An Inexpensive Backup Battery
1
AN INEXPENSIVE BACKUP BATTERY
for your
AIRBORNE R/C SYSTEM
Roy Bourke
MAAC 204L
Battery failure is one of the most, if not the most common cause of failure of modern R/C systems. On nice days we tend to extend our flying sessions, often to the point of exhaustion of the flight battery. A hot environment inside the aircraft in summer can deplete a battery's capacity as much as a cold environment can in winter. A single NiCd cell in a pack can go bad at any time, regardless of the age or history of the pack. And there are numerous other ways an airborne pack can lose its ability to power the receiver and servos in the middle of a flight. Frequent checks of the battery voltage under load may help to avert disaster, but voltage checks alone do not give an accurate measure of the remaining capacity of the battery, and can easily lead to a false sense of security.
The best way to survive a battery failure while airborne is to carry a backup (redundant) flight battery. There are a number of backup power systems available commercially which use NiCd cells as the power source, but these systems tend to be a bit costly, and have the same maintenance requirements (charging, cycling) as the main receiver packs, as well as the same weaknesses. A few years ago, I became aware of an inexpensive backup power system which that is sufficiently simple, reliable and easy-to-build that anyone who flies R/C should be able to install one in each aircraft he/she flies. The system, designed by George Steiner and originally published in R/C Modeler (I don't know the issue date) uses four alkaline cells for the power source plus a few readily-available electronic components, and provides automatic backup power for a 4.8 V (4-cell) flight battery. It can be plugged into any spare servo channel on the receiver, or if all receiver channels are in use, it can parallel the main power line to the receiver through a Y-connection on the switch harness.
In Steiner's circuit, 6 V (nominal) is available from the alkaline pack, but the two silicone power diodes in series will drop this voltage to about 4.5 V. As long as the main 4.8 V receiver pack stays above this voltage, no current will flow in the backup circuit because of the diodes. If the main pack fails, the standby pack will give a boost to the failing main power, and the resultant voltage drop across the power diodes will cause the LED to flash as the servos operate. The alkaline cells could power an airborne system for several hours but you really need auxiliary power only long enough to complete the flight, because if you notice the LED flashing when you retrieve your model, this will tell you that the main pack needs maintenance. And best of all, if your main pack never fails, the standby pack never kicks in so you get shelf life out of the alkaline cells (4 years or more). All you have to do is tuck the redundant power source someplace in your model, and forget it.
Steiner's original circuit called for 1N4001 power diodes (Radio Shack RS276-1101), but these diodes are rated at only 1 Amp and a stalled servo can draw more than that. The 20D2 diode is preferred because it can handle a 2 Amp current. There are a number of possible LED's that will work, one of which is the Len Lite LLL4203. The diodes, LED and switch together shouldn't cost more than a few dollars.
Alkaline AA sized cells work well, and weigh about 0.8 oz per cell (3.2 oz for a 4-cell pack), which is fine for larger aircraft but may present too much of a weight penalty for light (1/2 A) aircraft. Some weight saving can be achieved by using AAA cells which weigh less than 0.4 oz per cell (about 1.5 oz for a 4-cell pack). It is better from the standpoints of both weight and reliability to solder the pack together rather than using a battery holder. The weight of the diodes and switch are negligible.
With separate switches for the main and backup power supplies, you can test the backup circuit simply by switching it on without switching on the main receiver pack, and watching for the flashing LED as you operate the servos. If you don't like the idea of having to trigger two switches to fly your model, you can incorporate the backup circuit into the main switch harness, and switch both main and backup circuits from one double-pole switch. However, this would make it a bit more awkward to test the backup circuit for proper operation. Another option is to add a test plug to the backup circuit to facilitate periodic checks of the voltage of the standby pack.
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