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.
Memory Effect
The memory effect, also known as the battery effect, is well known to users of nickel-cadmium and nickel-metal-hydride batteries. In Ni-Cd and NiMH batteries, the memory effect is manifested in a reduced working voltage, which is observed in the discharge curves when the batteries undergo repeated shallow-depth discharge for a large number of cycles; the discharge voltage seems to memorize the depth of discharge of the previous cycling. This memory effect leads to a reduction in practical cell capacity at a fixed cut-off voltage and/or to a wrong estimate of the state of charge of the cell. True memory cannot exist if any one (or more) of the following conditions holds:
- batteries achieve full overcharge.
- discharge is not exactly the same each cycle, within plus or minus 3%
- discharge is to less than 1.0 volt per cell
The memory effect of NiMH batteries is much less than nickel-cadmium batteries. The simplest way to get rid of memory is to discharge the battery to 1 Volt per cell (VPC) and then charge it fully. You can keep repeating this procedure until the battery restores its maximum energy capacity. In contrast to nickel-based batteries, lithium-ion batteries are considered to have no memory effect.
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.
