R/C is without doubt a most satisfying and creative hobby. Ideas flow from many sources and when combined in the R/Cers mind, many fascinating results occur. My intention is for the following to be a part of more fascinating results.

When flying the normal sized three and four channel airplanes the battery packs supplied with the Radio Control system are entirely adequate for that purpose. IMAA flyers being a cut above normal can expect to find that in many applications the standard battery packs can be less than adequate. IMAA aircraft with wing spans in excess of 90 inches and weights exceeding 20 lbs. create aerodynamic pressures that require heavy duty or dual coupled servos, extra activating devices using more than four servos, etc. Each of these added work loads consume more and more of our battery capacity.

MONITOR YOUR BATTERIES
Knowing the state of charge of our battery packs is very important. Some flyers feel that a charge cycle of 14 to 16 hours, insures a full charge, may be in for an unfortunate surprise, sometimes something prevents a full charge from taking place. Two actions will insure against such surprises and should be a part of every flyers safety program. (1) Measure the voltage of the airborne battery pack before the first flight and any other time that the voltage level is in question. This measurement should be made with a load being placed on the battery. (2) The battery packs should be recycled at regular periods, discharged below the cutoff curve (see below graph) and recharged for a full charge period (14-16 hrs.) Do this every 6 to 8 weeks during a busy flying season.

The Expanded Scale Meters (ESV's) on the market are very good and are recommended. Big Flyers, however, use batteries that do not always conform to the standard 4.8 volt receiver and 9.6 volt transmitter battery packs. I use a small digital auto-ranging voltmeter available from Radio Shack or other electronic outlets. They range from $35.00 to $70.00. These meters read to an accuracy of 1/100th of a volt (.01 volts). Allowing you to read a 4.8 volt battery to a figure of 4.80 volts. I change the regular leads to a custom set that plugs directly into the charging jack or whatever other means you have to get to the battery leads. You also must install a 47 ohm resistor across the leads in order to load the battery with 100 milliamps of current. If you want a 200 milliamp load use a 22 ohm resistor (both 1 watt ratings)

47 ohms 1 watt = 100ma load
22 ohms 1 watt = 200ma load

DISCHARGE CURVE GRAPH:

A basic understanding of the Nicad Discharge curve is important to gaining confidence in your battery upkeep program.

Using a standard 4.8 volt nicad pack as an example you still notice that at full charge the pack voltage is 5.6 volts. (1.4 volts for each cell) Rather quickly the high charge will drop to around 5.2 volts. The volt drop will begin to slow down until it reaches about 5.0 or 4.9 volts. Then for some extended time it will remain at the 4.9 volt level, the more capacity (500 mah, 950 mah, 1200 mah) the longer it will remain at the 4.9 volt level. It is important to note the curve dropping off just after the voltage reaches 4.8v. It could be 5 minutes or maybe 15 minutes, or even a little longer with a larger capacity pack. However, you can't afford to take a chance because of the sharp failure curve beyond that 4.8 volt cutoff.

This is where your careful monitoring of that voltage comes in. By measuring the voltage of yours at intervals you know you are above the 4.8 volt mark and have enough battery left for a flight. If, however, you read 4.80 volts (that's why you must have a digital meter to read that 1/100th of a volt) you know that the failure curve is about to drop in the next 10 or 15 minutes and you must stop to recharge the battery pack. If you have a standby pack you would change packs at this point and be sure to read the voltage of the standby pack under load to insure that it is capable of supporting continued flight operations.

From this discussion you can see that if you are consuming increasingly larger quantities of current (larger servos, more servos,more applications requiring current i.e. lights etc.) you cut down the time that is available before you reach the cutoff curve. At this point consideration should be given for upgrading to larger capacity packs. 950 mah packs give quite a boost in capacity over the standard 500 mah packs and should increase your confidence with 1/5th scale size aircraft with the standard 4 or 5 control channels. For 1/4 scale aircraft you might well consider the 1200 mah size battery pack.

BIGGER AND BETTER:
Up to this point we have only considered variations to the basic 4 cell pack. There are two additional concerns that should be addressed, (1) providing additional power to the system for better control and more response, (2) provide for safety in case of cell failure. More than 30% of radio troubles can be traced to problems in the battery pack. Additional power is accomplished by adding an additional cell in the pack (5 cells) providing a higher voltage and an accompanying increase in power. By combining two of these more powerful type packs (Redundant) we get more power, double the capacity and provide a measure of safety by having standby power available should a failure occur in either pack while airborne.

Redundant applications are a little more involved. One applicationuses detection of failure and a switching mechanism that changes to a second battery pack when a failure occurs. This is rather involved technically so we'll leave it to the experts. It is much easier to mate two battery packs together in such a manner that they work as a single unit. Their capacities add to each other and if a failure takes place in one pack the other is in a position to continue operation and safely return the airplane to a landing.

Along with the obvious benefits we must attend to more detail. Nicad packs cannot be directly coupled together in parallel, as you might do with lead acid car batteries. The packs must be isolated from each other with diodes in order that they can't accept current from each other, i.e. should a cell short in one pack the other good battery would expend it's charge trying to bring the failed cell back up to the proper voltage. Diode isolation prevents this from occurring as well as allowing each pack to work independently of the other.

The diode in the circuit subtracts 0.7 volts from the output so with a redundant pack we must use 5 cells in each pack. We haven't lost anything here and everything works out to our advantage. We have two more powerful batteries working together increasing capacity, power and safety. Not bad!

Each pack must be charged with it's own charger so we have to provide a dual charging system and a way to hook it up. If you need a method to make an economical dual charger, refer to my article in the winter '85 issue of HIGH-FLIGHT. As both batteries work together and discharge along the same discharge path, you can perform the same monitoring technique described above. When starting out in the morning it would be advisable to measure each battery when you unplug it from the charger in order to insure both batteries are working correctly and again before you re-connect the pack to the dual charger. You don't want to find out the hard way that a failure has already occurred and you didn't know about it.

My idea of a good battery configuration is a redundant 950 mah pack made up of two 5 cell batteries separated by diodes with the same charging set up shown above. (available from INDY) I have set up the charging plug to take the place of a switching mechanism. When the flying session is complete, remove the flying plug and the battery circuits are opened. The jack is then ready for plugging in the charger system. Example:

This arrangement provides a total of 1900 mah of capacity and a higher available voltage. With the 0.7 volt drop over the diodes the output for the discharge curve would look like this chart.

 
5 cells at full charge 7.0 volts less .7 volts = 6.3 volts 
	at flat 6.3 volts less .7 volts = 5.6 volts
	at cutoff 6.2 volts less .7 volts = 5.5 volts 

So, with this arrangement we have a starting flight voltage of 6.3 volts available, with a capacity of 1900 mah. At the flat part of the discharge curve the voltage is 5.6 volts (where the 4 cell pack starts off). Then when we reach the cutoff curve (where we stop flying on the pack) the voltage is 5.5 volts and still providing a more powerful voltage. I also install two diodes in parallel. They are cheap and provides a second path should one diode fail. (Not vital but increases safety and its not that expensive)

I feel this approach is very important when flying a larger plane that consumes more power in its flight envelope. The redundant pack provides the insurance against the possibility of a cell failure, and increases the available capacity and power.

If your bird is big enough to demand even more power consider the "Dual Redundant Battery System" by Ken Spears in the Summer '85 HIGH-FLIGHT.

OTHER CONSIDERATIONS:
Another consequence of larger loads on the battery pack is a subsequent increase in current demand. With the larger servos the start and run currents increase dramatically. Current demands of 2 amps or more are a definite possibility. With the standard #26 wire used in our radio systems, a 2 amp load could create a problem. This can be reduced by using double wiring using two #26 wires in each place a single #26 wire is used in the harness up to the receiver. Also a double wire harness with two plugs and switches which can be connected to the battery connection on the receiver and the second connector to a spare servo connector on the receiver. This provides two current paths to the airborne system.

Another technique is to wire the black (neg) lead straight through the switch (don't switch the negative lead). Then run the Positive lead (red) through two switch poles. In this manner two make and break sections are available. (Redundant) By the way, keep your old 4 cell pack around and charged up. It's very handy for checking things out while working at the bench, breaking in the motors, taxi tests etc., before going to the flying field.

LED MONITORS:
There is a device on the market that could provide another method of monitoring the battery voltage. It is a small unit that has two LED's on it. One yellow and one red. When it is mounted in the a/c you can test the voltage by pressing a button. If the voltage is 4.9 volts or above the LED's remain out. If the voltage fails below 4.9 volts the yellow LED lights up, warning you of a lower voltage condition to be considered. If the yellow and the red LED both light up you have reached the cutoff condition and should cease flight operations until the battery has been charged again. This unit is made for 4.8 volt packs only.

I have designed a similar device that is sensitive to 1/100th of a volt and can be set to monitor the voltage in a 5 cell pack. It uses two blinking LED's to make the read out more intensive. It can be configured as a plug in unit or it can be configured as an integral airborne unit that can be actuated by the push of a button or run along with normal flight operations. If there is enough interest I may write it up for a future article, or if you are interested in such a device you can write me for a copy of the schematic.