Common questions related to setting-up large-scale gliders, are what batteries to use, the benefits of five-cell packs, and whether to use a redundant system. In this article, we’ll look at various battery set-ups, and the pros and cons of each.
Standard Installation – “Floaters” and Smaller Scale Gliders
Unlike other RC airplanes, gliders typically need significant nose weight, and batteries are a functional way to add forward weight. That allows the modeler considerable flexibility in choosing a battery set-up.
The standard installation requires a switch, (although some omit the switch and just plug the battery direct into the receiver) and a single battery pack. On smaller, usually non-scale “floater” sailplanes, this set-ups has been used with great success. The next step uses the same electronics, except that the battery capacity is increased to accommodate more servos. On a four-channel 3 meter sailplane, for example, a modeler may use the standard set-up, but with a 1500Mah battery – approximately twice or three times the capacity of the normally supplied pack. The added capacity provides additional insurance, sometimes needed if the sailplane will be flown for long periods. If the modeler wants to enhance servo performance, a five-cell pack is another option – more on that later.
Some three meter to three and a half meter scale gliders use a lot of servos – adding to the basic control surfaces flaps, speed brakes, and a retract. Of the three, the flaps create the highest load on the battery, followed by retracts, and then the spoilers. And regardless of whether there is a load, all servos draw current, and that is the rationale for using a higher capacity pack, even in smaller applications. How much capacity is needed? There is no rule, but “more than you need” is usually about right. The usual issue with high capacity batteries is making sure they are properly charged – and that requires an after-market charger – wall chargers won’t cut it.
How rapidly a battery discharges depends on different factors, including the type of servo and the pilot’s style of flying. A further consideration is where the glider will be flown – a glider flown on the slope where the control surfaces are constantly moving will lose capacity faster than if the same glider was chasing thermals.
Whether battery redundancy is necessary on smaller ships is open to debate. There is nothing inherent about smaller airplanes that makes them less likely to experience a battery failure, except that sometimes the installations are less complex. Outright battery failures are rare, and most modelers flying smaller gliders or power planes simply accept the risk of using a single pack, and rarely have a problem. Some argue that a small glider doesn’t “need” a redundant system. That doesn’t mean, however, that a redundant system wouldn’t be useful if a pack was to fail or otherwise lose capacity. Nor is battery failure limited to packs – connectors can come loose or switches can fail. In the end, it is a matter of choice – if the modeler feels strongly about protecting the investment, a redundant system provides additional protection.
But what is redundancy…redundancy is typically defined as having two batteries, but how those batteries are connected is left to the modeler. One method, used in the power community, consists of one (or two) receivers, two switches, and two batteries. One battery is connected through the receiver’s regular battery port, the other to an open servo channel, or “Y” with a servo. Diodes are not needed. Another method, used commonly with gliders is one receiver, two switches, two batteries, and a battery-switching device. A third and simple redundant system consists of a single switch, and a “Y” connector, feeding two batteries. While that system does benefit from using two packs, it is 100% dependent on a single switch, which eliminates true redundancy.
The first method is the least expensive, as no switching device is needed. Moreover, the system is truly redundant, as each battery operates independently, including how they are wired to the receiver. The logic behind this set-up is to provide instantaneous battery power without going to large (heavy) cells, while maintaining the protection of a redundant system. This system is most commonly used on giant scale aerobatic airplanes, but is becoming popular in other applications, including sailplanes. It is the set-up currently used in the DA-100 powered ISSA Pegasus. The caveat with this set-up is that the batteries need to be the same type and capacity. If not, the discharge curves are asymmetric and one will discharge faster than the other. The single most obvious advantage with this system is the cost. All that it requires are two switches, two of the same batteries, and possibly a “Y” adapter (if there are no open ports on the receiver).
The second method, utilizing a switching device, (such as sold by EMS or Electro Dynamics) only relies on a single receiver input. In these installations, the batteries are labeled “primary” and “back-up”. Unlike the first method, this installation requires only that the batteries are the same voltages – four-cell or five-cell, but the capacity may differ. Generally, the larger pack is the primary pack, but it would certainly work in reverse. When the primary pack is depleted, or experiences a load sufficient to drive the voltage beyond the lower voltage “safe” limit, the switching device terminates the primary and goes to the back-up. A small on-board light is usually used to indicate that the packs have been electronically swapped. One obvious advantage of using a switching device is that alerts the modeler that the primary pack has been depleted. The downside is the added cost of the voltage sensor, and because of the single lead to the receiver, that the system is not purely redundant. The other consideration is that the modeler must emphasize regularly checking the capacity of the back-up pack, because once the primary is gone, the redundancy is gone.
Using Redundant Systems
By now, one thing should be obvious – excess capacity is a good thing. Regardless of which system is used, capacity is cheap insurance. In some installations a $25 pack is barely 1% of the dollars the modeler has into the airplane. Imagine losing a $4k airplane over $25.
Maintenance and Testing
Regardless of which system is used, the benefits of redundancy are lost if the modeler doesn’t regularly check the packs. Each pack should be turned on and tested separately to ensure that it has enough power to drive the airplane and then switched off to test the other pack. Only when the modeler is comfortable that both packs are working should they both be switched “on”. If using a switching device, test the device by turning off the primary back and make sure the switching device moves to the back-up pack. The switching device’s light should come on when the primary pack is switched off.
Four Cells vs. Five
Few things have sparked more misinformed debate within RC than the pro and con of using five-cell packs. Five-cell packs were first popularized in power competitive aerobatics, as they gave the servos faster speed and higher torque. The downside was a slight increase in the current consumption (from the increased speed and torque) – a price that the pilots were more than willing to pay. But as servo technology has improved, torque ratings have increased, and so has servo speed. (Hitec publishes servo performance with both 4.8v and 6v packs). As a result, there are plenty of servos today that at 4.8 volts are faster and stronger than older “high-end” servos on 6 volts. However, with less expensive servos, a five-cell pack may be an inexpensive way to increase the overall performance without upgrading servos. With better servos, especially digital servos, the choice becomes largely a matter of preference as the holding power of the servo will likely far exceed the load (the exception possibly being large aerobatic gliders with large control surfaces). The better digital servos are fast, and with a five-cell pack some are very fast. Not all pilots like the feel of really fast servo, especially those with radios that don’t have exponential rates. Some of the higher end radios allow the pilots to slow down the servo speed, providing the higher torque of the five-cell pack, but with slower resolution.
What to Do?
Whether one chooses a four-cell or five-cell pack for most applications is largely a matter of preference. For aerobatics where the control surfaces may be subjected to heavy and prolonged loads, the increased torque may be a benefit. Hitec’s changes the gear trains on some of its servos to emphasize either speed or torque – thus the servos that end with “25” (speed) or “45” (torque) on some of their servos. To dispel a couple of myths about five-cell packs – one, they are compatible with all receivers. Receivers use a voltage regulator that operates well below the 4.8v of a four cell pack – which is why even when the batteries are near gone, the receiver can still process a signal. Two, regulators are not needed when using a five-cell pack. Some people opt to use them for their own reasons, but they are not necessary. We flew the ISSA Pegasus using two five-cell 1800SCE packs with no regulators, and while the servo speed was noticeably increased, there were no detrimental side effects.
Other Hints on Getting the Most From Your Batteries
– Use heavy-duty 22-Gauge wire/connectors
– Use heavy-duty 22 gauge switches
– Periodically cycle the packs to ensure that they meet or exceed rated capacity, and ALWAYS load test the packs with an Expanded Scale Voltmeter (ESV) before flying.
– Avoid excessive fast charging with low-priced chargers. The so-called delta peak technology requires that the battery reach full charge before the charge is terminated. The interval between achieving full charge and trickle overheats the cells and over time will degrade the cells. (The ACE Smart Charge and Sirius chargers both use the same technology, and both are safe for repeated fast charging.)