Li-ion battery has a higher cut-off voltage of around 3.2 V. Its nominal voltage being between 3.6 to 3.8 V; its maximum charging voltage can go to 4– 4.2 V max. The Li‑ion can be discharged to 3V and lower; however, with a discharge to 3.3V (at room temperature), about 92–98% of the capacity is used. Importantly, particularly in the case of lithium-ion batteries which are used in the vast majority of portable electronics today, a voltage cut-off below 3.2V can lead to chemical instability in the cell, with the result being a reduced battery lifetime.
Other Characteristics
To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.
Cell Voltage
The voltage of electric batteries is created by the potential difference of the materials that compose the positive and negative electrodes in the electrochemical reaction.
Cut-off Voltage
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.
Capacity
The coulometric capacity is the total Amp-hours available when the battery is discharged at a certain discharge current from 100% SOC to the cut-off voltage.
C-rate of Battery
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.
Self-discharge
Batteries gradually self-discharge even if not connected and delivering current. This is due to non-current-producing “side” chemical reactions that occur within the cell even when no load is applied.
Degradation
Some degradation of rechargeable batteries occurs on each charge–discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes.
Depth of Discharge
Depth of discharge is a measure of how much energy has been withdrawn from a battery and is expressed as a percentage of full capacity. For example, a 100 Ah battery from which 40 Ah has been withdrawn has undergone a 40% depth of discharge (DOD).
State of Charge
The state of charge refers to the amount of charge in a battery relative to its predefined “full” and “empty” states i.e. the amount of charge in Amp-hours left in the battery.
