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        135mAh Lithium Polymer Cell Tests

#1 Gordon Johnson May 18, 2002 02:10 PM

135mAh Lithium Polymer Cell Tests
 
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I just got my 135mAh LiPoly cells from Skyborne Electronics http://www.bktsi.com/ in Texas. The cells are $20 each. I got a high-end charger from them for these and NiMH and NiCd cells for $140. It has a heat sensor to monitor cell temperature. It will also work from either 110 AC or 12 DC sources.

First, a few specifics. The bare cell weighs 3.7g. After soldering a JST plug and wire on it and putting some lightweight clear shrink tubing around the whole thing to protect the fragile tabs, it weighs 4.1g. For comparison, a 120x2 NiMH with JST plug and wires and shrink weighs 6.9g, and a 120x3 NiMH weighs 11.8g complete. So, this saves a lot of weight. Here's a picture of an Excel table from the first static test. I'll post more complete results here and on my site early next week.

Basically, the LiPoly cell provides more thrust than the 120x3 NiMH everwhere except the first minute. It also has much Longer run time. And it does this at a weight savings of 7.7 grams.

5/20/02
I've updated the graph with one that's easier to read (not fuzzy) and that has the M20-LV motor test.

#2 Gordon Johnson May 18, 2002 02:24 PM

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Here's a picture (sorry it's fuzzy) of the cell before and after soldering and shrink wrapping. I don't know how durable the
+/- tabs are. Also, you don't want to risk them touching each other. So, I soldered leads on, and put heat shrink on the positive lead. Then, I folded them back on the top of the cell with the wires going out the other end and encased it in AirDyn's shrink tubing. It seems a lot more durable this way.

These cells are going to going to find their way into a lot of my planes.

#3 Dave Wulff May 18, 2002 03:53 PM

Gordon,

I visited this web site, but could not find this particular cell...am I looking in the wrong place? Also. is it possible to charge these cells with a less expensive charger, $140 is a bit steep, but I could live without some of the bells and whistles if there is a more manual charger that would safely do the job.

Dave W.

#4 fwilly May 18, 2002 04:34 PM

sweet!!! now we don't need a voltage booster. I also could not find it on their site but the 570mah looks promising for gws motors. whats the voltage per cell?

#5 KeithK May 18, 2002 08:24 PM

Gordon,

Once again, excellent work! And who says procrastination doesn't pay off? I can sit here reading Ezone and all my questions are getting answered:cool: What is the cut-off voltage on these cells? I hope the temperature cut-off is not for lithiums, I think $20 would be wasted long before they start to heat up. Didn't you post the original thread on the RFFS-100 conversion of the "micro-taxi?" If so, can't wait to hear how this cell works in it:p

#6 Gordon Johnson May 18, 2002 08:52 PM

Bob Selman is working on a charger. It will certainly be less than the one I got. But, I wanted to start using these cells right away. Dan Krieg is selling some larger two-cell packs of LiPolys and has a less expensive charger for them. It wasn't available when I ordered these cells. And, I don't know if it will work. I waited six weeks for these cells. They were delayed for two weeks in Customs in LA. These are not on Skyborne's site yet. When I ordered them they had not even seen them. After I ordered them I found out that Matt Keennon had used one of these. That's when I thought they would be a winner. Call or Email Kevin at Skyborne if you want to get any. He just got these first ones last Wednesday.

The heat shutoff is for charging. So far mine haven't gotten hot. The charger "surface" charges the cells for three minutes, then charges regular. I think it is one of the types that maintains the cell at a particular voltage while it charges, but I'm not sure about that. Skyborne said these would take about an hour to charge. I'm getting the voltage up to 4.15v at about 66 minutes. I'll try charging them longer, but what Skyborne sent said the incremental volts after the first hour will be small.

The nominal voltage is 3.7v. But these start operating at about 4.15v after I charge them.

#7 Gordon Johnson May 18, 2002 08:58 PM

I just found out that Skyborne has only 8 of these 135's left. I just took 4 more, so there are 4 left. They will be getting more in about 2 weeks or so (I don't know how many). BTW, these cells are Kokam brand from Korea.

#8 fill May 18, 2002 10:34 PM

Gordon,

Am I seeing a true living room flyer? How about this cell with the didel pager drive, and something like a penny plane? I like it, thanks for doing all the dirty work for us. Now the question is how long before these little charms hit the surplus stores???

Philip

#9 Gordon Johnson May 19, 2002 07:58 AM

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Here are some discharge curves that Skyborne did and included with my cells.

Top to bottom the curves are: black=0.2C, red=0.5C, blue=1.0C, green=2.0C. The LH scale is in incerments of 0.5v, and ends at 4.5v. The bottom scale is discharge capacity measured in mAh in increments of 20mAh, ending at 160mAh. Sorry, it's color and my cheapo scanner doesn't pick up everything.

Phil, I wouldn't hold your breath waiting for these cells from surplus sources. They are very very new.

#10 Dave Wulff May 19, 2002 08:26 AM

Gordon,

Thanks for turning this up. They seem like the perfect cell for the RFFS. 3.5 - 4 V @ 1 amp for 4 grams. That cuts the 3x50 mah nicads weight by 66%. At $20 they should sell a bunch, if they can get them.

Dave W.

#11 Graham Stabler May 19, 2002 09:17 AM

Well done Gordon, you've done it again!

I don't want to get any of you too excited but I think Matt Keenon managed a loop with one of his models using a KPOO and a home made gear box. Best thing was it wasn't intentional it just climbed so hard he had to pull back to miss the ceiling:D

Kokam have a web site and I guess it won't be long before others start stocking these rather cool cells. And never mind them being lighter than 3 cells there lighter than two.


Graham

#12 jberg May 19, 2002 09:35 AM

Quote:

Originally posted by dow
Gordon,

Thanks for turning this up. They seem like the perfect cell for the RFFS. 3.5 - 4 V @ 1 amp for 4 grams.
Dave W.

Dave,

2C are only 270mAh for 135mAh cells, unfortunately not 1A! 2C is about the usable limit for all Li-Polymer cells I know of.

Gordon,

which current did your motors pull during your tests?

#13 Graham Stabler May 19, 2002 09:47 AM

I'm pretty sure these cells can do better than 2C. I'm sure Matts biplane was pulling more than 270mA. Doubt it was 1 amp either mind.

Graham

#14 Gordon Johnson May 19, 2002 09:58 AM

Jochen,
I'm still doing tests, which I should have complete details on my site early next week. But, from the M20-LV 6:1 and 5x2.7 prop it pulls about 0.77amp near the beginning of a motor run. So far I'm finding very little more thrust for the HV version of the M20 on LiPoly compared to NiMH, probably because it's more efficient and doesn't pull as many amps. For the LV E-Charger version, I'm seeing between 4% and 20% more thrust (4% occuring at 1/2 minute and 20% occuring at 7 minutes) with the LiPoly compared to NiMH, presumably because these cells can deliver somewhat more amps. This may be the real win combination -- more power and less weight.

I'm also discovering that when letting the charger take the time to get a full charge, that the volts right before putting a load on are peaking at about 4.30v. A motor run from this very "full" charge puts out slightly more thrust. However, the quicker not quite full charge of about 66 minutes charge obtains about 4.15v and almost as much thrust. So, for flying the one hour charge is probably sufficient to be able to get back out there and fly.

So far the charging time is the only downside I can see to these cells.

Graham, I found out Matt had used one of these cells after I had mine on order and was dreaming of loops. When I heard he had used these cells and done a loop, I contacted Matt and verified they were the same cells. Then I began thinking thinking the loop wasn't a dream after all, but a definite possibility with the right plane.

#15 Dave Wulff May 19, 2002 12:44 PM

Jochen,

Thanks for putting me straight, however if Gordon is right about these cells putting out 770 mah, I think that will do nicely.

Dave W.


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