Ah, those wonderful little bundles of electricity that keep our radio equipment working, The NiCad! Let's take a look at the power source for the electronic equipment that keeps going and going and going...
Nickel Cadmium batteries, or NiCads, are one of the most common yet misunderstood items in electronics. Starting with the basics I'll try to explain how they work and how to take care of your NiCads. The battery packs in our radio equipment are made up of individual cells. Generally 4 or 5 cells for the receiver and 7 or 8 cells for the transmitter. Each of these cells rated at 1.2 volts and the capacity ranges from 500ma (ma - milliamp hours) to 2000ma. The ma rating is how much energy is stored. A 500ma battery won't last as long (hold as much energy) as a 700ma battery.
The construction of the NiCad is what determines what the rating will be.
CONSTRUCTION - NiCads are a metal can (the negative terminal) that has layers of a nickel based material rolled up inside, like a roll of paper towels and Cadmium which flows between the layers.
NiCads also have a "lid" (positive terminal) for the battery. Under this "lid" are vents and a spring system that act as a safety valve to vent off pressure when too much heat is produced. The number of layers determines the capacity. The greater number of layers, the greater the capacity. (This is why the old high capacity "SCE" type were so touchy.
The plates were so thin that they could not handle much abuse. The newer "SCR" and "SCRC" use more durable plates and as the usage of NiCads increases, technology continually produces better, more durable cells.) As the cadmium flows between these layers, the chemical reaction that takes place between the two elements produces electricity.
When a NiCad is charged, the cadmium separates from the nickel plates. As the battery discharges the cadmium bonds to the nickel plates, producing electricity and heat. NiCads naturally discharge at a rate of about 1% to 5% a day depending on age/condition/usage. Heat and vibration is what destroys batteries.
Even at a low discharge rate, heat is produced and even though you may not be able to feel it, it is being produced.
As the cadmium heats up, crystals form in it and these crystals cause the cell to loose capacity. There are chargers on the market (The Tekin BC210 "Reflex Charger" was the first to address the problem - RC Car stuff) that can dissolve the crystals. They use a pulse of "negative" charge thrown in during charging to break up the crystals. The idea is that by charging the battery with "pulses" and then "hitting" it with a high rate discharge pulse the crystals are destroyed.
The charger may hit the battery with a discharge pulse once every 4th or 5th charge pulse (this all depends on the charger being used).
There are also linear chargers on the market that do the same thing.
CHARGERS - There are basically two ways that chargers work.
The Pulse charger and the Linear charger. Pulse chargers work just the way they sound. They charge the battery with pulses of electricity. Quite often you can hear them make a buzzing sound as they turn on and off (many times a second).
The other type of charger is the Linear Charger. This type supplies the battery with a constant uninterrupted supply of electricity. As a NiCad is charged, the voltage rises. When the cell is fully charged, the voltage peaks and then begins to drop off.
This is where the "Peak Charger" or "Delta Charger" idea comes into play.
By following the voltage rise and drop, the charger can detect the voltage peak and turn off, thus giving a full charge.
Both Pulse and Linear chargers follow this voltage rise and fall and function the same way.
CHARGING METHODS - Ask 10 people who race remote control helicopters how they charge their batteries and you will get 10 answers.
Everyone has a "theory" on charging, but it basically boils down to 2 methods. Yes, you guessed it, Pulsed or Linear.
Simply put, a pulse charge has a tendency to yield a higher peak voltage or "surface voltage" giving the battery more "punch" but slightly less run time, while linear charging gives a slightly longer run time, but not the kick.
After racing for almost 8 years, I learned the tricks for charging racing batteries to meet the needs of the race using different chargers - pulse and linear. Now, how does this translate to our radio gear?
IT DOESN'T MATTER ONE BIT!!!
The discharge rates of our equipment is so low in comparison to what the NiCads in a rc helicopter go through (spikes up to 100 amps and equipment that can discharge at rate up to 40 amps) that it's like comparing apples to oranges!
Pulse charging is a bit harder on the cells and does produce more heat than linear charging does, but that's when you charge at a rate of 5 to 10 amps! I don't know of any field or bench charger designed for radio equipment that even approaches 2 amps charge rate, so your cells are safe. I think that the HiTech charger can be set up to fast charge at about 1800ma (1.8 amps) and the Alpha 4 can get up to 1000ma (1 amp) in both charge and discharge mode. A peak charger will yield more run time and there is less chance of overcharging compared to leaving the battery pack hooked up to the wall charger that came with your radio for days on end. Most of the newer chargers also allow "trickle charging" or "indefinite charging". What these do after peak charging is drop the charge rate down to a level that keeps the battery from self discharging, thus maintaining a full capacity charge.
DISCHARGING - Once again, as with chargers, there are two types of dischargers.
Pulse and Linear. They work in the same way as the chargers do (either by pulsing or constant discharge), but instead of following the batteries voltage down to a roll over voltage, they can be programmed or are pre-programmed to stop at a certain voltage. When discharging NiCads the minimum cutoff voltage should NEVER go below .5 volts per cell (2.0 volts for a 4 cell pack). This is considered EXTREME and the general cutoff voltage should be .75 to .8 volts per cell.
This works out to 3.0 to 3.2 volts for a 4 cell receiver pack and 6.0 to 6.4 volts for a transmitter pack. Dropping voltage below .5 volts per cell can cause the NiCad to go into what is known as "Cell Reversal" and can cause permanent damage to the cell.
MEMORY - For some reason NiCads develop a memory of how they are discharged.
If the cells are used for a period of let's say 12 minutes (average flight?) 10 times, they will develop a memory of 12 minutes. We put a larger tank in our heli and now get 20 minute flights and suddenly at about 13 minutes the batteries go dead! What has happened is that the cells have become trained to a certain usage period and when they are pushed longer than usual, they basically give up! (Ok, you got me, I can't tell you why they do it, I just know that it happens!)
CYCLING - The idea of cycling your batteries is to remove memory.
By putting NiCads through a series of charge/discharge cycles you can effectively remove the memory characteristics of your batteries. Approximately every 3 to 6 months (depending on the amount of flying done) the NiCads should be cycled. Charge the batteries to full capacity (peak charge) and then discharge them to .75 to .8 volts per cell. This is 1 cycle. Run at least 3 cycles to get rid of memory and even 5 or 6 cycles won't hurt. The discharge can be as simple as leaving your transmitter and heli on until the voltage drops to the desired level. The problem with this type of discharge is that in flight the servos and radio are being worked (higher discharge rates).
Just letting them sit until they run down does not imitate flight discharge and you can end up with batteries that go "soft".
You can also discharge batteries by connecting a 1 ohm 10 watt resistor across the terminals of the cells and follow the voltage down to cutoff with a volt meter. DON'T use the battery leads...they WILL melt! Also, the resistor will get HOT, so be careful. This is not the best method, but it will work in a pinch. Try to find someone that has a discharger or a charger that has a cycle mode in it. It will give the best results. NICKEL-METAL HYDRIDE BATTERIES Nickel-Metal Hydride (Ni-MH) batteries are the new kids on the block. These batteries are a direct result of the need for longer lasting, more efficient batteries for the personal computer.
They have a higher capacity and discharge rating than a NiCad of the same size.
TOUCHY LITTLE CELLS - The problem with the Ni-MH is that they are much more sensitive to the environment they are in.
Take a portable lap top computer...usually in a controlled environment, very little vibration, fairly constant discharge rate...These batteries work great under conditions like this. Your transmitter is similar in the way it treats batteries, low vibration, fairly constant temperature and discharge. Now, you helicopter is a completely different world for batteries.
The vibration level is high, the temperature can fluctuate from hovering to flight, the rate of discharge is constantly changing. Ni-MH batteries just don't fair well under conditions like this.
Vibration and heat seem to be what takes the greatest toll on them. A couple of the hobby shops in the area now no longer sell them as receiver packs, due to the loss of more than one helicopter/airplane.
Tim Knott, our club's president and 3D expert, feels that his two recent crashes were due to loss of receiver power. Tim was using Ni-MH batteries in both mishaps and has now gone back to the old bullet proof NiCads for his receivers.
Another problem with the Ni-MH batteries seems to be lack of staying "matched", ood battery packs will have cells that have similar discharge times and voltages so that the pack acts as one large cell. If one cell does not last as long as the other 3 the others are put under more strain and the result is a less efficient battery pack.
NiCads, in general, stay fairly well matched as long as they were closely matched from the start and then always used together.
The Ni-MH batteries seem to suffer quite badly from not staying matched, thus leading to odd discharge rates and times.
CHARGING - Ni-MH batteries seem to be very sensitive to charging. I don't know what the recommended/correct(?) method for charging is, but they seem to be touchy and not be able to take any abuse while the NiCad can take different rates/charges and still come out working well.
TELL ME MORE - I am not extremely familiar with Ni-MH batteries and what I have written here is what I've been able to find out by talking to others.
I have no first hand experience with them (they scare me and I know my NiCads!) and if anyone can shed some light on them, please let me know so I can pass it on to everyone. Author: Richard W. Kosar -
Date: March 27, 1996
