home

Ni-Cd and Ni-MH Battery Management

CHARGING HIGH CAPACITY BATTERIES

By Ray Datodi

Use of Nickel Cadmium batteries and methods of charging may be found in many technical journals and R/C modelling magazines; it is not the intention of this article to regurgitate all of what has been said before but rather to cover a few of the subtleties often missed when dealing with the subject of Ni-Cd and Ni-MH batteries.

Over the years the load on receiver packs has increased as larger models are being built requiring more powerful servos and, in many cases, an increasing number of servos. Larger servos and/or increasing the number of servos will result in a greater demand on the receiver battery pack load capacity. Modellers recognising this fact often opt for higher capacity batteries to handle the greater loads and wisely purchase battery packs ranging from two to three times the capacity of the pack supplied, or recommended, by the manufacturer of the radio system purchased. The only problem in taking that step is in the continued use of the battery charger included in the set at the time of purchase.

You may well ask, "What's wrong with that?", and on the surface there would appear to be little problem in using the original charger to charge the higher capacity Rx batter pack. What may not generally be known is that the recommended charge rate for Ni-Cd and Ni-MH batteries is 10% of battery capacity. Rx battery packs supplied by R/C equipment manufacturers generally range in capacity from 500 to 600 milliamp hours (mAH). The charger supplied with the set subsequently would be designed to deliver around 50 milliamp (mA). Many readers would have noticed that the charging instructions stamped on, say, a 500mAH Ni-Cd battery state "charge at 50 mA for 14 hours". Most manufacturers rate their batteries at the 10-hour rate. That is to say, that a fully charged 500mAH battery pack would deliver 50mA for 10 hours before being fully discharged. It must be noted here, however, that the charging requirement calls for 50 mA for 14 hours. Due to the inefficiencies in the electrochemical conversion, to fully recharge the battery, an additional 40% (approx.) of electrical energy is required to complete the charging process. Another way of viewing the charging requirement is to say that of the 50mA being delivered by the charger, approximately 36mA go to charging the battery, whilst 24mA are wasted through inefficiencies. (Purist chemical gurus among you, please forgive the simplistic analogous explanation).

If we now take this approach and consider using the same 50mA charger to charge a battery pack of equal voltage but 4 times capacity, i.e., 2000mAH, we would find that to charge this battery from a fully discharged state, using the "useful" 36mA from our 50mA charger, will require 2000/36=56 hours (approx). The balance of current from the charger, i.e., the remaining 24mA x 56 hours is being burned up in inefficiency. To avoid the protracted charging time, a 200mA charger is required to correctly recharge the 2000mA battery pack. Where modellers have found themselves in trouble is that many have considered an overnight charge as being adequate to restore the battery! Nothing could be further from the truth! The end result is a gradual and ongoing undercharge of the battery until one day a model is lost. I have seen it happen!

What will further compound the above scenario of the inadequate charging system is the problem of "memory" effect. "Memory" effect is the common term used in describing the reduction in battery capacity resulting from continued partial discharge and incorrect charging of a Ni-Cd battery pack. If a battery is not regularly deep cycled (fully discharged and recharged) crystal growth and crystal clumping occurs within each cell effectively reducing the overall battery capacity. Battery capacity is a function of surface area of the positive and negative plates in contact with the electrolyte. Crystalline growth and clumping causes a reduction in surface area, thus reducing the effective plate area within the cells. An analagous view of the problem is to consider a 200 litre barrel filled with golf balls immersed in water. The total wetted surface area of the golf balls would be representative of the plate surface area of a Ni-Cd cell with the water being the cell's electrolyte. If the golf balls were replaced with basketballs to emulate the effect of crystalline growth, the total wetted area would be greatly reduced, reflecting the phenomenon which causes the reduction in cell capacity of the Ni-Cd battery. Deep cycling the battery on a regular basis will reverse the process of crystalline growth and reduce the size of the crystal structure.

The deep cycling approach is the lesser of two evils! If the battery is not deep cycled the cell capacity is reduced by "memory" effect, a very dangerous condition for the R/C flyer. If the battery is deep cycled, then total battery life is reduced! Ni-Cd and Ni-MH batteries are capable of approximately 3000 recycles; unfortunately, deep cycling will reduce the battery life to about 350 to 300 cycles. So how can these shortcomings be minimised? A well-known technique called REFLEX charging has been proven to be the most effective way to charge Ni-Cd and Ni-MH cells. This technique charges the battery by a series of charging current pulses with an intermittent discharge current pulse - obviously there must be more charging than discharging pulses in order to charge the battery. This technique is capable of controlling crystalline growth in new cells and, in fact, restoring used cells suffering from memory effect back to maximum capacity for that cell. This charging technique, although simple in principle, does require quite sophisticated electronics to achieve the desired result. For the serious modeller wanting to ensure maximum reliability of his rechargeable batteries, the purchase of a charger offering the above method of battery charging becomes a worthwhile acquisition.

Every now and then one comes across a "little gem". In early 1999, I noticed an advert for a battery charger that seemed to have all the right characteristics. Not only did this charger claim the use of REFLEX charging, but also incorporated fast charging as a feature (the REFLEX method does in fact permit high current injection resulting in fast charging). An added bonus was that it is Australian made. Being unable to resist the temptation, I purchased one of these units and I must say, was amazed at the results it achieved in not only fast charging but in restoring battery pack capacity. After conducting numerous tests, I was convinced that this charger was without a doubt the best thing that had happened in a long time to R/C modelling.

The model I purchased is one of the early designs requiring a power supply to drive the charger from AC mains, or it could simply be connected to a 12V car battery. Since then, I have seen some rapid developments in the design, both electronically and mechanically, with added features such as small switch mode power supply for mains operation and auto-sensing of battery voltage, thus further simplifying the operation of the unit. Credit must go to Horst Reuter, the designer and manufacturer of the microprocessor-controlled "Smart Fastcharger" for a very well designed and engineered product. Having been in the electronics industry all of my professional career, I am well positioned to evaluate such a product and hasten to add that "Smart Fastchargers" offer incredible value for money. A number have already been purchased in Western Australia and for those of you interested in the unit, I have included a photograph (courtesy Ian Johnson, Conquest Advertising). The photo includes the accessories available with the unit which include a switch mode power supply for mains operation, a battery connect cable for direct connection to a 12V car battery and a cigarette lighter fitting for charging whilst driving.

For further information on the very latest version of this unit go to cats corner where you will also find ordering details.