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Top: receiver-type Ni-Cd packs with four cells each. Bottom: cells not yet made into pack. There is no such thing as a one-cell, 4.8 volt Ni-Cd pack! |
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Inexpensive Hobbico Digital Voltmeter Mk III places a load on the battery and tells you when it is time to recharge. |
Q-8: I noticed in your series that you refer to four- and five-cell receiver battery packs, but I have a single-cell pack rated at 4.8 volts and was wondering what voltage can it reach before needing to recharge?
A-8: Your battery pack may look like a single battery cell, but because it is identified as having 4.8 volts, it contains four individual cells. If you multiply the nominal cell voltage of a Ni-Cd or NiMH battery cell—1.2 volts—by 4, you get 4.8 volts. Most of these four-cell receiver battery packs have simple heat-shrink-wrap cases. If you look closer, you can probably see the outline of the four cylindrical shaped battery cells under the thin wrap.
Receiver battery packs usually comprise four Ni-Cd or NiMH cells. Some of the Pattern and helicopter pilots like faster servo response time and higher output torque, and therefore go to a five-cell pack.
As for what voltage to go down to before you stop flying and recharge, I recommend roughly 4.8 volts. A fully charged four-cell pack can reach as high as 5.6 volts. The time it takes to get from 5.6 down to the 4.8-volt mark is a function of the battery’s rated capacity. The higher the capacity (expressed in mAh), the longer it takes to get to that recommended minimum voltage. An average 600 mAh capacity, four-cell battery pack should take roughly two hours to reach that minimum.
Much of that has to do with the type of model flown. An RC sailplane will take far fewer commands than an RC Aerobatics model. Control commands use up battery power! When the time to reach 4.8 volts starts to get shorter and shorter, the capacity is diminishing and replacement becomes inevitable.
You can easily check the receiver battery voltage at the field with a meter such as the Hobbico Digital Voltmeter Mk III described in the "Battery Basics" article (on page 59 of the October 2003 Model Aviation). This inexpensive instrument not only places a load on the battery, but it tells you when it is time to recharge.