Load Considerations for Lithium Metal (Duralite) Batteries
In recent years there have been several new rechargeable battery types hitting the R/C marketplace. Not only are there Lead Acid, NiCad, and Nickel Metal Hydride, but now there is a Lithium Metal battery. These batteries use different chemistry systems to store electricity. Each type has advantages and disadvantages. The Lithium Metal batteries promise performance with a fraction of the weight of the other rechargeable battery types. An excellent article in the April/May, 1999 issue of Sailplane & Electric Modeler magazine ((800) 558-1544 ext.413) tells about these four basic types.
When you start your car at night with the headlights on, the light initially goes very dim, then brightens somewhat while the starter continues to run. A large amount of electrical power is required by the starter when it begins to turn. If you would measure the battery voltage at several spots between the battery and the starter while this is happening, you will see a dropping voltage as you move towards the starter, the spot where it really counts. The voltage right at the battery terminals will have the largest drop in voltage from no load, meaning that it has significant internal resistance. The wires to the starter and the starter solenoid also lose some of the starting power.
The same thing happens when the motors in the servos in your airplane start to run. Like the car lead battery, the R/C battery has to supply a large amount of power to get them moving. The switch harness, connectors, and the switch itself have to carry this high instantaneous current to the receiver bus, where it is fanned out to the various servos needing the current. Instantaneous high amounts of energy are needed in violent maneuvers such as a Snap Roll where several servos are simultaneously starting to move. Even landing puts a significant load on the battery because several servos are constantly starting, stopping, & reversing simultaneously. Currents in excess of 20 amperes for very short periods can exist in medium to large sized airplanes.
The main reason I am concerned with this high starting current in R/C is that it results in a significant drop in receiver voltage. If the voltage goes too low, even for a few thousandths of a second, the receiver will either glitch or, if PCM, momentarily go into lockup. A glitch or lockup during a low snap roll or landing would be disastrous. A less serious but still important effect of this voltage drop is that servos need the full voltage to attain their rated speed and power. Sluggish servos make a plane fly poorly.
This article is chiefly concerned with the ability of the latest type, the Lithium Metal battery made by Tadiran, to provide instantaneous high amounts of energy. The Lithium Metal battery is sometimes marketed as a "Duralite" battery. Of the four types of batteries, the Lithium Metal battery is the least able to supply this high instantaneous current because it has the highest internal resistance. But it still works quite well, just like the aging car battery that doesn't spin the starter so fast any more, but still starts the car!
Everything must be considered. The receiver dropout voltage point is a critical concern. I use JR radios. A JR 347 receiver, (NER-627X), goes into PCM lockup when the voltage drops to 3 volts. When I tried the same battery system in a larger airplane with more servos it wouldn't fail. Mysterious! I discovered that the radio in this larger airplane, a JR 10 receiver (NER-910XZ), had a voltage dropout of 2.8 volts, making it less susceptible to dynamic voltage drop lockups.
One person at our field was having terrible intermittent glitching problems when landing at the end of the day. We found that a 6" piece of thin wire soldered to the battery cable so the battery would fit under the tank was introducing too much loss. When his older 4 cell pack was about up towards the end of the day, the instantaneous voltage drops during landing were momentarily turning off his receiver during landing. Removing the additional wire & purchasing a new & slightly larger battery solved the problem. Servo leads & extensions come in two sizes, 22 gauge and thinner 26 gauge. ALWAYS USE the heavier 22 gauge wire and gold contact connectors.
A 4 cell NiCad battery pack as delivered with most radios spends most of its life around 5.2 volts. Most (hopefully) stop flying when the voltage is measured "under load" at 4.8 volts. Guess when most of the unexplained crashes occur. The "under load" is about .3 amp. When multiple servos start suddenly, I have measured currents over 20 amps in airplanes with only 5 coreless high speed servos. On the ground. The 5 cell NiCad battery pack universally used in larger airplanes spends most of its life around 6.5 volts. End of flight time for them is 6 volts. Notice that the margin is much greater in favor of the 5 cell. You will also notice that the 5 cell planes have fewer unexplained glitch or lockup problems.
The Lithium Metal battery pack has two cells in series, each producing a nominal voltage of 3 volts. They are only made in one size, with an 800mah (recently upgraded to 900mah) rating. To get a high mah capacity pack, sets of two batteries are connected in parallel for 1600mah, 2400mah, etc. The Lithium Metal battery pack spends most of its time around 6 volts. End of flight time is around 5.5 volts, maybe a bit lower. Notice that the Lithium Metal voltage margin lies somewhere between the 4 cell Nicad and the 5 cell Nicad. But with the Lithium Metal's higher internal resistance, it is a bit closer to the 4 cell's margin of safety. Paralleling the sets of two batteries proportionately cuts the internal resistance and increases the margin of safety.
Are the Lithium Metal batteries safe to use in R/C? Definitely yes, maybe.
What lowers the margin of safety?
1. Too small of a Lithium Metal pack(s). Use this rule. Take the weight of your plane in pounds. Multiply by 1.5. Add two zeros. E.G. 10 pound plane x 1.5 = 15. Add two zeros =1500 mah pack. 20 pound airplane = 3000 mah Lithium Metal pack. (If using NiCads, don't multiply by 1.5). Use a pack size nearest the calculated value.2. Use minimum length of wiring harness. An extra foot of wiring decreases the safety margin by .1 to .3 volt. Don't use old switches that may have corroded contacts.
3. Don't use receivers with poor dynamic margins. I have only tested some JR types.
4. A Snap Roll button causes the servos to move simultaneously. A Snap Roll by using the sticks shows about half the instantaneous dynamic voltage drop that the button produces. If ever experiencing unexplained glitching, move the controls slowly, preferably one at a time.
5. The worst margins occur at the end of charge. The instantaneous voltage drop is also greater, compounding the problem. The conservative calculations above will keep you higher on the curve.
6. R/C voltage regulators introduce an additional .2 to .5 volt instantaneous drop. The Lithium Metal batteries have a flatter voltage curve than NiCads, so the constant voltage benefit is marginal. Most R/C voltage regulators output is above 6 volts, meaning they won't regulate the Lithium Metal batteries anyway. The "Perfect Switch" from Custom Electronics has a regulated output of 5.7 volts and showed the only benefit of the regulators tested.
What raises the margin of safety?
1. A safety margin gain of between .2 and .5 volts results when you run 2 switches & harnesses from the Lithium Metal pack, one to the regular battery port on the receiver & the other to an unused servo port on the receiver. Similar gains result if using NiCads.
2. Many people advocate dual receivers, splitting the servo load, and multiple packs. All of these help proportionally. They also increase the number if items that might fail.
Testing for margin of safety.
1. Discharge the pack to 4.7 volts for 4cell NiCad packs, 5.9 volts for 5 cell NiCad packs, or 5.2 volts for Lithium Metal packs. Turn on the receiver. Hit the Snap Roll button with a "snap" so that the aileron, elevator, and rudder servos all simultaneously move. YOU MUST HAVE SOLID CONTROL WITH NO TWITCHING OR MOMENTARY HANGUPS.
2. Now disconnect the (each) battery from the switch harness and connect a 3' long "heavy duty" (22 gauge) servo extension cable between the battery and the switch harness. YOU SHOULD STILL HAVE SOLID CONTROL.
3. Keep connecting more 3' long extensions until you lose the solid control. Now you see your margin. I can add about 2 to 5 extensions before trouble sets in. The safety margin depends on the battery type, it's age, charge level, and how I snap the Snap Roll button.
4. Best battery dynamic performance came from a "full size" (& heavy) JR NiCad pack. It had about a .3 volt better safety margin than an equivalent capacity Sanyo lightweight NiCad pack, which itself had about a .5 volt better safety margin than the equivalent capacity Lithium Metal. Nickel Metal Hydride batteries have slight lower dynamic performance than the high density Sanyo NiCads.
Conclusion: Don't use a lightweight battery if weight is not a problem!
This report covers about 3 weeks of intense testing and graphing using the SAI equipment reviewed on page 154 of July, 1998 RCM. Dynamic graphs of good & bad performance are included on page 155 of the review.
----How to charge a Duralite with an Ultimate Charger----
(will NOT work with a Brand X peak charger!)
