RADIO CONTROL ELECTRICS
Bob Kopski, 25 West End Dr., Lansdale PA 19446
THIS COLUMN discusses some reader inputs on charging and the SSC, includes a charging look-ahead, shares some flightline experience, and continues with the cell-soldering subject.
The May 2000 column included some discussion about fast-charging motor packs in the workshop. Many E-modelers often want to do this, but to my knowledge there is generally no slick, easy, and inexpensive approach; every method I know of has a downside.
For example, one can use an ordinary field charger in conjunction with an in-shop auto battery. This works, but is not pleasant—for several reasons.
Another approach is to use a 12-volt power supply (one that plugs into any household outlet) and functions as an auto battery substitute. This works very nicely but is more costly.
One reader interpreted the text as suggesting that a 12-volt power supply could charge any pack, no matter the cell count and no matter the charger—including the simplest packs. This is certainly not what I meant to convey.
A 12-volt DC power supply only replaces the usual on-field auto battery as a source for whatever field charger you use. Putting it another way, any charger you use in the field would function exactly the same indoors with the line-operated 12-volt power supply. Of course, the chosen power supply must be capable of supporting the power demand, and the May column discussed that consideration. I hope this clears up any residue confusion.
Here's a thought for the future: I've been reading that the auto industry is planning to gradually change new-car electrical systems over to 42 volts. The familiar 12-volt car battery may someday be history—and so will all of your chargers!
This very big change will take a long time for a total transition, but it does appear to be coming, so I want to note an upside to this.
Consider that any battery can only be charged from a source that has internal voltage capability greater than that of the battery under charge. At present, all "greater-than-seven-cell" fast-chargers I know of are of the "voltage-boost" type. Such chargers contain the necessary electronics to multiply the input voltage (12 volts) to a higher value, then proceed to deliver current to the pack under charge.
In general, when the pack voltage is close to or greater than the source voltage, the input current to any "voltage boost" charger is greater than the charger output current. In fact, the higher the pack voltage, the greater this input current. This makes the charger electronics relatively more lossy (relatively inefficient), in part because some losses are associated with the mathematical square of the input current. All losses appear as heat in the charger itself.
Clearly, 42 volts would change this, and I'd expect input currents to be lower than charger output currents for the packs most folks use. (This is exactly what happens internal to the Universal Slow Charger.)
For larger cell counts with terminal voltages comparable to or greater than 42 volts, the input current would again rise above the output current. Roughly, this would occur at approximately 24 cells, instead of seven cells as is now the case. However, on average, this is a far better operational circumstance than today's auto standard allows.
The articles did not mention anything about the physical size and capacity (amp-hours) of the proposed 42-volt car batteries, so we'll have to wait and see. And the root reason for the change has to do with the rapid expansion of electronic content in cars, driving the need to reduce current levels and associated losses.
Flightline inputs on the Sport Speed Control (SSC)
I have two new inputs from my own flightline regarding the Sport Speed Control (MA 9-10/99).
A local modeler who has a wide range of Electrics has an old and well-worn Midwest HOTS that he often flies. It has a seven-cell pack, a cobalt 05, and beginning late last summer, an SSC with BEC in use.
This modeler likes to make up his own packs, and spends considerable time and effort in measuring and matching cells. He normally has several interchangeable packs ready to go, and he can get in a lot of flying rather quickly this way.
But he says that he also has a good deal of pack trouble, such that after "not too many" flights he would historically have to rebuild his tediously constructed packs because of "bad cells". Not so anymore.
All of his pack troubles disappeared since installing the SSC with the motor-cutoff function. Even though I normally fly just steps away on the same flightline, I was unaware of this taking place. But I'm also not surprised to hear this story, because it's not new.
Reader input supports what I've been reporting for months: the motor-cutoff function is a cell-saver—period! I'd really like to see some ESC manufacturer include this function in their product line. The first to do so could capture the majority market. Everyone deserves to have this beneficial feature built in to their ESC, no matter who makes it. It's not "rocket science" or even expensive. And it's easy to incorporate in "digital" type ESCs, I'm told.
One reader misinterpreted earlier statements in this column and thought I was touting the SSC itself directly as a battery saver; not so. You can accomplish the same thing as follows. What the motor-cutoff function in the SSC does is preclude that user error from occurring. And for most folks, this is really important.
Basically, it disallows draining the motor pack "too far" no matter how tempted you are to stretch a flight. This feature takes over and retards the motor as the pack approaches the end of charge. Ni-Cds are not "good to the last drop," and this will not allow any pack cells to be driven "reverse"—a condition considered to be a cell-killer.
Nevertheless, you can independently do the same thing manually with any ordinary ESC. All you have to do is back off throttle when you sense the pack is getting low. Of course, you must possess that "deep inner strength" needed to do this, resisting all temptation for "one more go-around"—the "gotcha" for most folks, it seems!
Perhaps the uncontrollable desire to sustain flight as long as possible has something to do with a modeler's genetic makeup. In any case, the motor-cutoff idea works well and is of great personal assistance where the "self control" alternative is challenged.
The second SSC input is a new observation. I noticed a nearby airplane with an SSC display a "motor burst" upon system power up, and I've never seen an SSC installation do that. In this case, the SSC/BEC installation used an "arming fuse" externally plugged into a connector on the fuselage side—the fuse was used as a power switch in the battery lead. (Note: a fuse in the battery lead of any BEC installation is not a good idea; if it blows, you lose radio!) There were no other switch or connector functions involved. The modeler "powered up" everything by plugging in the fuse, and the motor momentarily powered up—sometimes.
The radio was an old Futaba Attack AM model operating on BEC. As best I can tell, this intermittently happens with this particular radio and seems to be associated with the way the radio itself comes on (powers up). The modeler told me that this particular radio system did it with other ESCs as well.
I also observed that when there was no motor burst, one or more servos would glitch instead, again pointing to something about radio "power up."
Besides all this, I fly a different AM radio and never had this happen, so it may have nothing to do with the AM aspect per se.
If you happen to have this particular unpleasant situation, you can work around it, as did this modeler. Just use an arming switch in the battery lead and a plug-in fuse (or additional switch or simply motor connectors) in one motor lead. Arm the system, transmitter on, then connect the motor. This should work for any problematic ESC/BEC/radio combination—SSC or otherwise.
Soldering cells
Soldering cells was briefly discussed in the July 2000 column, with the promise of more to come. An expanded discussion is still forthcoming, but in the meantime I want to identify one suitable soldering iron for assembling packs: the Weller SP-40. This iron is inexpensive, is readily available, and has what it takes to do the job. I've seen it mail-order for $13.95 and bought one in a local hardware store for $16.95.
In the referenced column I made issue of the "thermal mass" of soldering irons. This parameter was distinguished from tip temperature. Any iron suitable for soldering cells into a pack must be massive enough—"have enough mass"—so that it does not cool much while soldering cells. An iron of the small-tipped type routinely used for circuit board assembly is not suitable, no matter the temperature.
The more-massive SP-40 works well for pack assembly. I'll be getting more into this later. If you're new to cell-soldering, you might want to wait for the upcoming discussion before you leap forward and put any cells at risk.
So ends one more column—and so begins my eighteenth year of historically happy association with Model Aviation—and you! Have a great E-aeromodelling summer and fall, and please enclose a SASE with any correspondence for which you'd like a reply. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
