Care and Feeding of Nicad Batteries
by Jerry Kelly
This concerns the rechargeable batteries used in radio control devices. The receiver pack is rated at 1.2 V per cell or 4.8 V for the pack while the transmitter pack is rated at 9.8 V or 1.225 V per cell. Go figure! They use the very same cells. Fully charged, a transmitter pack might read 11.4 V under load for just a few seconds, then begin in to drop more in voltage until after ½ hour or so it will read the “rated voltage” of 9.8 V. It will remain at this voltage longer than any other (except 0 V.) When the voltage reaches 9.1 V you should think seriously about not using it until recharged.
My preference is to use the charger supplied with the radio since “fast field” chargers cause the cells to heat up more than I think is safe.
Transmitter batteries in good shape can be relied upon to be very consistent in the time of operation because the load (drain) is constant. On the other hand receiver batteries will vary depending on how much the servos are used. If you turn the transmitter and receiver on, but do not move the sticks, the receiver will last much longer than the transmitter because the “idle” current on modern receivers is very low.
So how do you predict the flying time you can get out of your batteries? Simple:
1. The next time you fly, log the time of each flight.
2. When convenient, within the next couple of days, turn your transmitter on with the antenna raised. Turn on the receiver and operate the servos for about 10 seconds (or your guess of how much you use the servos in one flight)
3. Watch the meter on the transmitter, if it begins to fall end the test and log the overall time. If the servos begin to slow up before the transmitter meter drops, end the test and log the time. The sum of the flying field time plus the bench run is the limit of flying time.
4. Use a 25% safety factor if the total logged time was 2 hours limit your flying to 1 ½ hours.
5. Recharge the batteries right away.
6. Perform this test every 6 months.
An often asked question “If I charge my batteries overnight and don’t fly the next day should I charge overnight the next time I intend to fly??”
YES! There was a time when it was popular to leave batteries on charge when ever you are not flying so the batteries are ready all the time. I don’t do this.
It is best to keep batteries fully charged because foreign mater can form between the plates and drain the capacity of cells. This is called leakage. If the cell is fully charged it is more likely to burn away this foreign matter.
And then there is Ni-Cad memory. The warning is that if you fully charge a Ni-Cad and use it for, say only ten minutes, then recharge and you do this on a regular basis, the time will come when you want to use it for twenty minutes at which point the Ni-Cad will remember that it is only supposed to work for ten minutes and then quit. The recommended preventive medicine for this problem id to occasionally drain all batteries to near 1.1 V then recharge. This is supposed to break the memory.
Then there are those who say nonsense! This situation is like the person who uses his car on a regular basis for very short city trips and after a few years has to make a long trip in the summer. Well, during those short trips the radiator is clogging up and after a few high speed miles the motor over heats and the car stops. Memory?
In any case if you perform the Time-Check in the 3rd paragraph, you won’t have to worry about who is right concerning “Ni-Cad” memory.