by Paul J. Weijers, MAAC 42097

For those of us into electric aircraft. We know how important it is to maintain our battery packs at peak performance, The ideal instrument to do the maintenance is an automatic battery cycler. The unit will discharge, charge and indicate the condition of the pack. By running several cycles, a pack can be reconditioned. The problem is the prohibitive cost of a battery cycler. Here is a solution at very low cost. But instead of being fully automated, it requires some work on your part.

Specifications

Let us examine what this means, starting with the battery packs. 500 mAh indicates that a pack should be capable of delivering 500 milliamperes in one hour. or 250 mAh in two hours, or 50 mAh in 10 hours. The capacity as stated by the manufacturer assumes that the cells are discharged to It) volt. For the transmitter pack this means the voltage is down to 8.0 volt and for the flight pack the voltage is down to 4.0 volts. WARNING: Never fly until the packs reach these values. A maximum safe levels considered to be 8.8 volts and 4.4 volts respectively. In the case of this particular transmitter, the total time to discharge the pack to 8.1) volts is 500 mAh x 190 mAh = 2.6 hours. To avoid disaster, set the total ON time of the transmitter to 75% or two hours or less. Note that we consider ON time, not flying time. You have to take into account time spent making adjustments to your plane while still on the ground. This calculations fairly accurate since the current drain is constant and not subject to the changes of the controls. The situation is different for the flight system. While the receiver represents a constant load, the servos present a varying load, depending on whether they are idling or moving. The receiver requires 18 mAh. each servo requires 8 mAh when not moving. With a standard setup of four servos (throttle, ailerons, elevator and rudder) it adds up to 18 + (4 * 8) = 50 mAh when controls are not moving. However. each time a servo moves, its current requirement can go up to several hundred mAh. The exact amount is a function of the servo size and the amount of mechanical resistance the servo encounters. Since servo motions occur for only a second or so. The total drain is almost impossible to determine.

The good news is that it will take longer for the flight pack to discharge than for the transmitter pack. Therefore, any calculations of maximum SAFE ON time should be based on the transmitter specifications.

To charge the batteries, use the chargers supplied with the system. They are designed to charge at a rate of C/10. i.e. one-tenth of the capacity. Theoretically that

means charging for 10 hours. However, since the charge rate levels off towards the end, charging time should be approximately 14 hours. Overcharging can be disastrous. A good solution is to use a timer modified so that it tunes off after 14 hours but cannot turn the power on again.

These are the items you need to do the job. Most important, a small digital volt-meter capable of reading to two decimal places. Look for a sale and you will find them between $20 and S30. You also need a terminal strip. three 50 ohm, 5 or 10 watt resistors, one Lead with connector for the Tx battery pack and one lead with connector for the Rx pack. The parts are hooked up as shown. The components arc available at electronics stores. The exceptions are the connections to the batteries. Use a servo extension cable and remove the female connector. Note that you must connect directly to the Tx battery

because most transmitters contain a blocking diode so that you cannot to discharge the battery through the charge jack. The theoretical values are: 9.6 volts divided by 50 ohms equals 192 milliampere and 4.8 volts divided by 25 ohms equals 192 milliampere. Although the scheme can be made more accurate. these numbers are close enough for our purpose. For a 500 mAh battery it will take about 2.5 hours to discharge.

Connect the batteries and measure the voltage across the resistors. Initially make a measurements every few minutes. then every 10 or 15 minutes. Note the time when the measurements are made and the measured voltage. When the voltage across the Tx battery reaches 8 volts, disconnect the battery. For the Rx battery disconnect at 4 volts

You can now create a table that will look something Table I This data was from an old set of batteries and I let it slip below 4 and 8 volts deliberately to show the results. The Rx battery hit 4 volts at just 140 minutes or 2.33 hours. The capacity of the flight pack is 2.33 times 192 mAh equals 448 mAh. The Tx battery was run down after about 130 minutes resulting in a capacity of 416 mAh. You could try to charge and discharge several times to recondition the packs. but in this ease they were about 20 years old and not considered reliable. An alternate way of looking at the data is by making a graph. This shows that once the cutoff point has been reached, the voltage drops rapidly, suggesting that a safe ON time is up to about 30 minutesbefore the knee shown in the graph.

I have attempted to show that it is possible to maintain your battery packs without a heavy investment in peripheral equipment. I would appreciate any comments or suggestions you may have on this subject. 

You can reach me via  E-mail: kabouter@odyssee.net.