Fuel tanks for electrics

Model Airplane News, Jan 1999 by Marshall, Larry

People are familiar with fuel tanks for glow and gas engines; they're simple devices that hold a visible liquid, and the tubes in and out of them are easy to understand. When it comes to nickel-cadmium cells (the fuel tanks of electric airplanes), things become a bit l.ess clear. That's not bad in itself, as you don't have to understand the internals of a Ni-Cd to fly electric airplanes, but when you mix this lack of familiarity with Ni-Cd supplier nomenclature and the loose use of jargon when electrics guys talk about cells, it sometimes becomes a struggle. Both Greg Gimlick and I have received email from guys with cell nomenclature questions-some as a result of the charts published in Greg's October '98 column. In that column, the number and capacity of cells was reported, but not the type of cell. This is the "loose use of jargon" I was referring to, as for "people in the "know," the statement, "I used 121700s" is self-explanatory, even though it is an incomplete description of the battery used. So I'd like to discuss some of the basics of Ni-Cd nomenclature and jargon so we can minimize the confusion and maximize the fun of flying electrics.

CELL SIZE

There are really two properties of cell "size" that are important to us. First and foremost is the capacity of our "fuel tank" (cell). With both Sanyo and SR cells, the number associated with the cell name (e.g., SR1100, Sanyo 1700SCR) is the capacity of the cell in milliamphours (mAh). Using a Sanyo 1000SCR as an example because it makes the arithmetic easy, a 1000mAh capacity means that we can draw 1000mA from that cell for one hour before it will be depleted. There are some subtleties that I'm ignoring here, but they aren't important for our purposes. Milliamp-hour values are convenient for most Ni-Cd applications but are not particularly useful for us, as we typically draw many amps from our cells for a few minutes. But some simple unit conversions can give us more useful numbers. I take the easy way out and you should, too. If you simply take the mAh capacity of a cell, lop two of the zeros off it and multiply by 6, you'll end up with the number of amp minutes the cell can provide. For our 1000mAh cell, that would result in the simple arithmetic of 10x6 = 60 amp minutes. For a 1700SCR, we'd get 17x6 = 102 amp minutes, and 600mAh cells could provide 6x6 = 36 amp minutes.

With a capacity, it's easy to calculate the duration you can expect if you draw 20 amps from the cells. Sanyo 1700s will provide 102/20 = 5.1 minutes. This doesn't take into account throttle management, but most people calculate full-throttle duration and realize that actual flight time will be much longer.

Physical size of the cells is another matter and one not to be taken lightly. Unfortunately, Sanyo's nomenclature only helps a little in this area. The first letter(s) of the Sanyo labeling scheme refer to the diameter of the cell. A 600AA cell is an "AA" cell (14.2mm diameter) that you use in small flashlights. The "SC" in Sanyo 1700SCR stands for "subC"; it represents a cell diameter of 23mm (just under an inch). Another commonly used size is "A" (17mm). The problem with this scheme is that these labels tell you nothing about either the height or weight of the cell. A 650SCR is 26mm high, while a 1700SCR is 43mm high, and a 1700 weighs nearly twice what the 650 weighs. I've included a short table of some of the popular sizes of cells, along with their sizes and weights; this should help you select cells for your project.

CELL TYPE

All Ni-Cds are not created equal! Not only do they differ in their capacity; they differ in their internal structure. The important thing about these internal differences is that Ni-Cds can vary in the way they take a charge and how they give up a charge.

There are three classes of Ni-Cds. These are "regular," "extended" and "rapid-charge" cells. Sanyo identifies these by placing an "E" in the designation of an extended cell and an "R" for rapid-charge cells. So, a 1300SC is a regular sub-C cell, a 600AE is an extended Asize cell, and a 1700SCR is a sub-C cell that can be rapid-charged.

One difference between the cell types is their internal resistance. Internal resistance varies with cell size, too (larger generally means lower internal resistance), but for a particular cell size, regular cells will have the highest internal resistance, rapid-charge cells will have the lowest internal resistance, and extended cells will be in the middle somewhere. It's typically the case that extended cells will have a higher capacity for their size than either regular or rapid-charge cells.

Let's assume we want to build a 7-cell pack of small cells to power a Speed 400 model and look at how these types present potential tradeoffs. The cells typically used are either 500AR, 600AE, or 600AA cells. The small table presented here will help with our discussion. Note that the cost of the regular cells is much lower than the cost of rapid-charge cells and that the extended cells are between the other two in price; "cheap" goes to the regular cells. But the rapid-charge pack weighs 1.2 ounces less than the regular pack. That may not seem like much, but it's nearly 10 percent of the weight of a typical Speed 400 airplane.