Electrical Power and Model Helicopters
Today we have so many choices regarding electronic components such as receivers, servos and gyros that purchasing decisions can become confusing. We read specifications usually understanding what they basically mean. More importantly the limitations of general technical data often can cause compatibility problems or reduce the full advantages we sought to acquire. Further more, advanced components have in some cases left other key parts lagging behind, thus limiting the potential technical gains. This is the first in a series and will deal with batteries and the supporting switch harness.
The popular nickel cadmium battery has many advantages over other types. This type only will be the topic of discussion. Most importantly for the average person is its rechargeable feature. Generally speaking nicads can supply vast amounts of current with a minimal voltage drop and can be recharged up to 1000 times. The former is because they have a very low internal resistance when compared to other battery types. They also have the ability to recharge quickly at high rates. Like many things these batteries come in different grades or types depending on the operational requirements. Parameters such as charge/discharge rates, vibration resistance, and the relationship of electrical capacity to the physical cell size are normally considered during the selective process.
Just what makes a battery type different as far as capacity and vibration resistance might be a question you are now asking yourself. The battery capacity is generally dictated by the size of its nickel and cadmium plate construction. The bigger the plates the more energy a cell can hold. It would reason that increasing a cells capacity would require basically increasing its physical size, which is true. There is another consideration and that is of cell plate area. When the plate surface area is increased so is battery capacity. In order to fit more plate area into a given sized battery shell, the plates and/or the dielectric barrier will have to be thinner. In this manner they will be less durable and more prone to damage from overcharge and vibration.
Before I get too far ahead of myself certain technical terms should be understood. The capacity of any battery is measured in ampere hours. Loosely speaking, a one amp/hr classification or specification means that the battery in a fully charged state will supply one amp of electricity for one hour before becoming fully discharged. Due to the smaller size of the batteries we use, the ratings are given in ma/hr or milliamps hours. Just remember 1000ma equals one amp. What is an amp you ask? It is a measurement of electrical current or just how many electrons pass by a given point for a given time period. Since the speed of electricity is constant, it is the electron flow size that changes with changing current.
The AH rating means little if the discharge rate is not stated. If say a fully charged 1 AH battery is “discharge rated” at the half hour rate it can be said that it is supplying 2amps of current during this time period. Another battery type with similar AH specification rated at say a 4 hour discharge rate may well not supply 2 amps for ½ hour time period. Some battery manufacturers use a decimal C rating. The 1AH battery could be rated at .5C and would supply full capacity at the two hour rate of 500ma. Generally speaking the higher the battery discharge rate the lower the battery capacity. Maximum and recommended charge rates will vary with different battery types in much the same way as the discharge rates. Exceeding these recommendations causes excessive heat to build up in the battery which can cause physical damage limiting the lifespan and capacity. Some brands use one common discharge rate with all battery types which is so low that even the weakest choice will deliver full capacity on paper. This kind of thing does not help the comparative decision making process.
You might say what’s the big deal since nicads supply such high current before the voltage drops. The fact is everything has its limitations. Voltage is the potential force that moves or pushes electrons, so if it drops so does the current. The receiver and servos are designed with ratings at specific voltages. Granted there is a wide operational range but at some point the thing will stop working. Servos with a high current demand will not be as powerful when the voltage supply drops. Electrical power is measured in watts which is the current multiplied by the voltage or P=IV. So if we add an extra cell to the battery pack and up the system voltage the servos will move faster and more strongly. Sounds like nitro methane doesn’t it! I only wish it were that simple.
It is conceivable to get around voltage losses by adding an extra cell for a 6 voltage system. Servo speed and torque will be higher but at the risk of reduced reliability. If one servo is being worked heavily and the others are not, the losses associated with small wire size may not be as yet apparent. Some modelers will add 4.8 voltage regulators to compensate. These gizmos should be located at the receiver end of things. Personally I have found that a four cell nicad battery pack with the correctwiring size between it and the receiver to be perfectly fine with conventional 70-120in/oz servos. The voltage regulators for use with 6V packs work by using the extra battery cell energy reserves in a five cell pack to make up for the high current demands.A very important consideration for people into competition is the all up helicopter weight. Using bigger packs will cause a handicap. For these individuals a smaller, lighter, high discharge rated pack could be replaced every flight. Remember these guys operate on the edge and any small weight advantage helps.
Presently the newer servo types are becoming very powerful and fast, potentially consuming vast amounts of current. Switch harnesses and certain battery packs may not be up to the task at hand. The switch harness wire most people are using was designed two decades back in the past, when electrical current demands were much lower. The receivers were and still are for the most part based on these older requirements. The receiver connector pins have a maximum current rating. Remember the wire size at the switch is the same or smaller than that used on the servos in some cases, and all servos draw power through the single conductor from the battery to the receiver. The harness was designed to lower standards of the past, so it is conceivable that with four modern, high power flight control servos operating that the wire might now be over tasked. This brings us to the electrical resistance of a wire.
If the wire is too small to handle the electrical load, current flow will be hindered due to a voltage drop. In this case the wire will be seen as a resistor. Normally with standard servos this will be most prevalent momentarily during peak loading when the servos start and stop. The wiring connector pins have a limited contact area and are thus rated for a maximum current. The same limitation applies for the switch contacts which can in some cases be effectively increased. This switch is called a double pole double throw or DPDT. If the switch contact is rated at 4 amps and the wiring method correct, it can carry much more than this rating while the contacts are closed. The wiring diagram figure 1 should give a better idea of how this switch is configured or doubled up.
As you can see the wire chart current ratings vary with insulation types due to the heat created by the resistance of the wire. When wires are bundled together the heat concentrates so the rating is multiplied by the derating factor. I feel the 6-15 conductor block would be good for the models. Since this is the bare safe minimum based on heat, a 150% safety factor is applied which also reduces the voltage drop in the wire. From what I have tested so far it appears possible for a 3 amp or larger drain with some high current servos if used entirely through out the machine. This would give 3amps multiplied by a 1.5 safety factor results in 4.5 amps. If we use neoprene wire (semi-rigid) in a 24 gage it would be too small since its rating of 6 amps multiplied by the .7 derating factor equals 4.2 amps. It appears that 22 gage wire is the way to go. To quick charge a flat 2000ma pack in one half hour requires about 5.6 amps. The bundled #22 gage wire may be marginal for this task as the 150%safety factor cannot be utilized.
Figure 5 shows the comparison of the improved switch harness with its heavier wiring. This switching method is used mainly for redundancy and added reliability. Due to the lower voltages used on our models as compared to the usual 120/220volt current rating of such a switch, arcing when the contacts are initially closed or opened will be much lower. These DPDT, two position switches have superior point contacts rather than sliding contacts and may be purchased at Radio Shack or other electronic supply stores. Pick something rated around 5-10 amps and you can’t go wrong. Figure 5
For people concerned with the present wire and plug contact limitations, two switch harnesses from an upgraded switch to the receiver can be installed to effectively double the current carrying capabilities of the power supply wiring. Figure 1 depicts this alternative method. Since the plug contact pins have a limitation, full advantage is possible since two plugs are inserted into the receiver. The second power supply plug will be installed into a spare servo slot. Connector pins come in different grades. The gold plated connector pins are best, offering higher corrosion protection thus ensuring the best long term, low resistance junction. I have seen the cheaper silver colored connector pins actually turn the copper wire green with corrosion at the crimp area. The largest wire size I managed to use with the standard contact pin is 20 gage. Usually standard as supplied OEM switches range from 26-22 gage. Number 22 seems ideally matched for heavy duty use. As a point of interest the smaller the gage number the heavier the wire. A large multi strand conductor can be installed at the battery including application at the same end of the switch harness, but remember different connectors will have to be substituted for the larger wires to fit into. This will allow very fast charging rates for high current quick chargers through the switch harness.
In any event use multi stranded wire and never a single conductor due to vibration issues. You should be made aware that one radio brand programs the user selectable fail safe position using volatile ram inside the receiver. To do this a special PCM switch harness must be installed. If a normal switch harness (Futaba, Jr, Airtronics etc) is used then the user programmed fail safe position will be lost and it will only hold the last good signal position (hold). A small electrical current is always present through the correct PCM switch to maintain the information in the receiver’s volatile RAM. This is only applicable to specific products in the Airtronics line as far as I know. The receiver plug from the switch has a third wire which I believe is to power the memory. See figure 2 for an indication of how a wire size relates to electrical current carrying capabilities.
Since the terminal voltage of a nicad remains almost constant over a wide range of charge levels, its measurement will not provide a valid state of charge indication. Thus the only positive method of accurately determining the state of charge is to discharge at a fixed current flow and measure the time it takes for the battery to reach an average of 1.1 volts per cell.
It is possible to determine the approximate state of charge by discharging at .2C for about ten seconds and measuring the terminal voltage with an accurate voltmeter. If the voltage is more than 1.25 volts per cell then the battery is at least half charged. If the cell average is less than 1.25 volts per cell then you best charge the thing up or be very careful. See figure 3. You will also notice how the available battery capacity is reduced at colder temperatures. Winter flyers please pay close attention to your battery.
If the battery and switch harness are upgraded, the ESV (expanded scale voltmeter) loading should be increased to give a better indication of the battery charge. Many of these were designed for the smaller battery packs and loaded at about 150ma. We have to remember that these devices were manufactured for the average plank flyer with a 600ma/hr battery. Myself and others use up to 1800ma./hr packs and the 150ma load value is no longer relevant. I personally load the bigger battery packs at about 400-500ma. Using the standard switch harness with this load size gives a reading lower than the actual remaining battery capacity. This is because the small gage wiring harness causes the error. Taking the reading and loading at the battery directly can give a higher more accurate voltage than at the charge lead or receiver. The total peak electrical load from all the servos can easily exceed the 500ma value. A 10ohm resistor across the battery should be sufficient for 1000-2000ma/hr packs. The ceramic resistor must be able to handle 10 watts or more and can be found at common Radio Shack stores. Your old ESV resistor can be replaced or you can mount the new 10 ohm resistor in its own plastic box with two plug sockets for a multi meter and a third for the battery pack. In the name of accuracy what you may not want to do is combine the new 450ma load with the existing load in the ESV, since current draw will be higher. (It will still work though) Resistors in parallel will have a lower total resistance than the highest resistor value. This is because the electrical current has two paths to flow through (450+150=600ma). See figure 4 for the wire diagram and picture 4A for an example of a load box.
If and when you modify your electrical system quality workmanship is required. Special materials, proceedures and tools are required. If these are beyond your scope it is best left to other people or specialized after market companies. The safe operation of your helicopter depends on the most carefully constructed electrical system possible. As technology and time advance several of the above features hopefully will come standard from the radio manufacturers. The information is general and so each case must be evaluated individually because many servo and battery combinations are possible.
Now on to battery servicing. The good fashion of long term storage for a nicad is either fully charged, or fully discharged with the individual cells shorted out. Since our packs are shrink rapped we are left with one practical method. Top charging in the off season every 30 days will maintain the capacity of your packs. Using the 50-60ma constant voltage charger that came with your radio system is perfect for this chore. Just before the flying season starts it is best to cycle the battery to check its capacity. This must be done immediately after the top charge using a known load and timer. An automatic battery cycler has these two features implemented so that you don’t have to hang around watching matters. Your battery should have at least 85 percent or better of the rated capacity to be considered airworthy.
The battery can sometimes be reconditioned if it does not meet accepted capacity standards of 85%. Usually this can be a temporary loss which is easily corrected. It is mainly due to an imbalance in individual cell capacities due to different self discharge rates, charge efficiency etc. Some have even called it battery memory. Reconditioning (deep cycling) entails fully discharging the pack and shorting individual cells when the individual cell voltage is about .5 volts. After letting the battery sit for several hours, the shorting straps are removed and the battery fully recharged. This is followed by an acceptable discharge then subsequent charge. Since many will not go this route of shorting individual cells, acceptable results can be effected by discharging to an average of less than 1.1 volts per cell. It may take a couple of tries but eventually things should improve. Many commercial hobby type chargers will automatically cycle the pack recording the capacity or energy drained from it. These constant current cyclers work very well indeed.
Discharging at the two hour rate seems to be a nice well rounded everyday figure to use. Connectors should be checked for corrosion and wire integrity. Don’t forget the transmitter since it could cut your first flying day short. The nice thing about some transmitters is they warn you of a low battery voltage condition with a beeper well before a catastrophic failure. This normally gives more than enough time for even the slowest of us to land. The helicopter receiver or governor may have a battery fail safe feature built in which will cycle the throttle as a warning of low battery voltage. This is fine if you have the altitude, can hear it, and are not upside down hovering.
Many battery cyclers know how to reduce the charge rate (by sensing voltage) when nearing full charge in order to prevent battery damage from overheating. At this time they switch to trickle charge to maintain the battery at full charge. Some people leave it in this mode until they go flying. The charger that comes with your radio system does not but supplies such a low current that it can be left on overcharge for many hours. Some battery cyclers charge by way of a timer and a selected battery size, then switch to trickle. The timer parameter is based on a fully discharged battery so in effect it is possible to excessively overcharge a partially discharged pack. This observation is based on a high fast charge rate with a smaller pack. Some chargers have a main charge current rate select switch for these cases.
Sometimes a pack dies before its time. This is caused by a defective cell shorting internally. Since the other cells usually are perfectly fine it seems wasteful to throw the pack away. Single cells can be replaced economically and safely with ones of the same specification. All that is needed is a soldering gun and new shrink rap. After replacement the pack must be cycled to verify its performance.
Remember the more servo over kill you put under the hood the more attention to details the helicopter electrical system will require. I hope I have given you some food for thought.