Electrical Connections By James Goss      I am sure you are aware that a lot of crashes are due to problems with the battery system that provides electrical power to your plane. Most in flight batteries will be a 4.8-volt supply at any number of ampacity ratings. Some modelers will use a six-volt pack to obtain more torque and speed from their servos, but by and large most will be 4.8-volt systems. The 6-volt systems will give your servos a 20% increase in torque.  If you measure your 4.8-volt pack you will probably measure a voltage greater than 4.8-volts such as 5.2 or greater. This is their surface charge and will be reduced to 4.8-volts if a heavy load is connected across their terminals. Your voltmeter only places about a 200-milliamp load on the battery, and this is only if you have an expanded voltmeter with an internal load resistor built in. A 4.8-volt battery pack has four cells connected in series, each cell is rated at 1.2-volts. So here is a source of problems that may occur if a bad connection develops internal to the battery pack. This article deals with the number of connections that exists between your battery and the receiver that the battery supplies power for. Before you read on any further, try to determine how many electrical connections exist between the positive and negative terminals of the battery and the servos that the battery drives for a typical installation.      In another article of mine entitled Redundant Systems (it may not have been published yet) I discussed how important the wiring between the battery and receiver is and some of the ways to make it more reliable. As you can see from trying to count the number of connections above, there is quite a few and any one of them can bring your plane down. To start with, a 4.8-volt battery has 8 connections and a 6-volt has 10 connections. These connections are internal to the battery and we have no control of them unless you make your own battery packs. If you make your own packs you will use standard solder or maybe silver solder to make the connections. This is ok but it does expose the cells to high heat. Most manufactures will use resistance soldering (spot welding) to make the connections. This is reliable and with the jumper straps it is easy to place the cells in a very tight pack. Most modelers will use pre-built battery packs so we will not count those connections.      Leaving the positive terminal of the battery we will normally find a switch. The switch will have a plug on it so we can plug the battery in. Both the positive and negative side of the battery is switched so we count two connections for the battery leads and two at the switch input connector for a total of four connections. Contact between the pins in the plug will create two more connections for a total of six. The switch uses two sets of contacts so this requires four more connections for the wire soldering to the switch terminals and two for the contacts themselves for a total of twelve at this time. (I have always said that it would be a safer system if we only switched one side of the battery, the negative or positive side, and parallel the two sets of contacts in the switch for a redundant switch system. That way if one set of contacts became open the other set would still deliver voltage to the receiver.) The switch has an output connector for connecting to the receiver, and we now have a total of fourteen connections. We need to add two more for the pin connections between the plug and receiver, now we have sixteen. The receiver bus has male connections for accepting the battery; this is two more for a total of eighteen. The receiver bus also has male terminals for its output to the servos; this brings the total to twenty. The servo has two connections in its plug so we now have a total of twenty-two connections plus two for the pin connections, or twenty-four connections in our battery loop. How many did you count?      If you count the internal connections of the battery there are thirty-two connections in the total battery loop that can fail us and bring our beloved planes to the ground. Who would have thought that there would be thirty-two electrical connections in this little battery loop? If you count the internal connections of each cell (where the electrolyte connects to the case of the cell) the total comes to forty connections. Since we have no control of the internal battery cells, other that buying good name brand batteries, we will only count twenty-four connections.      With twenty-four connections it is easy to understand why voltage drop may occur in our flight systems. Remember that each connection will add resistance to the current flow and a tiny voltage drop will develop across each resistance. This battery loop forms a series circuit and in a series circuit all the individual voltage drops will add up to the source voltage. Ideally when the switch is closed all the battery's voltage will be applied to the servomotor. But due to voltage drop on the conductors and connectors a portion of the voltage will be lost. Having a grand total of forty tiny voltage drops may result in reduced voltage at the servo. The power that a servo develops varies with the square of the voltage, so a little voltage drop will produce much less torque by the servo. If you are controlling large surfaces on giant scale or on fun fly planes that have bookshelves for ailerons, you may want to check for voltage drop.      If we had a way to check for voltage drop at the servo while the servo was under full load, we would know for sure if the servo was developing all the power it was designed for. To do this we would need a storage voltmeter that could store the lowest voltage that it measured during a given time. I don't think you will find a device such as this at Radio Shack, but they would have the parts to build it. In the November 2001 edition of RC Modeler magazine on page 42, you will find the exact device I am talking about. It is called an E-Trap and is designed to measure the lowest safe voltage that your servo receives. If you set the E-Trap for 4-volts as a reference, and the actual voltage at the servo drops to 4-volts, the E-Trap will turn on a led and let you know the voltage was indeed too low at the servo. This would be a great tool to have on your workbench.      You may wonder why you can't use a digital voltmeter to do the same thing. A digital voltmeter would not lock in because it takes time for them to settle down and give a fixed reading; they just hunt around for a while. The load on a servo is constantly changing while in operation. If you would like to check the torque of a servo on your workbench it is easy to do. Anchor your servo in position and connect to the receiver with a wye connector. Connect your voltmeter to the other wye connector. Connect a spring scale calibrated in ounces to the arm and pull the spring for 40 ounces. If the servo is rated for 44 ounces of torque it should move the scale. Remember the further you get from the shaft of the servo, the less torque the servo will develop if it is operating another horn such as a bell crank. So it would be better to connect the servo arm by a push rod to a bell crank that has the same length arm and the spring scale to the other arm of the bell crank. If you can maintain constant torque on the servo you may get a stable enough reading on your voltmeter to read voltage drop, but I doubt it. The E-Trap would be ideal for this test. Here is the schematic of the E-Trap for those of you that do not take RCM.