John J.C. Kopera
25 June 2004
Inside the NiMH Battery
The Nickel Metal Hydride (NiMH) battery has become pervasive in today’s technology climate, powering everything from cellular phones to hybrid electric vehicles. The NiMH battery started its life as an evolution from the nickel hydrogen battery used in aerospace applications. Because of their exceptional cycle life and reasonable specific energy, nickel hydrogen batteries were attractive for aerospace applications; however nickel hydrogen batteries have poor volumetric efficiency and require tanks of compressed hydrogen gas and platinum catalysts. NiMH batteries are the result of configuring a battery using metal hydride hydrogen storage materials as one of the battery electrodes. NiMH batteries have been in development for well over twenty years, but were mere laboratory curiosities before the development of advanced metal hydride electrodes that were capable of being charged and discharged in a cell environment without failure. The basic work performed in multi-component metal hydride alloys has paved the way for the current generation of high performance, long life NiMH batteries in use today in nearly every application and those produced by Cobasys.
What is a Battery?
Some definitions are in order here:
An electrochemical cell is a chemical reactor containing reactive and electrically conductive materials which react in a controlled manner to produce direct current electricity.
In a primary cell or battery the reaction is generally not reversible; i.e. after discharge the cell or battery cannot be recharged by supplying current in the reverse direction, or it may be recharged to only a small fraction of the initial amount of energy available from the first discharge due to the non-reversibility of the chemical reactions inside the cell. Primary batteries are generally used once and then replaced with new primary cells or batteries.
A secondary cell or battery contains chemical substances that allow a reaction in reverse of discharge to occur when charging current is supplied to the cell. Thus, after a discharge, the cell can be restored to nearly its original amount of energy by application of the charge current in a specified manner. This discharge/charge activity may occur for several cycles to many thousands of cycles depending on the specific battery technology.
The voltage or electrical pressure generated by an electrochemical cell depends on the types of chemicals involved in the reactions, while the energy (in watt hours – Wh) depends on the amounts and nature of the chemicals comprising the cell.
The ability of the cell to provide power (in watts – W (voltage x current)) is determined by many factors including the chemical makeup of the cell, cell construction, temperature, etc.
A battery is technically a string of electrochemical cells connected in series to achieve a higher voltage. However, for convenience we shall call a cell and also series and parallel connected cells a battery as is a common convention. Batteries may be referenced both by the chemistry of the cells as well as the voltage developed.
Basic Components of an Electrochemical Cell
The anode is the electrode where oxidation takes place and electrons are fed out of the cell into the external circuit. The cathode is the electrode where reduction takes place and the electrons from the external circuit return to the cell.
In a primary cell, the anode is also the negative electrode and the cathode is the positive electrode. In a secondary cell, when on charge the negative electrode becomes the cathode and the positive electrode becomes the anode. Because of the reversal of roles in a secondary cell, the electrodes will be referred to as either positive or negative (which never changes) and the direction of current flow (charge or discharge) will be specified.
The electrolyte serves as the path for completing the electrical circuit inside of the cell via the transport of ions from one electrode to the other.
The reactants making up the electrodes (the active material) may be gaseous, liquid, or solid. The electrolyte may be a liquid or a solid.