Power Parameters

Power Parameters

This is simply the number of cells in your flight battery pack. Stored in Planes database. The most popular values are 7 and 10 cells. It is common to hook packs in series to make larger sizes. If you need to model two packs in parallel, create a new cell type with half the Cell mohm and twice the MAH and name it something like "2xRC2000".

This is the nominal internal cell voltage. Stored in file ECALC.INI . This is not the voltage you would measure under load, since this would also include the drop due to the internal resistance. A NiCad normally starts at 1.30-1.35 volts, levels off at around 1.25, and at 1.20 is about empty! The simple model of a NiCad cell is an ideal voltage source and a series resistance. During the 'plateau' of the discharge curve the voltage drops a few hundredths of a volt. We have measured 1.25 as a typical value but some feel that a slightly lower number is more realistic.

This is the equivalent cell resistance in milliohms. Stored in Cells database. The lowest resistance is obtained with end-end soldered cells. Cell resistance should include the resistance of the cell interconnections. The original data assumes minimal (end-end soldered) resistance. Bars or heavy braid will be somewhat more (+0.5 milliohm) and welded tabs could be substantially more (>1 milliohm). You can measure this yourself with a good DVM, ammeter, and load. Basically you need to measure under load and no-load voltage quickly so that chemical processes do not interfere. Use at least a 10-amp load. Cell resistance is voltage difference divided by load current divided by number of cells. See our web site for information on end-end packs.

This is the equivalent resistance of the ESC (Electronic Speed Control) plus all the wiring and the connectors, fuse(s), and switch(es). Stored in Planes database . The resistance of ESCs is a function of design and number and type of FETs. Also, brushless controllers have inherently higher resistance because they must switch both sides of the winding and this requires p-channel FETs with higher resistance. High performance brushless controllers have a bunch of FETs.

This is the typical milliampere-hour capacity of a cell. Stored in Cells database . This basically means that the cell can supply that many milliamps for one hour before the voltage drops substantially. This is generally higher than the manufacturer's rating, but will vary as a function of the charge method, discharge rate, and temperature. This number is used to estimate the motor run time at the calculated battery current. MAH is normally measured assuming a cutoff voltage of one volt or less. With high cell-count packs you can't afford to let the pack get down this low per cell without risking damage to the lower-capacity cells. You can expect the capacity of a higher cell-count pack to appear less due to the higher probability of a spread in cell capacities.

This is a description of the selected cell. Stored in the Cells database . These cells are mainly Sanyo. The most popular are the N-1700SCRC and RC2000 for sport flying, and the KR-600AE and N-500AR for the smaller "Speed 400" models. This brand has a proven track record. Cells with an 'R' in the suffix are especially suited for the high charge and discharge rates needed for electric flight. Other cells have a higher energy to weight ratio but at the expense of lower current handling and/or charge rate.

This is the Cells database control. If you click the big button, you jump to the Cells database. The smaller buttons step you back and forth through the database. Use CTRL_C as a shortcut to access the database. This selects the cell information on the screen which includes cell resistance, cell capacity in MAH, and a description of the cell.

This is the current supplied by the battery. It is equal to the motor current at full throttle and less than the motor current at partial throttle. For motors wired in parallel, the battery current is the total to all motors. At part throttle, battery amps will be less than motor amps. This seems wrong, but what is happening is sort of a transformer effect. The watts out of the battery are roughly the same as the watts into the motor. However, the ESC essentially lowers the average motor voltage. If you have the same wattage, you must have more current. How can this be? When the ESC switches off the motor. The current in the motor continues to flow via a special diode or equivalent in the ESC.

This is the estimated battery duration at the indicated current. This number is often much lower than the flight time you would like. The key to success is intelligent use of the throttle.

This is the percentage of total energy that is delivered to the propeller. This does not include any propeller losses, but includes losses in the cells, wiring, ESC, and the motor itself. Generally, efficiencies below 45% for ferrite motors and 50% for others indicates an overloaded system. Use the System Efficiency graph to figure out where the problem is.

This is the power supplied by the �ideal� battery, not including losses due to internal cell resistance. This is the denominator in determining system efficiency. In the ideal cell model, this is the cell open-circuit voltage times the current times the number of cells.

This is the power loss in the battery, wiring, and ESC. Most of the loss will be in the pack itself, so the lower the cell resistance the better. It's not a bad idea to check your wiring with a DVM with the motor running to look for excessive voltage drops. At 20A, you should generally see less than 0.02V across connectors & switches.

This is an estimate of the pack weight based on the weight per cell from the Cells database plus a small amount for solder and shrink wrap. End-end soldering yields the lightest packs. Braid works well but a lot of solder can wick into the braid. You can save a little weight by removing the wrapper(s) but you must be REALLY careful how you handle the cells and how you insulate the pack.

This is the current at which the % system efficiency will be maximized. This always occurs at a lower current than the maximum motor efficiency current. For sport flying you don't want your full-throttle current to be this low. It's better to prop for something higher than max. sys. eff. current, and use your throttle.

This is the duty cycle of the (high rate) ESC. This is controlled by a �slider� control. This reduces the average voltage seen by the motor(s). Clicking the arrows steps 1% and clicking in the white area steps 5%. Set your speed for twice stall speed and then back off the throttle until you get a 5-degree climbout. This is a good upper bound on your expected flight time.