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Definitions Nicad cell facts Cell failures The cell in use Charging Loss of capacity Checking capacity Cycling The average modeller buys a radio set expecting years of trouble free service with very little attention. The amazing thing is they usually get it, until one day there is a severe dose of the twitches or total loss of control. Cries of interference or (more likely) whose on my *@!? Frequency? are heard, especially if all appears well when tested on the bank. Even a battery voltage check may not help as a no load check is not in itself much of a guide to nicad condition. There are therefore certain things modellers should know about nicads, and this article attempts to present them in an understandable manner. The modeller needs no more complicated kit for this than a standard multimeter with a voltage range of at least 10v dc and a current range of 500ma (100ma minimum). These are relatively cheap to buy.
Typical cell life is around 500 to 1000 charge/discharge cycles so a five year life on 2 cycles/week is perfectly possible. Overcharging at the standard rate does not appreciably affect the life of a cell, unlike lead/acid cells. Charge retention for new cells, fully charged and stored at 20oC is 75% after 1 month, 50% after 3 months and 15% after 6 months. It is therefore advisable to fully charge your nicads just before you use them. Again unlike lead acid cells, storing nicads fully discharged does no harm and new cells are supplied discharged. However, new and stored cells will need several charge/discharge cycles to achieve maximum capacity. Cells do not function well when cold and will show an apparent loss of capacity when cold, typically being 100% capacity at 20oC but only 75% at 0oC. The apparent capacity is also affected by the rate of discharge, the higher the rate the lower the capacity, typically if at 50ma the capacity is 500mah then at 90ma this falls to 475ma. The capacity of a nicad is the total usable energy available in a fully charged cell. It is determined by discharging the cell at a rate 1/10th its rated capacity, i.e. for a 500mah cell at 50ma, until a voltage of 1.0v per cell is achieved. The discharge period should be 10 hours. NOTE discharging to lower than 1.0v per cell causes permanent damage to the cell. This discharge rate is known as the C/10 rate. Charging is usually done at a standard rate of c/10 + 50%, i.e. for a 500mah cell at 50ma for a period of 15 hours, to allow for inefficiency in the charging process. There are three categories, permanent, reversible and functional. Permanent failures are usually open or short circuits in the cell, no voltage, dispose of cell safely and replace. Reversible failures are usually seen as loss of capacity due to shallow discharges and can be improved by several charge/discharge cycles down to the 1.0v per cell limit. Functional failures are when the capacity becomes too small due to age/total usage where the cell should be replaced and semi-retired to a less demanding application. In a model, nicads are used to power the receiver and several servos, switching units, speed controllers, etc. all of which consume power, particularly servos when working and switches which have to be electrically held. Keeping all control linkages, rudder hinges, etc. loose and free, using switch functions that need only be momentarily held, etc. all help to reduce this power consumption and increase battery duration. Avoiding excessive use of the rudder also helps! Typical tests on Futaba gear gives:
For each case there is a volt drop of 0.1v for each running servo. Nicads should be charged at the standard C/10 + 50% rate. Checks have shown some chargers fall short of the required 50ma rate, a quoted example being a futaba charger which ran at 47ma at the start of the charging cycle of a 500mah battery, falling to 40ma by the end of the cycle. Hence the additional 50% allowed on time! Note the LED on the charger only proves the LED is working, it is not a guarantee the battery is charging! Keep the plug contacts clean and check your charger periodically to ensure there is in fact a current flowing. Do not fast charge your batteries if you can help it. Although modern cells can take the high currents involved without causing catastrophic failure (i.e. exploding) it will still drastically shorten their life and reduce their reliability. In cells in regular use loss of capacity happens gradually without being noticed. As cell age they still show a voltage of 1.2v, but their capacity decreases with reduced running times. This is why a no load check is of limited use. Capacity is checked at the C/10 rate and ideally at 20oC. Fully charge the battery for 15 hours and then discharge it at the C/10 rate until 1.0v per cell is reached. This is achieved either using a variable resister set using the multimeter, or using a fixed resister whose value is calculated using ohms law. An example of the calculation is for a receiver battery rated 500mah and 4.8v the resistance would be 4.8v/0.05a = 96 ohm (100 ohm is the nearest standard size). Similarly for a transmitter battery rated 9.6v, 500mah it would be 192 ohms (nearest standard 200 ohms). The resisters should have a minimum ˝ watt rating. The battery must be discharged to the 1.0v per cell level AND NO FURTHER IF YOU WISH TO AVOID PERMANENT DAMAGE and the time taken recorded. The capacity is then 100% if the time taken is 10 hours, 80% (i.e. 400mah) if 8 hours, 60% (i.e. 300 mAh) if 6 hours and 50% (i.e. 250 mAh) if 5 hours. We would recommend replacement when the capacity reaches 70% (in our example 350 mAh) as although in theory you can go on until reaching 50% what is the point when you know you have a failing battery anyway. Although transmitters usually incorporate a battery condition monitor, it is still sensible to check the capacity by this method. The receiver battery totally depends on this type of check. If you never check your batteries then it is not a matter of will they fail, but when! Care needs to be taken when cycling batteries, particularly as some transmitters have internal components that can be damaged if to much current is drawn. With this in mind do not use more than the c/4 rate (nominal 4 hour discharge rate) to discharge, and as an accurate capacity check is not required, cycle down to 1.1v per cell only. This means for a 9.6v transmitter battery a 76.8 Ohm (75 Ohm preferred size) resistor should be used, or for a 4.8v receiver battery a 38.4 Ohm (39 Ohm preferred size) resistor, both minimum 1 watt rating. In fact you can use a 100 Ohm and a 50 Ohm safely. This discharge can serve as a rough capacity check. REMEBER IN ALL DISCHARGES IT IS VITAL NOT TO GO BELOW THE 1.0V PER CELL FIGURE AS BATTERY DAMAGE WILL RESULT |
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Copyright © South Manchester Model Boat Club 2002 This site created and maintained by Steve Booty Or address snail mail to our Secretary: Mrs J Booty, 26 Amberwood Drive, Baguley, Manchester M23 9ND Or telephone 0 161 998 1204
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