CHARACTERISTICS
OF RECHARGEABLE
BATTERIES
Introduction
This chapter will highlight the most important electrical and physical characteristics of the three most popular chemistries used in rechargeable batteries:
Nickel-Cadmium (Ni-Cd)
Nickel Metal-Hydride (Ni-MH)
Lithium-Ion (Li-Ion)
Definitions of Terms
A cell is an electro-chemical device capable of supplying the energy that results from an internal chemical reaction to an external electric circuit.
A battery is composed of one or more cells, either parallel or series connected to obtain a required current/voltage capability (batteries comprised of series connected cells are by far the most common).
ESR (Equivalent Series Resistance) is the internal resistance present in any cell that
limits the amount of peak current it can deliver.
The Amp-hour capacity of a battery (or cell) is its most important figure of merit: it is defined as the amount of current that a battery can deliver for 1 hour before the battery voltage reaches the end-of-life point.
The "c" rate is a current that is numerically equal to the A-hr rating of the cell. Charge and discharge currents are typically expressed in fractions or multiples of the c rate.
The MPV (mid-point voltage) is the nominal voltage of the cell, and is the voltage that is measured when the battery has discharged 50% of its total energy.
The measured cell voltage at the end of its operating life is called the EODV, which stands for End of Discharge Voltage (some manufacturers refer to this as EOL or End
of Life voltage).
The gravimetric energy density of a battery is a measure of how much energy a battery contains in comparison to its weight.
The volumetric energy density of a battery is a measure of how much energy a battery
contains in comparison to its volume.
A constant-voltage charger is a circuit that recharges a battery by sourcing only enough current to force the battery voltage to a fixed value.
A constant-current charger is a circuit that charges a battery by sourcing a fixed current into the battery, regardless of battery voltage.
