Maximizing the life of a NiCd Battery

Maximizing the life of a Nickel Cadmium (NiCd) Battery

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The NiCd battery cell has a finite life that is measured in cycles of charge-discharge. Each cycle is a bit of life.

The typical NiCd battery cell can be recharged from 100 to 500 times, depending upon how it is treated. Improper charging will quickly shorten the life of a NiCd battery to 100 cycles or less.

The common causes of early failure:

NiCds that are continually over-charged over an extended period of time will have excessive cell heating and venting. When the battery cells vent, they loose electrolyte. Loose enough of the electrolyte and the cells stop working. Always remove the pager from the charger when it is fully recharged. If using a rapid or fast charger (4 - 6 hour), remove the pager from the charger when the NiCd battery is charged. Further charging will shorten the life of the NiCd battery. Do not "store" the pager in the charger. Do not leave the pager in or on the charger over the weekend. Continued over charging with a 14 hour charger will do just as much damage to the battery as a fast charger. Always observe the time recommendations for the charger used.
NiCd batteries that are stored, used or charged under extreme temperature conditions will die an early death. Do not leave the pager on the dashboard in the hot summer sun. Do not leave the pager in the car over a winter night. Allow the pager to warm or cool to room temperature before recharging. A NiCd cell will not accept a full charge at high temperatures. If the pager feels warm or hot to your hand, let it cool down before using or recharging.
Regular discharge to less than 1.0 volts per cell will cause deterioration and failure. Discharge until "flat" causes a reverse current to flow through the weaker cell(s) with resultant individual cell voltage reversal causing premature battery failure. Turn the pager OFF and recharge the pager when the low battery alarm sounds. Do not over-charge. Turn the pager OFF when it is not being used.

The myth of "Memory"

"Memory" is a unique condition that describes lost capacity in a NiCd battery. Memory can occur, but only if the following three conditions are met:

Discharge must be to exactly the same capacity point each time, +/- 2 percent.
Discharge is to never less than 1.0 volts per cell each time
Over-charge never happens.

Memory was noted on the early spacecraft where the battery was managed by a computer and always discharged to the same exact level (25%) before recharging. After several cycles, the battery "forgot" the additional (75%) capacity. The memory condition was resolved by reprogramming the computer to allow for a slight over- charge. Please note, a slight over-charge, NOT discharge.

What this means, in practical terms, is that the conditions for memory to happen in a pager battery will not occur because the battery is not managed by a computer. Most NiCd failures are wrongly attributed to memory.

The most common problem (wrongly attributed to memory) is voltage depression due to long term over-charge. The individual NiCd battery cell will drop 0.1 - 0.15 volts due to continued, repeated over-charging that extends over a period of several weeks or a few months. The total voltage depression that occurs in a three cell pager battery, 0.3 - 0.45 volts, can cause the low battery alarm (3.1 volts) to sound long before the NiCd battery is depleted.

This type of over-charging is not unheard of, especially if the user is in the habit of placing the pager in the charger whenever it is not being worn. Always follow the recommended time limits of the charger.

Deliberate, excessive discharge of the pager battery is NOT needed to prevent "memory" problems. Excessive discharge can and will cause premature cell failure due to reverse current flow through a cell.

Summary

Do not overcharge the battery. Do not leave the pager on either trickle charge or fast charge for extended periods of time.
Do not deliberately discharge the battery to avoid memory. Observe the pager low battery alarm and recharge promptly and correctly. Do not leave the pager turned ON until the microprocessor will not even "beep" when the power switch is cycled.
Protect the battery from extreme temperatures, either in use or during charging.
Even when all the rules are carefully followed, NiCd batteries do wear out and need to be replaced. Observing the precautions will maximize the service life of a NiCd battery.
Plan on the regular replacement of NiCd batteries that are in a daily discharge/recharge cycle.