All cells consist of two electrodes separated by an electrolyte capable of supporting a reaction between the electrodes, but not of transporting electrons. When a wire is connected between the electrodes a voltage difference exists, electrons flow through the wire and the reaction proceeds until the reactants are exhausted. The voltage across the cell is largely a function of the specific electrodes and electrolyte, The amount of current and it's maximum duration depend on the construction of the battery.
A variety of electrode/electrolyte systems are used in electronics. Specific chemistries have different power densities, recharge characteristics, maximum currents, costs, etc.
The most important characteristics of batteries are:
In principle, all batteries are rechargeable, but in practice, attempts to reverse the reaction may cause an undesired reaction to take place instead and/or may deposit materials in the wrong crystal form, wrong phase, or wrong place. Recharging, and especially overcharging, may alter the electrolyte. With many chemistries, recharging produces small amounts of explosive Hydrogen gas that must be vented for safety reasons and, once vented, are lost from the reaction system.
There is no battery chemistry that offers the combination of high power density, high current capacity, good recharge characteristics, low cost and long shelf life. The classical Carbon-Zinc batteries and more recent Alkaline cells are pretty good in most respects, but are not rechargeable. Lead Acid batteries are rechargeable, but have low power density and a tendency to leak caustic chemicals. Nickel-Cadmium batteries are not outrageously expensive, and are rechargeable, but they have poor shelf life, only fair power density and a limited number of recharges. Exotic and expensive technologies are sometimes used to overcome the limitations of the less expensive batteries.
see http://www.buchmann.ca/ for extensive information on chemistries, construction, capabilities, limitations and anything else that is likely to be needed with regard to computer batteries.
Return To Index Copyright 1994-2002 by Donald Kenney.