The Battery Clinic
Red Schofield | [email protected]
When old meets new ...
Since I've taken the quiet, clean path, I've modified a dozen or more glow-powered models for electric flight. It was simple: replace the glow components with a motor, ESC, and battery. Check for the correct CG location and then fly. That was, until I inherited an antique design: Gilbert Morris's 1941 Kerswap.
Bob Strub, of the On Top of the World RC Club in Ocala, Florida, who is a master modeler, built the Kerswap. It was set up for electric, lacking only the motor, ESC, and receiver.
I had what appeared to be a good choice for the motor: a 1588 Kv Maitian brushless that I won in a pilots' drawing at a local electric meet. Combined with an inexpensive Welgard 18-amp controller and an FMA Direct 2S 2100 Li-Poly battery, I had the Kerswap ready for its maiden flight.
Hitec HS-55 servos with an FMA Direct M5 receiver drove rudder and a half elevator. I was concerned that with only a half elevator, the model would have a rolling tendency. I was unprepared for the way the Kerswap responded to rudder commands.
Probably because of the pylon and polyhedral, I tended to way overcontrol. Small inputs and patience were the only things required to avoid what seemed, at first, to be erratic action. I merely had to add a tiny amount of rudder and then wait for the airplane to do its thing. I noted no adverse effects with the half elevator.
I would be interested to hear from others who have electrified one of these pylon-configuration, old-time free-flight (FF) models, remembering that they were designed to climb nearly vertical under power and then transition into a nice, flat glide.
Throttle-Lock Fixes
In response to my request for information in the January 2009 issue about programming a transmitter to have a throttle lock, I have several responses that cover the Airtronics Stylus, Spektrum DX7, Spektrum DX6, and Futaba 9ZAP. The setups are detailed and would take up a whole column by themselves. If you are interested in this information, send me an e-mail (or a letter via snail mail) and I'll send the details. Thanks to the AMA members who responded: Alan Buckner, Bill Inman, and Luis Alvirez.
The Ugly Truth About Battery Ratings
Frank Donnelly wrote:
“I have a NiMH, 4.8 volt receiver pack that's rated for 1000 mAh. It's very small since it's comprised of Sanyo AAA cells. I could purchase a NiMH, 4.8 pack with the same mAh rating in larger cells such as AA all the way up to C. The physical size of the pack would get larger and it would weigh more but electrically it should still supply 1000 mA for 1 hour.
“But something just doesn't make sense to me. How could such a small pack have the same capacity? If it truly does, why wouldn't we use these small packs in large aircraft? For that matter why would physically larger batteries even be built if a small one can provide the same capacity at the same voltage?”
In some instances, the rating game has been assumed by marketing types rather than engineers. When the engineers had pushed the capacity as far as they dared without significantly compromising the battery's life, the marketing wonks were unsatisfied and started playing games using "nominal" vs. "minimum" for capacity ratings. That gave their products a perceived advantage over the competition, until the other companies started the same game. Product failures or poor performance is usually hung on the engineering or manufacturing people—not on marketing or sales.
The AAA cell rated at 1000 mAh is what we used to call a "cram job" in the business. Cramming this much into a cell that size required other design parameters to be compromised. Along with those concessions was the cell's service life. The separator was thinner, winding pressures were increased, and the winding mandrel was smaller—all of which contributed to early shorting. The can thickness was decreased, which compromised the sealing of the cell. So whenever you see a cell that seems to have an abnormal amount of capacity for its size, you know that compromises were made.
A NiMH AA cell with a normal capacity would be roughly 1000 mAh. A C-size cell labeled at 1000 mAh is probably from the culls of a production lot that didn’t make anywhere near the capacity that would be expected or had some other capacity-debilitating problem caused by poor process control.
Rather than scrapping the cells, they were peddled to a secondary market where product quality was not of prime interest. Products were relabeled and sold under various brand names, and the buyer often got what he paid for.
The takeaway is simple: when a cell’s capacity seems abnormal for its size, compromises were almost certainly made in its manufacture. Expect lower service life, higher failure rates, and poorer high-rate performance from such cells. Buy from reputable manufacturers and be wary of marketing-driven capacity claims.
Usually these are no-name brands made in China. Cells may be labeled C-size but actually contain AA cells inside a plastic shell to make them C or even D size.
B-29 Crashed and Burned
Our club experimenter is way ahead of us when it comes to taking on challenges. The most recent was a B-29 built from a Guillow’s kit that was intended for display rather than flying. Then John Castronover thrilled us with its first flight.
The second flight was even more thrilling if you are into ugly. John’s model was on final approach after a nice flight, when it snapped into the ground from close to 10 feet up.
Club members were quick to run to the crash site, where the B-29 was consumed by flames. A member with forethought had grabbed his small fire extinguisher and put out the fire, but not before the B-29, two of the four brushless motors, and four ESCs were destroyed.
Forensic analysis indicated that it was one of the ESCs—not the battery—that had shorted, which caused the fire. John’s prognostication was that one ESC failed as he applied power to stretch the landing, allowing one motor (outboard) to stop, resulting in the snap into the ground.
Powered-Glider Wing Flutter
I encountered a perplexing issue. On the first test flight of a Jolly, a powered glider from Northeast Sailplane Products that was set up with the recommended motor—an AXI 2820/10—it developed a serious wing flutter under full power. I noticed this immediately after launch, as full power was applied and it came up to speed.
The first suspect was the ailerons, but checking those showed that there was no slop and that the gap was sealed. On a subsequent flight, I used less power (it had way more than needed, as it turns out). That flight was great and required little, if any, trimming. Then I took it up to full power and was rewarded with more violent flutter. Enough of that.
I learned later, according to Northeast Sailplane Products’ Web site, that performance was fine with a seven- to eight-cell Ni-Cd/NiMH battery; I used a 3S 2500 Li-Poly. Dropping back to a 2S Li-Poly pack fixed the problem.
Using a small drill bit (.080 inch) I grabbed from the tool drawer to align a motor’s mounting holes to the firewall, I inadvertently snapped off the drill bit. Where did the piece go? I found out when I tried to turn the propeller to assure that there was no interference with the outrunner.
The drill bit was sucked in and attached to the magnets in the motor. My only choice was to take it apart and remove the obstruction.
There is a tiny (very tiny) snap ring on the shaft as it exits the housing. Using my finest needle-nose pliers (my small snap ring pliers were too big), I managed to pull off the snap ring and carefully set it aside. The motor’s outer casing was easy to remove, and there, nestled between the magnets, was the rogue part of the drill bit.
With the stray piece removed, I reassembled the motor; the last step was to reinstall the snap ring. Sproing! It went into that never-never land of the shop where things disappear forever.
Oh well. I wouldn’t need it if the motor were mounted so that the shaft were used for the propeller attachment, rather than reversing the motor and using the AXI propeller adapter system.
Replacement snap rings are not available from the distributor, but I got some from McMaster-Carr. They come 100 in a package; if you need one or more, let me know.
Is balancing a gimmick to sell balancing chargers?
A Li-Poly battery vendor claims that it is. His batteries are matched and never need balancing.
Considering that Li-Poly incidents have all but disappeared coincident with the introduction of balancing to the hobby, you have to wonder.
Steve Anthony of Hangtimes Hobbies, a quality pack vendor, wrote (and I agree 100%):
“We’ve learned (some the hard way) that cells have individual characteristics. After a few rapid charge and discharge routines the small normal differences in capacity and impedance can easily become significant enough to force one cell in the pack into an overcharge state before the other cells come to full voltage.
“While it’s true that some of the Pacific Rim manufacturers have different production or assembly shops, one huge issue is that virtually all of them share the same raw material suppliers. Debris and impurities in separator, tolerances on materials, alloys, grading, etc., all done outside of the control of the cell manufacturer, makes cells cannot be the same.
“Assuming the vendor is matching cells at assembly, it’s still a pack. Cells on the outside of the pack will see a different operating environment, temp build up, cooling rates, physical contact with its containment, handling pressure on the outer cells jackets. A few hours into service, it’s no longer matched.
“To proclaim this technology as not requiring the common sense expedient of using a balancing charger is, in my humble opinion, ludicrous and dangerous.” /SA
Sources:
- Red Schofield
- 12219 NW 9th Ln.
- Newberry, FL 32669
- www.hangtimes.com/redsbatteryclinic.html
Transcribed from original scans by AI. Minor OCR errors may remain.
