Author: Red Scholefield
Edition: Model Aviation - 2011/01
Page Numbers: 87,88,89
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The Battery Clinic - 2011/01

Manufacturer recommends against using LiFe pack

Hobbico LiFe Pack Revisited: In the September column I wrote about the Hobbico LiFe battery pack for receiver/servo use; my concern was the higher voltage. Readers have since contacted me to let me know that they have used this pack with no problems.

Nick Gaynor wrote:

"I'm following up on our previous emails, and I have a comment on your September 2010 Model Aviation Battery Clinic column 'Is a two-cell Lithium battery too much for your receiver/servos?'

"What I would like from Futaba is an explanation, hopefully technical, as to why LiFePO4 batteries are not recommended.

"Ironically, perhaps even funny, is that in Futaba's S.bus video, they feature their S-Bus system being powered by guess what ... the Hobbico LiFe Source 6.6V battery pack.

"I, for one, am going to start using the LiFe packs to directly power my receiver electronics, Futaba or Spektrum or any other brand for that matter. And of course, only 6‑volt rated servos, but that's just common sense.

"The current argument of people using 6‑volt NiCd packs (which when fully charged do peak at about 7.2 volts, like the LiFe packs) successfully for years is the main factor of my decision. I am mindful, of course, of the steep LiFe voltage discharge curve below the 6‑volt point. The easy solution of course is not to let your pack become too discharged, but doesn't that apply to any pack?"

I posed the question again directly to Futaba service, and personnel remain adamant that the LiFe unit should not be used with their system despite Steve Kaluf (former AMA technical director) showing the pack in the Futaba video of its new S-Bus system. I asked them about that, and their reply was that Steve must have picked one up to use with the system without their knowledge.

Since I have gone to 2.4 GHz, I had a receiver and some servos to donate to "science." I fully charged the LiFe pack and then set it up to power a Futaba FPR127DP receiver, two Futaba S148 servos, and two Futaba S3003 servos.

I set my Futaba 9C transmitter to "servo" function, where it slowly ran the servos from one extreme to the other. This went on for more than two hours, with an occasional deliberate stalling of the servos (where they drew approximately 900 mA).

I found no problem. In fact, the current being drawn was only slightly more than with a four-cell Sanyo KR-1100-AAU pack. There was no evidence of heating. A discharge comparison of the two packs shows a significant difference in voltage delivered.

Are we comfortable using Hobbico's LiFe packs without a regulator? Hobbico thinks that we should be, and I have found no reason not to be, despite Futaba's stand. No other RC system suppliers seem to have a problem either.

Buffalo Li-Poly pack evaluation

OK Hobby sent me one of its Buffalo Li-Poly packs to evaluate. It is a 3S 3200 mAh pack, rated at 40C.

Discharging at a 40C rate gives you less than 90 seconds of flight time. Most pilots look for a six-minute flight minimum with their electric-powered models, which equates to 10C discharge. That brings up the question of why rate something at 40C if it won't see that kind of use?

"Testers" in this market tend to feel there is more "marketing" than "engineering" in these ratings. The one thing everyone seems to agree on is that the C rating is an indication of the internal resistance of the pack: the higher the C rating, the lower the internal resistance. That means higher voltage delivery, particularly at higher rates.

The battery weighs 10.09 ounces without connector; a Deans Ultra plug was installed for testing. The open-circuit voltages, as received, were: cell 1 = 3.919 V, cell 2 = 3.928 V, cell 3 = 3.920 V. This showed the pack was delivered in the normal storage-charge state.

Let's look at the Buffalo Li-Poly pack's three discharges done with a West Mountain Radio CBA II, and the charging done with FMA Direct Cellpro 4S and 10S systems. After three cycles, the following data was produced:

  • Charge at 3 A input: 3.226 Ah; Discharge at 3.2 A to 9.0 V = 3.143 Ah
  • Charge at 3 A input: 3.212 Ah; Discharge at 10 A to 9.0 V = 3.067 Ah
  • Charge at 3 A input: 3.019 Ah; Discharge at 9.9 A to 9.0 V = 3.138 Ah

After the third cycle, one cell read 2.90 V while the others were 3.20 V, which indicated that that particular cell might have a lower capacity. When the 3 A charge input was applied, the same cell that read 2.90 V at the start of the charge took in 3.182 Ah, so it was good and fully drained. Its internal resistance, as measured on an FMA Cellpro 10S, read 6.9 mΩ (milliohms).

The pack was next bench-run in a model with an AXI 2820/14 motor, Turnigy 40 A ESC (set to a conservative voltage cutoff), and a 12 x 6 APC-E propeller. High-throttle start readings were 31 A and 356 W. Total run time was 7 minutes, 5 seconds when the voltage cutoff engaged. Output before cutoff was 240 W. This equaled 3.183 Ah delivered as shown on the AstroFlight wattmeter — what you would expect in an actual flying environment.

Other modelers' testing on Buffalo packs indicates that those rated at 40C are actually closer to 20C. This is based on the 140°F heat that developed, a temperature we know can have a serious impact on Li-Poly cell life. The 80 A test went to 160°F. The 100 A test went to 160°F when it was terminated and appeared to be headed for 180°–200°F.

Ongoing field testing

I am going to continue field-testing this pack in models with the same motor/ESC combination. One is a powered glider spinning a 13 x 6.5 Graupner folding propeller, and the other is the bench-test aircraft (Stick 25e). Flights will be terminated before a noticeable drop in power is reached.

My conclusion so far is that the battery is strong and offered labeled capacity at higher rates (10C). I would say it is worth a try at $33.17. Now for the long-term field-testing in a real RC environment.

I expect to put in four or five flights each week on the test pack (with help from my fellow electric-flying club members). Others have done more extensive tests on Buffalo Li-Poly packs. One series on a 2S 2500 mAh unit indicates that the packs are more realistically 20C capable; temperatures at rates above 20C exceeded the 140°F that was deemed destructive to the packs.

I wish there were a way to get good life data on these packs other than simply flying them and occasionally monitoring capacity. By the time meaningful data is accumulated, the technology has often advanced. In addition, drawing conclusions from one sample isn't very meaningful: too many variables need considering, and that takes many test resources and a fair quantity of samples to provide an appreciable test matrix. One would think that the manufacturers (not the distributors) would make data available to substantiate their claims.

As with numerous products on the hobby market, we will have to judge by our modeling friends' collective experience what is good and what is not so good.

If you want to see results of tests that competent electric-power modelers have performed on a variety of Li-Poly packs, go to the Battery Graph Vault (see Sources).

9‑volt battery-pack construction

While rummaging around in my "junk" drawer, looking for small metal pieces to use for a magnetic hatch, I found some depleted 9‑volt battery packs in a recycle can. The metallic cases married to the magnets nicely. However, in removing the cases I found different ways that a 9‑volt battery is made.

One sample uses cylindrical cells similar to an AAAA. These are interconnected via pressure contacts held in place by the crimped metal case. Another sample was made from plastic-clad button cells, with tabs to a connector plate; it also holds the cells together with the crimped metal can. The last sample was a rechargeable 9‑volt with six button cells. Energizer uses the AAAA cells, and a Walgreens-labeled pack uses the more conventional technique for building a 9‑volt pack. Cells from the 9‑volt rechargeable pack can be broken apart for an ultrasmall ~90 mAh battery.

CAPCAM — pictures of your flying

Do you want pictures of your flying? Bob Schumann came up with a simple idea to obtain them. He wrote:

"I am attaching some photos of my 'CAPCAM.' It is a vehicle for a mini video camera to take pictures of you flying your RC plane.

"I got one of these $25 cameras and Velcroed it to the bottom of my electric-powered Radical RC Inter 66 and took several shots of our field. After a while they all looked the same so I thought about taking pictures of folks flying. Panning with the plane worked out OK, but I wanted pictures of me flying my plane and no one at the field offered to do that.

"I took a freebie hat and cut a hole in the brim that would accommodate the camera body while pointing to the front of the cap. This gave me a self-following mount for the mini cam and the results turned out great! Not often does a 5-minute solution work as well as this did.

"Spelling out how it works is easy: you should never take your eyes off your plane while flying it. This arrangement meets that test as long as you wear the hat bill forward! (Is this permissible in today's society?)"

There are even smaller cameras than the one shown, available for much less money, and they do a good job. With some fiddling, you can use this mounting/tracking system with the smaller cameras too.

I wish you all a great building and flying new year. While you are warm and cozy making balsa chips in your workshop, we are braving the elements, trying to fly year-round where our building season starts immediately after we crash our last airplane.

Keep those cards and letters flowing. No e-mail connection? Drop me a note to "The Battery Clinic" at the address in the Sources section. Since it's our flying season, it might take me a day or so to answer you.

— MA

Sources

  • The Battery Clinic

12219 NW 9th Ln. Newberry, FL 32669

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