A rechargeable battery, such as one of nickel-cadmium construction, to which electrical energy is being applied is generally considered to have reached the state of so-called full charge when the applied energy has an easier time breaking down the alkaline electrolyte than converting the uncharged material at the battery plates into a charged state for storage as chemical energy. At that point, the pressure within the battery casing is rapidly rising as the electrolyte breaks down into gaseous compounds. Because the chemical breakdown of the electrolyte is exothermic, the internal battery temperature is also rising rapidly. At the same time, the cell impedance begins to drop due to a combination of increasing temperature and the lowering of the potential energy required to electrolyze the chemical electrolyte. Thus, attainment of the fully-charged state can, at least theoretically, be determined by monitoring any of three parameters--i.e. the internal cell pressure, the internal cell temperature or, by measuring the battery voltage as a constant charging current is applied, the cell impedance.
The sealed nature of most rechargeable batteries currently available in the consumer market renders the internal cell pressure an unavailable parameter for sensing the charge state of a battery. Similarly, absent a temperature probe placed at an appropriate location within the battery interior by the manufacturer, temperature measurements are indirect at best and, in addition, cell temperature is oftentimes influenced by other factors not directly related to the charge state. Moreover, at high charge rates a temperature-based termination procedure is often too slow or inaccurate to effectively locate a repeatable point of peak charge.
Considerable attention has therefore been directed to identifying the fully-charged condition of a battery under charge by monitoring its voltage as a substantially constant current is applied to the battery. Some of the heretofore-known methods or apparatus for thereby determining the state of charge are difficult or unrealistically costly to implement, requiring significant amounts of dynamic processing power and data storage. Others provide termination decisions of questionable repeatability or that otherwise lack reliability, providing no better than an educated prediction or guesstimate of the attainment of full or peak charge. Still others provide for termination when the battery has attained what is likely to be almost a full charge, trading the ability or expectation of accurately identifying the fully-charged state for a mere reduction in charge rate, as the point of peak charge nears, to one sufficiently low to permit continued long-term or extended duration charging and thereby lessening the risk or likelihood of damage to the battery at the cost of a longer total charge time. However, when it is desired to employ high rates of charge--as is generally most preferred to enable charging of a battery and permit its reuse in the shortest possible time--it is essential that charging take place and continue at a high rate until the battery has attained a substantially peak charge, and no longer, in order to avoid damage to the battery.