By Fred Workley
Just look around the hangar and you will see several applications for nickel-cadmium batteries (NiCads). They are often used in nondestructive test (NDT) equipment. Eddy current and ultrasonic test units use NiCads. The Rapid Damage Detection Device-RD3 (digital tap hammer) an alternative to the coin tap for composite materials uses a NiCad that when recharged from a fully discharged condition will operate for 200 hours.
The history of NiCads
NiCads were first used about 15 years ago in camcorders and other home electronics. Shortly thereafter they earned a reputation as being difficult to use. This bad reputation in home electronics was born in part out of consumer ignorance.
In general, batteries are seldom used until they are completely discharged, but it is actually good for NiCad batteries to be fully discharged. NiCads are able to hold a constant voltage until the battery charge is almost gone. Many consumers were unaware of these facts and actually shortened the lives of their NiCad batteries by recharging them before they discharged fully. This type of incorrect charging can cause memory accumulation in NiCads.
What is memory accumulation?
Scientists have many different hypotheses for what memory accumulation is:
- Accumulation of undischargeable active materials during cycling.
- Formation of an alloy of the cadmium (Cd) and nickel (Ni) used in the electrode (Ni5Cd21).
- Formation of a higher order nickel active material such as g-NiOOH.
Whatever the scientific explanation, in effect a battery experiencing memory accumulation remembers how far it was discharged and will only charge to the level at which it was last discharged. This results in a temporary voltage drop.
For example, if a battery is repeatedly discharged to only 30 percent of its capacity during normal operation and then recharged, the battery will only accept a charge to 30 percent of its capacity.
What causes memory accumulation?
Memory accumulation is often caused by the way the the equipment using the batteries is designed. If a battery accumulates memory, the battery's operating time can become extremely short. When batteries are used with equipment that is designed for NiCads, or with equipment not affected by voltage drops, memory accumulation is not a problem.
What are the problems?
When the minimum operating voltage of the equipment powered by a NiCad is set at a high level, memory accumulation will occur even if the equipment is operated until it stops. As a result, voltage drops more quickly and operating time becomes shorter. The degree of a battery's voltage drop depends on the discharged current, minimum operating voltage, battery type, and ambient temperature.
Battery operating time drops to a certain level and stabilizes at that level.
Does memory accumulation mean a battery's life is over?
You can still use a battery even if it has accumulated memory. A voltage drop due to memory accumulation is just like a voltage drop due to the changes in the ambient temperature or discharge current. It is only temporary. Generally memory accumulation has very little effect on the equipment using nickel-cadmium batteries.
How to avoid memory accumulation
The most cost-effective way to avoid memory accumulation is to utilize all of the battery's available capacity (it's like driving your car until the tank is empty). Then, replace that battery with a spare and recharge the expended battery. Ultimately, this saves money since batteries will last significantly longer. Each time a battery is discharged and charged, it loses a cycle and or day of operation.
One of the benefits of NiCad batteries is their longevity and resilience. NiCad batteries can be stored for long periods of time and brought back to their full charge.
NiCad batteries can be stored at temperatures ranging from -30 degrees Celsius to 50 degrees Celsius without essential deterioration in performance.
Either charged or discharged NiCad batteries may be stored indefinitely. However, the charging capacity of stored discharged battery is easier to rebuild.
Hand-held vs. aircraft batteries
The battery in hand-held test equipment is a small sealed NiCad battery that is internally protected with a fuse. Aircraft batteries consist of positive and negative plates, separators, electrolyte, cell vent, and cell container. (For more on aircraft batteries refer to AMT May/June 2002 issue.) This discussion is not intended to address Emergency Locator Beacons-ELTs.
Understanding how the NiCad works will help you to ensure that your test equipment using NiCads is always ready to do your inspection tasks. Keep 'em Flying.
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