Author: Red Scholefield
Edition: Model Aviation - 2011/05
Page Numbers: 92,93,94
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The Battery Clinic

Red Scholefield | [email protected]

Models you don't see everyday

THE ZEPHYR Electric Fly-In Festival in Florida, hosted by the Zephyrhills Can-Am Flyers, gets bigger every year. The 2010 edition featured some great flying in almost perfect weather, with no zephyrs.

Several models at the event caught my eye. A huge twin-engine Double Ugly Monster Stick by Bill Witson also got caught in a tall tree. It was rescued, but not without a fair amount of damage. I don't know if they got one of the dishwasher-sized Hacker motors out of the tree or not.

Another twin was an 8-foot-span PBY. According to the owner, the only problem was that the limited propeller clearance made it a challenge to use enough propeller to fly the airplane.

Honors in the beauty department went to Don Heath's Percival Mew Gull. Designed by Seagull Models as a 120-size ARF, it had been converted to electric power.

Neil Harris of Graves RC in Orlando created the twin-ducted-fan-powered Ugly Hog Wart Stick. You haven't seen a hobby shop until you've visited Graves. If you are ever in the area, it is well worth the visit.

SLK Electronics has added to its line of battery-care products an item that will monitor your A123 (LiFePO4) flight pack: the MilliAmp Monitor/Analyzer, or MAMA. Following are a few of its features.

  • Keeps track of current in and out of a pack.
  • Is compatible with all two-cell A123 (LiFePO4) packs.
  • Gives visual indication of remaining capacity.
  • Remote indicator (option) can be located for easy viewing.
  • Low-throttle blips (option) warn of low remaining capacity.
  • Handles up to 10 amps continuous and 25 amps peak.
  • No need to unplug; can remain connected for several months.
  • Automatically adjusts to pack capacity.
  • Keeps track of partial charging.
  • Draws 5 milliamps with receiver on and fewer than 100 microamps at idle.
  • Measures 2.05 x 1.10 inches and weighs 12 grams (less optional 16-gauge wires).

In addition to charging its LiFeSource receiver packs, the Hobbico LiFeSource balancing charger can handle other one-, two-, or three-cell LiFePO4 packs. The cover of the unit provides a good idea of what it can do. This charger operates from either 120 volts AC or 12 volts DC, making it useful in the shop or at the field. An automatic-start function with status LED and audible tones make it user-friendly. Solid-state polarity and current-overload safety devices ensure trouble-free operation. An adapter is included for the two-cell receiver pack. In addition to the balancing port, connection to the pack is either via a Deans Male Ultra Plug or a socket that accepts normal receiver connectors.

The question still remains: Are LiFe packs too much for our receivers? Letters from users suggest that there are no problems. Nick Gaynor offers those who are concerned an option. He wrote:

"I'm using some 2S LiFe packs that I made as directly connected receiver power for a Reactor. So far the results have been excellent.

"I'm using a Futaba R617FS 7-channel 2.4 GHz FASST receiver and five Futaba S9650 digital servos. I never let the battery get below 6 volts before recharging.

"The reason I write is that it occurred to me that a diode (rectifier) can be used to drop the LiFe pack voltage by 0.5 to 0.8 volts or so. That might satisfy those afraid to connect the fully charged (7.2 volt) LiFe batteries directly to their receivers. Have you heard about anyone else doing that?

"I connected a LiFe battery through a diode to a wattmeter with a 10 ohm load. At a 550 mA load current the diode dropped 0.79 volts. The resulting voltage from the 6.6 volt LiFe is shown in the attached photo as 5.81 volts."

Darren Braun is much more cautious about using LiFe packs without some form of voltage reduction. He wrote:

"I have enjoyed your column for many years now. I may be able to help shed some light on the issue of applying 'overvoltage' to various microelectronics, based on what I do for a living.

"Futaba states that it is not recommended to supply voltages greater than 6 V (unless noted otherwise i.e. some servos only 4.8 V) to their products and this simply has to do with reliability.

"The ICs (integrated circuits) themselves inside Futaba products are fabricated on processes at foundries that are designed and characterized for specific ranges of supply voltages. Operating outside these conditions does not guarantee reliable operation.

"Take for example the timing of a digital circuit. Hold time gets worse as voltage increases.

"There is no guarantee there is enough margin to satisfy hold time in the severely overvoltage condition. And timing for any voltage greater than 10% over nominal voltage will not be checked by the IC designer. However, they will check and ensure it operates within a range (usually +/- 10%) around nominal IC supply voltage which corresponds to the characterization range.

"So you may find your receiver works fine at 6.5 V or even 7 V but there is no guarantee all will do so and for how long."

More reader letters

Earl Haury wrote the following about "sick" Li-Poly packs.

"Let me add my thoughts/observations as to why things cannot be all good with charging/balancing related to pattern packs and the common chargers we use.

"So what about 'charge abuse'? My observations suggest that balancing equipment doesn't always do what is expected. Many balancers function by drawing charge current away from the high cell(s) which allows the low cells to 'catch up.'

"If one cell performs abnormally the balancer may continually shunt the other cells, leaving the weak cell undercharged. Many times balancers are set to terminate at cell voltage and/or total pack voltage and not by current, so they may not fully charge all cells. Chargers that rely solely on cell voltage termination are likely to leave some cells undercharged. Also chargers set at too high a charge rate can cause cell imbalance and heat damage.

"Part of the problem lies with the balancer's capacity to divert that charge current—usually 300-500 mAh—which isn't sufficient when charging at 5 A. If the charger primarily terminates on total pack voltage, the high cells are pushed too high until the average reaches the target pack voltage.

"This will often leave the weak cell low and some too high and won't be noted unless the individual cell voltages are observed. (The way to charge packs with a large imbalance is at a rate that doesn't exceed the diversion capacity of the balancer—takes time.)

"On a good note, most systems will terminate charge (with notification) if the high cells are pushed too far—still not good for them (especially without consideration for temperature) but an important safety feature that should prevent charging eruptions as well as providing notification that something isn't right.

"Then there are chargers that balance by either charging each cell independently or by adding current to the low cells to help them 'catch up.' This is a good strategy and that appears ideal except for issues with connectors/leads.

"A common problem is that both balance methods can be compromised by the balance leads/connectors themselves which can result in the balancer actually misbalancing the pack.

"The connectors are small and of materials that will oxidize over time. They also tend to lose tension with use. Both the looseness and oxidation result in a poor connection. A marginal connection will prevent the balancer from 'seeing' the correct voltage and will become worse as balance current flows.

"Ever have a 'balance lead not connected' or 'incorrect cell count' report? These are indications that the connectors are not doing their job. This all is 10X true if charging at 5 A is attempted through these faulty connections!

"Additionally, the balance leads themselves are small gauge wire of low strand count (not very flexible) which often results in several strands breaking from the connector with use. Eventually the wire might break off—in the meantime it's a resistor in the balancing circuit! I've also seen extensions (have all the same problems) where only a couple of strands were captured in crimping to the connector!

"The good news is that much of this can be avoided with maintenance and observation. Contact cleaners will help keep the connections clean, then some female connectors can be removed from the housing (one at a time if on the battery) and 'bent' back closed when they loosen.

"Check connections on new leads/extensions and always fix the problem if some lead 'wiggling' is needed to make the charger happy."

Mike Robinette has done a lot of work with first-person-view flying and has had some innovative ideas for inexpensive airframes; one he calls the "Micro Grey Ghost."

He explains:

"The wing is from a ParkZone Super Cub, 48 inches. I strap 3M Extreme tape to the wing before attaching the balsa/basswood booms. The fuselage is balsa and can be interchangeable with other fuses for other purposes.

"This is a trainer version, normally there would be a CCD camera mounted in the nose and 900 MHz video gear installed to fly FPV (First Person View). It flies super stable and travels with the wing attached quite well in a smaller vehicle.

"It is powered by a Turnigy 3530/1100kv motor swinging a 10 x 5 APC prop and Turnigy 30 amp ESC, all from HobbyKing. I use a Zippy 3S 3000 mAh pack in this version for long training flight times; normally I would use a 2200 mAh 3S pack with the video gear installed.

"The twin boom design is perfect for FPV and makes balancing at the CG a breeze."

Flying season is upon us: give those batteries a good preseason check. Measure the open-circuit voltage; if it is not at least 1 volt per cell, the pack (Ni-Cd or NiMH) is suspect.

Slow-charge the batteries, discharge to check capacity, charge again, and check again. If they won't deliver at least 80% of the rated values, use them in a model you don't care about. MA

Sources

Red Scholefield 12219 NW 9th Ln. Newberry, FL 32669 www.hangtimes.com/redsbatteryclinic.html

Transcribed from original scans by AI. Minor OCR errors may remain.