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Guys, we have a problem:
Nicads are being phased out due to the toxicity of cadmium, as discussed in an article this week in the Wall Street Journal (google thier online site). The European Union is banning them this year. The internal resistance of SR500Max cells has increased, in my measurements, from 19 milliohms to 22 milliohms, which is why they have been on sale at half price. I’d guess this owes to a change in manufacturing venue. Over the last ten years, manufacture of nicads has moved from the US to China.
So we will use nickel metal hydride [NMH] cells. It would help to gather our results on the various cells as to weight, internal resistance, watts per gram and general robustness. The work we’ve done for E-36, for example, shows the KAN 400 cell is quite nice at 7 grams and 55 milliohms. I’ve just ordered some 2/3A NMH cells (same physical size as the SR500Max nicad) to test: the Gold Peak 1100, KAN 1050, and Ellite 1500. I will test these at a high discharge rate and post the results here. Does anyone know of any other hot NMHs? Those of you doing battery work, please share! Mr. Ivers, what say you?
Chuck, I will be ordering and testing the following 6-cell NiMH batteries:
KAN 700AA This is a 13.9 gram cell. No available performance data, so I will have to test. This pack may be good for a medium size class A model.
ELITE 1500A (22.95 grams) I see this cell also on your list. I will get some and try to confirm your results. The available performance graphs look very good. This is a super cell, but needs a powerful motor and a big airplane to match.
Dick
Hi Dick,
Good! We can compare results. My test apparatus is a timer, resistor R and peak charged battery in a series loop with a peak hold voltmeter, with leads reversed, across the battery. I set the timer to 10 seconds — midrun for class A. The voltmeter reads about -8.4V before triggering the timer, and after cutoff it holds -E, the opposite of the midrun voltage E. The current is E/R, the power is IE, and the specific power is IE/m, where m is the mass of the battery in grams. If a series of tests with different resistors R1, R2 … Rn is made, linear regression in (I,E)-space yields the internal resistance as the opposite of the slope. Using a single test, internal resistance of a single cell can be approximated by r=(1.32-E/c)/I, where c is the number of cells in the battery tested. I’m sure your test apparatus is more sophisocated but mine is accurate enough that a comparison will serve to mutually verify our work.
For comparison, the test mentioned above was applied to a very old Sanyo AN500AR red-jacket nicad cell which weighs 18.1 grams. The old cell required two cycles to revive after which it showed an internal resistance of .0175 ohms from the quick formula. The potential was .95 volt after 10 seconds with an apparatus resistance (timer, resistor and wire) of .0464 ohms. This old Sanyo red-jacket is the best of all nickel cells — all others must be compared to it. The NMHs have not arrived yet but the apparatus is ready for them when they do.
Thus far:
Sanyo N500AR old red nicad: .0175 ohm, 18.1 grams, 1.32 volt open.
KAN 1050 pale green NMH: .0184 ohm, 22.0 grams, 1.32 volt open.
Both are 2/3A dimensions. The biggest difference is the weight. The open potential is assumed and the sole precision point is at 20 amps.
Graphs for Sanyo N500AR single cell attached. Data from Chuck’s post. Calculations and graphs from Mathcad. Note: Calculated peak power is 24.9 watts at 37.71 amps. However, this theoretical peak power is not realizeable because the current is too high and terminal voltage too low. At this current level the cell would be cooked from self heating. Nevertheless, this cell is the Gold Standard with a specific power output of:
24.9 watts/18.1 grams = 1.38 watts/gram
1) Thanks Dick! Yes, the maximum power of the cell, modeled as E=Eo- R, with Eo the “open” potential, R the internal resistance, and I the current, will be P(I)=EoI-I^2 R, a parabola opening down in (I,E)-space. Its maximum, Eo^2/(4R), occurs at a potential of Eo/2, about .66 volts, half the open potential, which would fry the battery. I like to operate at about .86 volts, where the power is a bit less but the battery will be happy enough to last a couple seasons.
2) The NMH batteries have arrived. My tests are not complete but preliminary results suggest that the Elite 1500 is a dynamite cell. It is more powerful then the N500AR but weighs more so its power to weight rate is about as good at the same current — I’m testing at about 20 amps. Internal resistance is very low about .012 milliohms as compared to the N500AR red-jacket’s .018 milliohms. (The later N500AR green-jackets are nowhere near as good.). The Elite 1500 will give two or three powerful runs without recharging. The second and third 10 second bursts are stronger than the first! The Gold Peak 1100 is quite good but not as good as the Elite. The KAN 1050 is a poor third. Results seem to depend on the method of charging. All cells seem to work better when peak charged very warm but not hot — C/10 trickle is not competitive. Also, cycling is best, with the cells giving the most power at the second or third cycle when new or after a period of storage.
Thermals
Here are the numbers:
battery_________________weight, gram___Ri, ohms___P/W, watt/gram
N500AR old red nicad______18.1_________.0175_______1.42
N500AR green clone_______18.1_________.0215_______1.15
KAN 1050 NMH___________22.0_________.0184_______1.11
Elite 1500 NMH___________22.5_________.0118_______1.69
Gold Peak 1100 NMH______21.9_________.0137_______1.49
The weights are approximate in some cases because I had soldered on
connectors and had to estimate. The NiMH cells weigh considerably more
than nicads of the same nominal size, 2/3A. Power to weight was
computed from the impedance-matched maximum formula,P/W = Eo^2 / (4 Ri w )
where Eo is the open potential, taken here as 1.34 volt, Ri is the internal
resistance in ohms, and w is the weight in grams. This power occurs at
a very heavy load at which the potential is .67 volt, half the open
potential, and the current is half the shorted current. Such an
operating point would fry the battery. A more reasonable use would
offer maybe 75% of the power tabulated.The upshot is that the KAN and the green clone suck, the Gold Peak is
quite good, and the Elite kicks butt. If you just drop the Elites in a plane
that used to use good nicads of the same size, you better watch out,
boy. If you don’t retrim for the greater power you will most surely
plant a balsa tree.Thermals
It looks like the last few posts to this thread were lost.
The last part of the discussion was about possible changes to the Electric Class A rules. I found out later that it’s too late to propose changes to the 2009/2010 rules. The AMA rules are on a 2 year cycle. Therefore, the next round of proposals would start in January 2009. If those changes were approved they would not go into effect until January 2011. That’s a long way off.
Dick
I thought I was living in the twilight zone. I have never experenced a disappearing thread before. It sounds like we will fly “A” at the Nat’s and hopfully see a variety of models. It sounds like a lot of fun to me.
We were flying in fairly high winds last weekend with short DT’s, and my son DT’d his T-Bird under power. I have heard the term “lawn dart” used in the past now I understand. The wing folded at the top of the loop and it flew into the ground under power. Can hardly wait for the flying season!
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