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Prepare to Charge | Back to Article Index

Radio Control Model World - Nov '95

by Stan Yeo

INTRODUCTION

Over the years there have been a number of articles on Nickel Cadmium batteries. Most have been quite informative, some have been very technical and some posed more questions than they answered. The purpose of this article is to discuss Nickel Cadmium batteries, in what I hope is a not too scientific way, their capacity, storage, charging and how to evaluate their serviceability.

CAPACITY & DISCHARGE CHARACTERISTICS

The rated or nominal capacity of a Nickel Cadmium battery is the maximum current a fully charged cell can deliver continuously for a 5 hour period, i.e. a 500mA hr cell is capable of delivering 100 mA for 5 hours before it is fully discharged. Unfortunately the full capacity can only be realised under certain conditions as the ambient temperature and the discharge current have an effect upon the available capacity. The higher the discharge current the lower the capacity. Discharging a cell at 2 x its rated capacity reduces it by 10% whilst discharging it at 1/2 the rated capacity will yield at least 10% more. This is an important point to consider when selecting the capacity of the batteries to fit in your model. Another important point to consider is battery voltage under load and its ability to deliver high currents. The higher the current demand the lower the terminal voltage. This means that under high current demand situations it is possible for the battery voltage to fall below that which is required to operate the circuit. In an airborne pack this would result in a receiver glitch or temporary loss of control. On the bench a typical three function model with all three servos moving at the same time draws a current of 300 - 400 mA. More if the controls are not very free. In the air this current would increase significantly due to the aerodynamic loads on the control surfaces, consequently it is advisable to use at least a 500 mA hr battery pack where possible.

The temperature at which the batteries are being discharged also has a significant impact upon the available capacity. If the temperature is too high or too low then the capacity can be greatly reduced. Fortunately the optimum discharge temperature of 30 deg. C is sufficiently close to average UK temperatures to have little impact upon the capacity of our batteries but on a bitterly cold winter's day it is worth remembering that the capacity of our cells could be reduced by up to 20%.

Graphs: Capacity v Discharge Temperature and Voltage v Discharge Current

STORAGE

Nickel Cadmium batteries self discharge during storage. Basically the higher the temperature at they are stored the quicker they self discharge. If the batteries were kept in a freezer at -20 deg. C then after three months they would still be holding 85% of the charge whilst those kept at room temperature would only hold 20% of their charge. Although Ever Ready and Varta technical information do not mention the affect of humidity on charge retention during storage common sense dictates that the batteries should be stored in a relatively dry atmosphere as recent research by Roger Todd (RCMW June 1994) suggests that it is the ingress of moisture that is responsible for the dreaded black wire corrosion.

Storing your models in a dry garage or shed is probably the best place to keep them, particularly in winter. It is not so ideal in the summer though as the temperature in un-insulated buildings can soar to extreme levels should the sun decide shine for more than a short time. At 30 deg. C Nicads can loose 25% of their charge in less than a week consequently in warmer weather it is advisable to give your equipment a top-up charge just before you go flying.

Graph: Capacity Loss v Storage temperature

CHARGING

There are four basic methods of charging Nicads based on the charging current. These are:

1. Standard charge

2. Accelerated charge

3. Fast charge

4. Trickle charge

Only with the trickle charge can the batteries be left on charge indefinitely without the risk of shortening their life.

1. Standard Charge

The standard charge is carried out at a current equivalent to 10% of the nominal capacity of the battery, i.e. for a 500 mA hr battery the charging current would be 50 mA. A full charge takes 14 hours. The extra 4 hours representing energy lost during the charge cycle. Contrary to popular belief it is not safe to leave batteries on standard charge for more than the recommended time. Over charging will reduce the life of the batteries (Ref. Varta technical handbook). For new cells, cells that have been in storage for a long time or deeply discharged cells a 24 hour charge is recommended to re-condition them as a normal 14 hour charge is insufficient to bring them up to full capacity. Standard charging is the most common method of charging Nicads as it is the only method that fully charges the batteries. As the charging current is increased so the charge efficiency is reduced. Standard chargers are the type normally supplied with radio control outfits fitted with re-chargeable cells.

2. Accelerated Charge

Accelerated charges are carried with a charge current of between 10 and 30% of the nominal battery capacity. This, as previously mentioned is not the normal charging method but should you wish to use it then the charge time can be calculated as follows

Charge Time = Battery Capacity x 1.4

Charge Current

When charging Nicads there is a constant danger of overcharging them consequently with the Accelerated and the Fast Charge it is recommend that the chargers are fitted with automatic timers and the charge starts with the batteries in the fully discharged state i.e. with a cell voltage of 1 to 1.1 volts. This is necessary because unless the batteries are fully discharged there is no way of knowing what percentage of charge remains and therefore no way of calculating the charge time. A 10% overcharge can result in permanent damage to the batteries.

3. Fast Charging

Fast charging is carried out at currents corresponding to 80 - 120% of nominal capacity. Normal practice is to charge at the nominal capacity current i.e. at 500 mA for a 500 mA hr battery pack for 60 to 75 minutes. With fast charging it is even more imperative that the charge is tightly monitored as the difference between a full charge and an overcharge is only a few minutes. Better quality Fast Chargers will discharge the batteries before charging them for the predetermined time or monitor the cell voltage and switch off automatically when it detects that the battery is fully charged.

There are other methods of fast charging being developed, one is Pulse charging. Here the battery is given a high current pulse for a few seconds followed by a stabilisation period at the standard charge rate. This is followed by a short high current discharge before the cycle is repeated. The main purpose of pulse charging is to increase the charge efficiency of the high current charge without damaging the batteries. There are a number of pulse charging methods and with the help of a friend we have developed our own. Our method is to charge the battery at the fast charge rate for a nominal time of 20 seconds and then discharge the battery at 25% of this rate, down to a predetermined voltage level. When the battery reaches this level the charge cycle is then repeated. If the battery is only partially charged the time taken to reach this predetermined level is minimal so the duty cycle of the charger is close to 100% but as the battery becomes fully charged so the discharge time increases. Whilst this technique can increase the overall charge time for fully discharged batteries it does have two main advantages over other methods of fast charging.

1. The batteries do not have to be discharged prior to charging saving valuable charging/flying time.

2. The charger does not require a timer or sensitive sensing circuitry to prevent overcharging.

In addition, by monitoring the discharge cycle time and comparing this with the charge cycle time it is possible to make a reasoned assessment as to the state of charge of the batteries. The discharge to charge ratio can also be used to detect problems with the cells themselves or the wiring harness as any fault will show up as a shortened discharge cycle. One further bonus is the discharge cycle performs a similar function to that of the discharge during cycling. Nicad cycling is discussed later.

Trickle Charging

Trickle charging is the only charging method where the batteries can be left on charge indefinitely without fear of overcharging. The purpose of trickle charging is to keep a battery fitted to electrical equipment fully charged, ready for emergency use or, as in the case of computers, to supply power to the memory circuits when the computer is switched off in order to retain start-up information. Trickle charging is carried out at currents equivalent to 0.03 - 0.05 x nominal cell capacity i.e. 15 - 25 mA for a 500 mA hr cell.

BATTERY CHECKERS AND CYCLERS

Battery Checkers measure the voltage of a battery pack under load and are very useful for checking the serviceability of a battery pack in the field. Because of the very flat voltage discharge curve characteristics of Nickel Cadmium batteries Checkers are not particularly reliable as indicators of battery charge state nevertheless they haved saved many a model from certain disaster. Checkers however can identify battery packs or switch harnesses that are in a poor condition i.e. cells that are in need of cycling / replacement or leads that are suffering from the dreaded black wire corrosion. Any resistance in the battery or leads will reduce the voltage that is being measured and the Checker will give a low reading that needs investigating.

Batteries that are stored in a semi or fully charged state oxidise internally and it is this oxidisation process that reduces the available capacity of the battery. This is the so called memory effect of Nicads. Discharging the cells at currents of 0.2 - 0.4 times the nominal capacity of the cell breaks down these oxides so that when the cell is recharged capacity is hopefully restored back to normal. This is why periodic cycling is so necessary. Over the life of a Nickel Cadmium battery there is a steady deterioration in performance and whilst a cell can fail at any time this deterioration can be monitored by regular cycling (say every three months) and recording of the results. In the Varta technical handbook cycling is referred to as conditioning.

The normal procedure adopted to cycle a battery pack is to fully charge it at the standard rate. Then discharge it under controlled conditions i.e. use a Cycler, down to 1.1 volts per cell. The time taken for the discharge is recorded and compared with previous results to see what if any deterioration has taken place since the last cycling operation. If there are no previous results cycle a known good pack of the same capacity and use that as a standard. If there has been a deterioration repeat the cycling operation and check the discharge time again. Should the battery pack fail to recover to at least 80% of its original new capacity investigate the cause (it could be the switch harness etc.) and take the necessary remedial action i.e. change the leads or replace the complete battery pack.

CHARGE EFFICIENCY

Charge efficiency, as well as available battery capacity, is very much dependant on the ambient temperature and the currents involved. The higher the temperature the lower the charge efficiency likewise for the current. This can be overcome to some extent by extending the charge time i.e. from 14 to 16 hours for a standard charge, but at higher than standard charge currents this is not always possible due to other factors coming into play. The standard charge is the most efficient charging method and the one that is recommended, unless time is a criteria, as it provides the maximum available capacity after a full charg

Graph: Charge Efficiency v Charge Temperature.

SUMMARY

Well I hope you have found this article informative and I have not baffled you with too much science. Most of the information contained in it was extracted from the Ever Ready and Varta technical handbooks on Nickel Cadmium cells. Nickel Cadmium batteries are very reliable but as with most things a little care will improve reliability and prolong active life or at least warn of impending disaster!

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