Lipo informatie uit de e-flight mailing list.

Lipo informatie ontvangen via de e-flight mailing list.


------------------------------

Date: Sat, 24 May 2003 22:49:17 -0400
From: "Don Stackhouse @ DJ Aerotech" <djaerotech@erinet.com>
Subject: [EFLT] Roadkill on lithium (part 1)

This message from "Don Stackhouse @ DJ Aerotech" <djaerotech@erinet.com> brought to you by EFLIGHT!

With all the discussion of Lithium-polymer batteries going around various e-mail lists these days, it looks like a good time for me to "spill the beans" on our own fairly extensive tests of these batteries in our "Roadkill Series" profile indoor/backyard/parkflyer electrics.

We've had a LOT of inquiries in this year about the use of various types of lithium batteries in our models, but until recently we were reluctant to endorse anything in particular. We had concerns about how well suited the available batteries were for our particular kits, how well they would perform in general, and about the availability of simple and reliable but economical chargers for lithium cells (we had serious philosophical objections to having to tell someone they needed a complex multi-hundred-dollar charger to charge the batteries in their relatively cheap and very simple model). There were also the obvious safety concerns, based on all the rumors of fires, etc..

As far as chargers go, we've had outstanding results with the Plantraco LPC-400. It can charge 1, 2 or 3 cells at 100, 200 or 400 milliamps, comes with an AC adapter, has internal checks to make sure you connected everything properly and made the correct settings in two switches on the front for the charge rate and number of cells, has a built-in cooling fan, is extremely simple to use, has a very nice charging profile that takes excellent care of the health of the batteries, and does all of this for a very economical price. We've used it very successfully for everything from 45 mah cells to 3300 mah 3-cell packs.

The only drawback to using the LPC-400 for the 3300 cells was the several hours it needed to charge them at its max rate of 400 milliamps. However, we've found that at least for Roadkill sized models with their cells of 1200 mah or less, you get so much flight time per charge that it's rarely necessary to recharge during a flying session. In addition, the Li-poly batteries hold their charge literally for years, unlike nickel-based cells. It is not necessary to re-peak them just before flight. The Li-poly batteries I flew for the entire evening flying session at the Bowling Green indoor demo at Toledo this year were not recharged at all during the evening, still had most of their charge left at the end of the evening, and had last been charged as much as a week or more before that evening.

We have some other chargers we're investigating as well, including some of the new ones from E-tec, and it looks like there are now a variety of cheap, simple and safe chargers for at least the smaller sizes of Li-poly cells.

The fire safety issues we've heard of for the most part seem to be related either the older Lithium-metal cells, or to folks charging Li-ion and Li-poly cells with nicad chargers, or using older chargers with inadequate error checking and then setting the number of cells and/or charge rate settings incorrectly. The newer chargers seem to do a much better job of addressing these issues, such as the checking to see if the charge and number of cells settings agree with the electrical characteristics they sense for the pack being charged. Nothing is perfect of course, we still need to be diligent with our charger setups and we need to monitor packs while they are being charged, but from what we've seen it appears to now be a manageable issue. I've heard of nicads exploding and even causing fires too, so the issues folks seem to be so afraid of with the lithium cells are not as new or unique as a lot of you seem to believe.

As far as maintaining balance between the cells in a pack, It does not seem to be a significant issue in actual practice, at least in our experience on these models. I've charged individual cells one at a time, but I don't bother with that any more. I do occasionally check individual cell voltages with a voltmeter, but so far they've always matched with each other within 0.01 volts, even on packs that see high discharge rates such as in the Ryan ST during one of my aerobatic workouts.

So what about cells for these models?

The old Lithium-metal cells simply were not suited for our size of models (much too big and heavy), as were the lithium-ion cells we investigated.

Regarding the currently available sizes of Kokam cells:

Two cell packs of the little 145 mah cells don't seem to have the power output and the necessary max current capacity, except for the most efficient MPS-1A models that don't need more than about 40% throttle for cruise. The Cub, A6M2 Zero and the Curtiss-Wright Junior may be OK based on this. Flight times might be acceptable, as long as you're willing to put up with a significant loss of full-throttle power. The weight reduction helps a lot, but the power loss more than offsets this. The max current limits on the Kokams indicate that on models that use higher power settings than that for a significant percentage of the flight, the cells might be damaged. We're still investigating that, but so far it appears that the best application for the Kokam 145 cells is as a single cell in an RFFS-100 equipped plane, or similar, which rules out most of the Roadkill models for the present time.

Two cell packs of the larger 500 mah KoKam cells do OK in our twins and MPS-2A powered models, about the same performance as the 230 mah NiMH cells but with more flight time. The shorter-winged models with a single MPS-1A do not seem to like the weight of the KoKam 500 mah cells. The Me109 Joe's son Terry was flying at Bowling Green had one of our new MPS-2A twin-motor systems with a pair of Kokam 560 cells. It's lighter than that same airplane with a 230 mah NiMH pack but with almost the same power at full throttle, and as anyone there could see it's a very happy airplane with that setup.

Note, the Kokam cells do seem to be good quality products from what we can tell, it's just that the presently available cells do not seem to have the best electrical properties for the needs of our models, at least in comparison to the other presently available options. We've heard of some new Kokam cells that should be available soon and that should be a better fit, and we're looking forward to testing them as soon as they are available.

Regarding the new E-tec cells that are now available: These seem to have significantly more current output than the Kokam cells, both in terms of what they can tolerate without damage and in what they will actually produce in service (officially about 4-6C max continuous and up to about 7C for brief periods), and come in sizes that seem better suited to our company's current models.

A two cell pack of the E-tec 250 mah cells is fully 1/2 ounce lighter than a 7-cell 110 mah NiMH pack (our standard NiMH recommendation for indoor flying with a single MPS-1A). Full-throttle power at the beginning of the run is slightly less (about 90%), but with the weight savings the overall performance seems to be about the same or slightly better, and the flight times are far better. Power at cruise is about the same as good 110 mah NiMH cells, and power available at all throttle positions near the end of the flight is a little better. I recently flew the Roadkill Series Curtiss-Wright Junior with a 2-cell 250 mah E-tec pack outdoors in measured winds averaging 10 knots with frequent gusts to 12 and occasional gusts to 15. This required about 75% on the throttle stick position (we can normally cruise the Junior at about 1/4-1/3 throttle stick position in calm air), and with the old 110 mah NiMH pack I would have expected about 7-8 minutes at the most at these power settings. The E-tec cells lasted a very solid 22 minutes! It appears that the E-tec 250 mah 2-cell pack may be the best presently available all-around choice for most of our models powered with a single MPS-1A.

Flying this same Junior in calmer conditions resulted in a 32 minute flight, most of that at about 40% throttle.

Earlier this week we tried the RK Series Sopwith Camel and Fokker Triplane with the 250 mah 2-cell E-tec batteries. Two Li-poly cells is a little less voltage than a 7-cell NiMH pack, so that's a little less rpm from the prop and therefore a little less power. For most of our single-motor models the 1/2 ounce weight reduction more than makes up for this, but in the case of the Camel and the Triplane there is a lot of stuff being dragged through the air on not very much wingspan, so there is a lot of both induced and parasite drag. Both of these models, especially the Triplane, like lots of power and end up spending most of the flight at pretty high throttle settings. Their short span also makes them very sensitive to weight, which can offset some of the extra power from going to an MPS-2A and a larger NiMH battery. Performance with the 2-cell 250 mah E-tec batteries was very promising. There was still no great surplus of power, but we got flight times of over 20 minutes, where with the 110 mah NiMH batteries times of 4-6 minutes were more typical. The Triplane seemed to be especially happy with the new setup, where that 1/2 ounce weight savings made a huge difference for its tiny wingspan (the smallest span of the entire RK Series). Handling was noticeably livelier, and its already tight turning radius was even tighter.

However, I flew the Triplane again last night, and found that with the outside air temperature of only in the middle 50's (Fahrenheit), it had lost some of its enthusiasm. Just like NiMH, the lithium cells lose some of their amps when they're cold, and they lose some voltage as the discharge cycle proceeds. Initially they're a little over 4 volts per cell, but they stabilize after that initial peak burns off at somewhere in the upper 3's. The voltage then seems to decline more or less linearly until the safe minimum voltage approaches at around 2.7 volts per cell (that's the "official" minimum voltage with no load; I'm told you can actually go a little lower than that under load because they will rebound to 2.7 after the load is removed), at which point the voltage seems to drop fairly quickly. The catch is that it takes volts to turn the prop at a given rpm, and if the voltage drops, the full-throttle prop rpm drops with it, and with it the amount of load the prop puts on the motor, which then takes the watts down. Eventually that full-throttle wattage is too low to sustain flight, and the plane has to land. In our experience using a 2-cell pack this occurs long before you drop to the 2.7 volts per cell limit. It's a good idea to have a programmable ESC like the Castle Creations Pixie 7-P where you can set the throttle cutoff voltage at a level that will protect the cells, but unless you get really high in a thermal (not a good idea anyway with a 20 inch airplane!), the plane will tell you it's time to land long before you've pulled the battery voltage low enough to risk damaging the cells.

The down side of this is that you're only using part of your battery capacity, but with so much flight time in comparison to what we used to get with the NiMH cells it doesn't seem like a problem. However, in the 55 degree air last night with the 2-cell 250 mah pack, the Triplane started out with lots of reserve power and wildly aerobatic performance, but after only about 5 minutes the draggy little Triplane was at full throttle, and proceeded to spend the next 5 minutes with just enough power to fly before landing at a little over 10 minutes total. The battery still had most of its charge left, but because of the voltage drop plus the temperature effects I could only use part of that.

The solution in the case of "current hogs" like the Triplane seems to be a 3-cell pack. The extra voltage from the third cell means that even when the individual cells are getting low, their collective voltage is still enough to fly the plane with authority. I tried this in the Triplane tonight with excellent results. The temperature was slightly warmer (about 60 F), but otherwise the conditions were similar to last night. It was dusk, the wind was calm and there were no measurable thermals. This time I was able to cruise at about 40% throttle instead of 75-80% for most of the flight, only going above half throttle for the last 5 minutes or so. Total flight time was about 24 1/2 minutes, which included aerobatics throughout nearly the entire flight. The airplane became noticeably anemic in the last minute or two, but the voltage after I landed was still around 3 1/2 volts per cell. Let me tell you, nearly half an hour of aerobatics in rapidly failing daylight with the tiny little Triplane is more than enough, thank you ! The rudder is especially effective, so it will do very clean hammerheads ("bridges" for our friends on the other side of the pond) from nearly stall speed. This is not some timid little electrified dandelion seed, especially with all that power available from the 3-cell pack.

The down side of the 3-cell setup is that you have to be careful with the throttle, especially at the beginning of the flight, or you can significantly shorten the motor's brush life (guess how I know). Normally an MPS-1A on two Li-poly or 7 NiMH 110 mah cells pulls about 6 or 7 watts at full throttle, but on 3 Li-poly cells it can pull about twice that. The cells are still within their specifications when doing that, but those poor little brushes in the motor do take a beating. Don't worry, you can use an occasional burst of up to full power, but keep it to a few seconds if possible. And yes, the performance at full throttle is indeed awe-inspiring.

An even better setup might be an MPS-2A twin-motor system, but with the smaller 5" prop of the MPS-1A single motor system. By using the same prop this will not pull any more watts, so the twin motor system won't overload the little cells, but by dividing the current between two motors you can run full throttle without frying the motor brushes. The available power at the propshaft might actually be a little better, since the motors will be a little more efficient at their lower individual currents. I'm planning to try this setup very soon.

I also have logged a fair amount of time recently on the Roadkill Series Ryan ST prototype with an MPS-2A twin-motor system (with its stock 6-5 prop) and an E-tec 2-cell 700 mah pack. This is the same plane we flew at Bowling Green, and we had the same powerplant setup in the FW-190A we flew there. Both planes flew the entire night on the same charge I'd given them two days before, without any re-charging between flights. This pack weighs 0.1 ounces less than the 230 mah NiMH 7-cell pack we normally recommended for a 2-motor model, but provides a HUGE 55% increase in full-throttle power compared to the 230 NiMH pack at full throttle (5800 static rpm vs. 5000, after the initial peak burns off and the rpm stabilizes about a minute or so into the run), and provides flight times in excess of 35 minutes. I'm not sure how much in excess, I haven't had a chance to run one all the way to the end yet, but end-of-flight voltage measurements suggest another 10-15 minutes or so. In the tests at the Ft.Wayne, Indiana golf dome they started turning the lights out in the dome about 35 minutes into the flight, and at that time the 2-cell pack still had 7.2 volts. In comparison, the 230 NiMH 7-cell pack goes about 8-9 minutes at those power settings. Note, that's not 35 minutes of "economy cruise", but rather it's regular sport flying with periodic takeoffs and landings to a full stop, and loops and other maneuvers throughout the flight. There is no question in my mind that this is the best available option today for models with the MPS-2A or a pair of MPS-1A's. I'm really looking forward to trying this setup in a P-38, or the super-efficient Electra. BTW, the MPS-2A is also available as an upgrade for any of the single-motor RK models except the Junior (it doesn't need it, and there isn't enough prop clearance above the tailboom for the MPS-2A's 6-5 prop), and the MPS-2A should work especially well in the single-motor WW II warbirds.

The MPS-2A pulls about 14 watts static on a 2-cell 700 mah E-tec pack. I've tried it on three of the 700 mah cells, and got a whopping 29 watts out of it (and the current was still within what those cells can handle), but the motors heated up pretty quickly. A 3-cell setup should be awesome for aerobatics if you don't mind replacing motors fairly often, and/or if you can use a little common sense and restraint with the throttle stick. I'm planning to try a RK P-38 soon with a pair of MPS-2A's and a 3-cell pack. I'm thinking it might be wise to add some carbon fiber in strategic places on that one!

I've done static tests of a 2-cell E-tec 1200 mah pack in the Boeing B-17 with four MPS-1A's. Full-throttle power was 20% better than we get from the 6-cell 600 mah NiMH pack we normally use in this airplane, and the 1200 mah E-tec 2-cell battery is 0.9 ounces lighter than the 6-cell 600 mah NiMH pack. We've also had some flight tests with this setup recently, and it's looking just as promising so far as the static tests suggested it would be.

Our standard practice is that when we find something that makes our models perform well, we try to get set up as dealers for it so that we can offer it to our customers as a convenience. We are now carrying 2-cell packs of the E-tec 250, 700 and 1200 mah cells, and working on getting some 3-cell packs. We are also adding the Plantraco LPC-400 charger to our product line, and we're currently testing some others. I've held off making this post to the mailing lists because we kept selling out of the E-tec cells as fast as we could get them in, and I know some of you folks get very annoyed when someone advertises something and then doesn't have it available. However, we're getting more of these cells again, and besides, there's only just so long I can tolerate sitting on exciting data like this.

Joe and I are so thrilled with the results of these tests that we are now in the process of shifting our emphasis to Li-poly batteries for all of our Roadkill Series models. We will still sell the NiMH batteries as long as our present inventory holds out (and conceivably longer if the marketplace so demands), but from what we can tell, the handwriting is on the wall. For some applications, particularly for larger models and for powering radios, the nickel-based batteries can still make sense, but it's clear that for models like our Roadkill Series airplanes, the new Li-poly batteries are rapidly becoming the dominant technology.


Don Stackhouse @ DJ Aerotech
djaerotech@erinet.com
http://www.djaerotech.com


*** Any complaints or problems? Send an email to monitor@ezonemag.com
*** For help with list commands go to http://www.ezonemag.com/pages/mailhelp.htm
*** For the list rules go to http://www.ezonemag.com/pages/mailrule.htm