The infamous label *memory effect* has haunted nickel-cadmium cells since the mid 1960s. It was originated to describe a phenomenon observed in the power system of a US satellite. The memory effect implies that a Ni-Cad cell can forget how to deliver all of its charge if repeatedly partially discharged to the same level. In other words, if a rechargeable 2-hour battery runs 5 minutes each day, it will turn into a 5 minute battery.
Memory effect seems to be brought on by certain condtions. First, the battery must be discharged to a cutoff voltage *above* 1 volt per cell. Second, it must be repetitively discharged to the very same shallow level. Third, it must never receive an overcharge. With all three conditions, it is possible to create a permanent loss of some battery capacity.
Since the mid 1960s, memory effect has been the *cause* of almost every conceivable rechargeable battery *malfunction*. To avoid it, fearful users have vigilantly enforced complete discharge of the Ni-Cad battery on each cycle. But in reality, battery manufacturers have NEVER been able to validate a memory effect in all batteries brought back to them! The overuse of the term may be a case of confusing *memory effect* with a similar phenomenon called *voltage depression*.
Voltage depression refers to a drop in Ni-Cad discharge voltage. It is usually caused by an unfortunate combination of poor design and product misuse which lowers anode oxidation efficiency during discharge. Again, the battery must be partially discharged to a cutoff voltage *above* 1 volt per cell. But in addition, however, the battery must be excessively *overcharged* for extended periods of time. When these conditions are both met, products may experience voltage depression and effectively reduces r
untime. MORAL: AVOID EXTENDED OVERCHARGING!
Voltage depression was a problem with early Ni-Cad technology in the 1960s with primitive anode construction. Modern Ni-Cad fabrication techniques have virtually eliminated the problem now.
The ability to prevent voltage depression lies in the hands of power supply designers. Keep discharging the battery until cell voltage drops to 1 volt (viz, 5 volts total for a 5-cell PowerBook Ni-Cad). Users should vary the depth of discharge or the length of time they use their PowerBook. However, voltage depression is effectively a *non-issue* because it is effectively eradicated after the first deep discharge/recharge cycle. If it appears at all, it will shorten only the first run after that deep d
If the 5-cell PowerBook Ni-Cad is discharged below 5 volts, it is possible that one cell becomes substantially lower in voltage than the rest. In that event, the overly discharged cell may try to recharge itself with current from other cells in the pack. This often destroys the cell and prevents the battery from taking a charge, making it worthless.
This is obviously less a possibility if all cells in the battery are identical, that is, from the same manufacturer and the same production run. High quality manufacturers always try to match cells in this way. It is best avoided by preventing deep discharge of the battery substantially *below* 5 volts. MORAL: AVOID DEEP DISCHARGE!
Extended periods of high temperature reduce Ni-Cad lifetime. High temperature can be caused by operating the PowerBook out in the direct sun or other hot environment. These temperatures degrade the nylon separators in the battery: every 10 degrees Centigrade in average operating temperature cuts the Ni-Cad life in half. MORAL: AVOID OPERATING ABOVE ROOM TEMPERATURE (23 DEG C = 73 DEG F)!
