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Old Aug 14, 2002, 10:27 AM
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Kokam Lithium Polymer Batteries!

I just received an email from Fred at FMA. He is back, but he is a bit under the weather at the moment. He said he is working as quickly as possible to get information posted on the web site.

He also said that he had a chance to look at the 7,500mah cells. And that they should be able to handle 35 - 40 amps!!!!!
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Old Aug 14, 2002, 10:45 AM
Former bpp dude
Joined Oct 2001
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That sounds like a great replacement for RC2400 & 3000nimh cells! I am loving the 1575's in my sporty. I fly for 20 minutes right before work & then 20 minutes right after. No battery pack change or charges in between! The 1020's are working very well in my housefly & the 2070's are great for calm flying in my Hornet. I have high expectations for the 3000's
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Old Aug 14, 2002, 11:58 AM
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damn.
That's alotta millies(amps).

Toast
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Old Aug 14, 2002, 12:10 PM
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If I remeber correctly the 7500 weighs around 4 oz. Put 2 packs of 2 in parallel and you gots a 7.2 volt 15,000mah battery than can produce nearly 90 amps. Oh, and it weighs 16oz.
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Old Aug 14, 2002, 03:33 PM
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Joined May 2001
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Asperkins,
Good idea to start this thread.

Did Fred give you any prices?

To answer another question, yes the Poweflites are Kokam's on the Skyborn site. Yes the 570's are going in my TM.

Gordon
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Old Aug 14, 2002, 03:43 PM
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No prices directly from Fred yet. Sounds like we may have them this week tho.

Keeping fingers crossed.....
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Old Aug 14, 2002, 06:06 PM
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I just got an e-mail from FMA. The cells are due end of this month/beginning of Sept. Webpage should be updated "soon".
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Old Aug 14, 2002, 06:06 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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Let us know when prices are available. I want to do a comparison. Competition is good.

Gordon
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Old Aug 14, 2002, 11:27 PM
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Hi Gang. Feeling a bit better this evening. I will start the ball rolling with a very brief update that is based on spending most of a week with Kokam in Seoul. This included a survey of manufacturing and all processes. I did negotiate a blamket order that will give us the best pricing that can be had on the Kokam cells. We have a few cells in stock. Since I haven't been to the office this week, I am not sure how many are there. However, we will have an initial stocking order in asap. The cells we got in earlier in quantities of 25 to 100 primarily for promo were: 45 mah; 145 mah; 530 mah; 1020 mah; 1575 mah; 2020 mah and 3270 mah. I know the 530s are gone. We also had ordered 880 mah, but they did not arrive. We will review and establish tier pricing for our dealers and OEMs. We are presently installing a new server for FMA Direct that will speed up accounting processes immensely. Thus, I am loathe to lean on Tim to drop any of that to get new material on the site until early next week. It seems incredible that we just put in the old server, brand new, in 1998 and here it is not able to keep up.
As soon as possible, I will post pricing for the bare cells as you have been getting them. We will also offer the cells with a pc board on them that lets you solder leads without concern. A third option will be with a special PCM that has a super BEC so you can power receiver and servos while powering the motor directly. We willl also have two and three cell packs. The pack designer at Kokam did a bang-up job on this and it looks great.
I am running out of steam ,so will add one more bit tonite and call it quits. You were so kind to send me your questions before I left for Korea and Kokam answered them specifically:
1. What is the maximum safe discharge for these cells?
A. Kokam states it as 2C in just the same way Sanyo, Panasonic, et al do for other chemistries. However, tests are showing that the cells handle up to 5 or near 6C without overheating. We are testing now to see if or how much higher current drain reduces service life. I know some R/C car users accept 50 or so runs on a pack, but don't feel we airplane drivers will. I feel that life will not be reduced much since the cells do not heat much even at high current. As a bonus answer: the Kokam cels are very much safer than competing cells. The plastic envelope does not explode no matter what you do to the cells. Kokam is sending me sample cells that I saw in their QC facility (Blockhouse) where samples from each block are abused to death. A nail is diven through some; others are subjected to deliberate overcharge; and still others are shorted. The worst that happens is that the envelope swells up like a sausage. But nothing bad happens. I know of one such test on Li Ion that blew a metal garbage can into shreds!
One fact is now known that was reflected in a question from Troy Goff. The Kokam cells perform so well because the envelope is under a very heavy vacuum that keeps the plates all close in contact with the electrolyte and the separater. If the envelope is punctured, the vacuum is lost and contact is less. This reduces capacity.
2. Is there a difference between steady-state discharge and pulsed discharge.
A. Not meaured; but it appears from PWM operation with ESCs that capacity is, if anything, enhanced by pulsed discharge.
3. Also, what is your definition of pulsed?
A. PWM speed control is a perfect example of pulsed.
4. What is your definition of steady state?
A. The test facility at Kokam is steady state. That is, the load is constant, not pulsed. A motor being driven directly with no ESC is relatively a continuous load.
5. What is the fastest, maybe not the best, way to charge the cells?
A. The cells should be charged at 1C, nothing more, nothing less. Lower than 1C does not hurt. Higher than 1C may lead to less than full capacity. We haven't determined exactly how much capacity is lost by faster charge. It may be only a few percent. Future results will be reported.
6. What is the self discharge rate?
A. One of the beauties of Li Poly is that the discharge rate is < 5% per month at room temperature. This is much better than, e.g. Ni Cds. At more elevated storage temp, discharge rate is slightly higher.
7. What is the impact of storage temp on self dischrge.
A. See 6, above. I will provide a curve as soon as I can get to it. For now, Kokam takes 80 % of full capacity as the end-point of self discharge. At 20 degrees C, the cell loses 5 % in six months; 10 % in 3 months at 45 degrees C; and 15 % in one month at 60 degrees C.
8. What happens if you short a cell without protection?
A. If you remove the short timely, nothing. If left in a shorted condition, the cell may vent or at least swell. That ain't good, but it is better than exploding. You may also burn the terminals in two.
9. If you have a pack that is 2-3 cells in parallel, what is the best way to charge it?
A. Same as for any chemistry: the cells must not be charged in parallell. Any charging arrangement must separate the terminals so that cells are charged in series, never parallel.
10. What, if any, memory do Li Poly cells develop?
A. None
11. What is the power density of Li Poly compared to Ni CD? Ni Mh?
A. Li Poly has about three times the power density of NiCds and about twice that of Ni Mh. Li Poly is very much superior to Ni Mh in that they do not heat up or drop voltage at the same current drain. A direct experience with the Flex Flyer had twelve 2250 mah NI Mh cells depleted after < 10 minutes. The Ni Mh barely flew the airplane and were too hot to touch after ten minutes.

I leave you with one fun experience: During a visit to our Japanese distributor in Kobe City, we were on an empty street near the docks next to a huge Mitsubishi warehouse at 4:30 AM to fly the first turboshaft powered model helicopter in Japan ( maybe anywhere)... Two , two-liter coke bottles for fuel tanks. Highly successful, I left my friend Hiro walking aroung a foot off the ground that morning!

Sleep well!
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Old Aug 14, 2002, 11:50 PM
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Louisville
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145 Posts
Pardon my ignorance fellas, but I'm stoked about these batteries and have only one question. (At the risk of sounding foolish, here goes....)

As long as I don't exceed 1C charge, could a NiMh/NiCd charger with peak detection be used on these packs?

Please, go easy on me if that's a real stupid question.

Thanks, Michael
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Old Aug 15, 2002, 12:17 AM
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ooh.... september... seems to far away...
great information! helps a bunch.
Toast
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Old Aug 15, 2002, 12:46 AM
Former bpp dude
Joined Oct 2001
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4throttles, you cant use a regular charger for the Li-ions cells. The problem with Li-ions is that they never really peak. Single cells should never go beyond 4.2-4.3v. SO for a 3 cell pack the voltage should never go beyond 12.6-12.9 volts. If you ever look at how a Li-ion Charger works, they all start out at around 1C then taper off until its as low as 1/50C at the end of a charge. This is why it takes 90min-2hours to charge most Li-ions. 70% of the charge is done within the fist 30-45 minutes, then the rest is slowly added in. Before I had Li-ion chargers, I used to use a regular charger in 10 minute increments, adjusting the current so that the voltage never crossed 12.6v. Big pain in the ass and if you forget to check the charger you could burn down your house (with old Li-ion tech). I would expect that FMA will be offering a low-cost Li-ion charger for these cells. I use the Orbit charger & love it. Its worth the 200 dollar investment as it is worth every penny.
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Old Aug 15, 2002, 06:39 AM
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Wow sounds brilliant! I have 3 Kokam 145s at the moment, and they seem brilliant so far.

What cells would be good for 20A? The 3270s? If so, how much do they weigh? If they're around 2oz each then they would be great for speed 400 setups.
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Old Aug 15, 2002, 11:18 PM
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Gaithersburg, MD, USA
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Trizza: The 3270 mah probably will handle that but at a stretch. I haven't had a chance to see what happens when you paralllel two 3S packs, but hope it will let it handle over 30 amps. Certainly the 7.5 AH coming will. FMA is considering an in-line module that takes the programmed output from Supernova or other programmable charger and conditions it to handle Li Poly cells. In other words, you program the charger for the next highest # of Ni Cd/Ni Mh and the 1C charge rate and the module then does current regulation and voltage cut-off at full charge for the Li Poly. You get to keep your investment in our outstanding Supernova charger. Any interest?
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Old Aug 15, 2002, 11:31 PM
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Rochester,MN
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Quote:
Originally posted by electroman
Trizza: The 3270 mah probably will handle that but at a stretch. I haven't had a chance to see what happens when you paralllel two 3S packs, but hope it will let it handle over 30 amps. Certainly the 7.5 AH coming will. FMA is considering an in-line module that takes the programmed output from Supernova or other programmable charger and conditions it to handle Li Poly cells. In other words, you program the charger for the next highest # of Ni Cd/Ni Mh and the 1C charge rate and the module then does current regulation and voltage cut-off at full charge for the Li Poly. You get to keep your investment in our outstanding Supernova charger. Any interest?
Yes.
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