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Old Jul 04, 2003, 03:02 AM
Don Kimball
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[EFLT] Lithium Polymer Cell Discharge Graphs

This message from Don Kimball <dkimball@pacbell.net> brought to you by EFLIGHT!

I have am trying to find voltage versus time graphs for E-Tech, Kokam, and
Thunder Power lithium polymer cells. I have found one high rate discharge
graph (6A, 8A, 10A) at Bishop Power Products
http://www.b-p-p.com/i/etec1200tests.jpg for E-Tech 1200mAh cells. I am
seeing a lot of bold claims about the power output of lithium polymer
cells, but there seems to be a suspicious lack of high rate discharge
graphs. Note, the E-Tech 1200mAh graph shows the voltage of a 2 cell series
battery is equivalent to a 6 to 7 cell NiCd or NiMH battery under high
rate discharge. Thunder Power claims that their 2 cell series pack is
equivalent to a 7 to 8 cell NiCd pack under high rate discharge, but there
are not graphs to back up their claim.

E-Tech, Kokam, and Thunder Power all claim 5C continuous discharge, and 8C
10second peak discharge for many of their cells. Can someone please point
me to the voltage versus time graphs to support these bold claims?

I am trying to determine if the power to weight ratio of lithium polymer
cells are higher than NiMH or NiCd cells. It appears that power to weight
ratio of the improved KAN950 NiMH cell is still higher than the lithium
polymer cells.

Don Kimball


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Old Jul 04, 2003, 03:02 AM
Samuel M. Kennard
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RE: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from "Samuel M. Kennard" <sam@kennardind.com> brought to you by EFLIGHT!

<< E-Tech, Kokam, and Thunder Power all claim 5C continuous discharge, and
8C 10second peak discharge for many of their cells. Can someone please
point me to the voltage versus time graphs to support these bold claims? >>

I found this.

http://www.allerc.com/lipoly.htm

Sam Kennard



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Old Jul 04, 2003, 03:02 AM
Joe Minton
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RE: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from "Joe Minton" <jminton@adelphia.net> brought to you by EFLIGHT!

Don:
I (and others) have found that the peak Ampere output claims of most of the
LiPoly suppliers are exaggerated. I (and others) find that the original
Kokams are good for a constant 2.4C and sag noticeably after less than one
minute at 3C. They'll deliver 5 or 6C for 15 seconds or so before their
output falls off. The E-Tec cells are fine at a constant 5C and much more
for short periods.
Mind you these "C" ratings are for static conditions. The 'real' current
drain is less in flight. The numbers given above reflect the load
measurements taken statically while the performance was judged in flight
where the actual loads are less.
Because the LiPolys are so power dense (light), I recommend that you use
plenty of cells instead of asking them to produce the kind of
Power-To-Weight ratio of the best NiCads - which they simply cannot do.
The main practical advantage, at this time, of LiPolys is time-in-the-air.
Weight savings is the second and shall become more important as the maximum
constant "C" rating (real) goes up.
I may get some heat for saying this. So be it. I stand by what I've said
and invite those with differing opinions to make the same tests I have ----
try these batteries in the air and see how they do.

BTW: I shall never buy another NiCad or NiMH cell. I firmly believe that
LiPolys, together with brushless motors, are the most significant advance in
powered modelling since the advent of the glow plug ----- and I am old
enough to judge this matter.

Best Regards and Best of Luck --- Joe Minton



I have am trying to find voltage versus time graphs ---- .
Don Kimball


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Old Jul 04, 2003, 03:02 AM
Rick Page
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RE: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from Rick Page <rick-page@shaw.ca> brought to you by EFLIGHT!

There are plenty of graphs in RCGroups but there is no easy way to find
them.
http://www.rcgroups.com/forums/forum...s=&forumid=129
https://www.fmadirect.com/site/Detai...726&section=28 For Kokam
http://www.aircraft-world.com/prod_d...ets/lipoly.htm ETec
http://www.flightenergy.com/ Thunder Power still not providing data

The cells are changing every few months anyway, so we can't keep up.

Rick.


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Old Jul 04, 2003, 03:02 AM
Rick Page
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RE: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from Rick Page <rick-page@shaw.ca> brought to you by EFLIGHT!

>http://www.flightenergy.com/ Thunder Power still not providing data

but http://www.rcgroups.com/forums/showt...threadid=92664

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Old Jul 04, 2003, 03:02 AM
LVRCFlyer@aol.com
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Re: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from LVRCFlyer@aol.com brought to you by EFLIGHT!

Discharge curves will be published for the Thunder Power cells/packs in most of the major RC publications worldwide over the next few months. Packs and cells were just released a few weeks back for this testing. Ive seen the in house curves direct from the manufacturer, but he has yet to publiush them until others have a chance to test for themselves.

The new Thunder Power 2100 mah packs were designed for for factor and performance to replace Kan/HeCell/Sanyo 2/3 sub A cells. A few weeks back at SEFF in GA, I had a chance to fly a number of models back to back using the KAN 950s (10 cell pack) and the 3s Thunder Power 2100s.

Using the same motor/prop and a whattmeter, the TP packs delivered a tad more voltage under a 20 amp load than the 10 cell KANs - this also meant a few more watts at a few less amps. In flight, the 3s LiPo packs only weighed 4.5 ounces vs the 10 cell KANs at 7.8 ounces so there was a noticeable increase in performance on the 20 ounce airplane.

This was the case in a number of airplanes this size I flew that weekend. Less weight, the same or even a tad more power with twice the duration - it excited everyone who flew em! Even the manufacturer was suprised by the results as he had conservatively rated the 2100s for 10 amps continuous and 16 amps burst. This has now changed to 12.6 amps continuous and 22 amps short term burst! Im flying these same packs in a Mini Laser 3D with a Hacker B20-15L motor. Many guys saw me fly this airplane at SEFF. Full throttle current draw with a 9x6 prop on the 3s packs was 23 amps - needless to say with 250 watts the vertical on this airplane was unlimited! With throttle managment, flights averaged 20 minutes...so no, I did not abuse them at 23 amps for a large part of the flight - only in vertical. Still, the performance boost over 10 cell HeCells was amazing!

Performance curves for these new cells should be published sooner as they have received so much praise and attention since SEFF. Watch the Thunder Power thread on the EZone, and keep an eye out in the magazines coming up!

Jason

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Old Jul 05, 2003, 03:01 AM
Don Kimball
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Re: [EFLT] Lithium Polymer Cell Discharge Graphs

This message from Don Kimball <dkimball@pacbell.net> brought to you by EFLIGHT!

At 11:56 PM 7/3/2003 -0400, you wrote:
>This message from LVRCFlyer@aol.com brought to you by EFLIGHT!
>
>Discharge curves will be published for the Thunder Power cells/packs in
>most of the major RC publications worldwide over the next few months.


Good news. With this data I can optimize my aircraft set-up around the
battery. I believe that the battery is the weakest link in electric flight,
so I like to set up my aircraft around the optimum use of the battery.

>The new Thunder Power 2100 mah packs were designed for for factor and
>performance to replace Kan/HeCell/Sanyo 2/3 sub A cells. A few weeks back
>at SEFF in GA, I had a chance to fly a number of models back to back using
>the KAN 950s (10 cell pack) and the 3s Thunder Power 2100s.
>
>Using the same motor/prop and a whattmeter, the TP packs delivered a tad
>more voltage under a 20 amp load than the 10 cell KANs - this also meant a
>few more watts at a few less amps. In flight, the 3s LiPo packs only
>weighed 4.5 ounces vs the 10 cell KANs at 7.8 ounces so there was a
>noticeable increase in performance on the 20 ounce airplane.


Reducing weight while keeping the power the same has an excellent effect on
the flight envelope. The stall speed decreases, the rate of climb
increases, and the angle of attack decreases. In addition, reducing the
weight usually increases the roll rate, pitch rate, and yaw rate.
Consequently, you can go through the same acrobatic routine with
substantially reduced power. I am not sure, but I believe that a 15% weight
reduction results in performing the same acrobatic routine with 28% less
power (weight^2). Less power results in less current from the battery, that
in turn results in a higher battery voltage, so that the current can be
further reduced. In this case, we have a desirable self-reinforcing
situation, so the flight times under acrobatics increase substantially. In
fact, with the weight reduction, the increase in flight time will be
substantially more than predicted by just the mAh rating increase alone.


>This was the case in a number of airplanes this size I flew that
>weekend. Less weight, the same or even a tad more power with twice the
>duration - it excited everyone who flew em! Even the manufacturer was
>suprised by the results as he had conservatively rated the 2100s for 10
>amps continuous and 16 amps burst. This has now changed to 12.6 amps
>continuous and 22 amps short term burst! Im flying these same packs in a
>Mini Laser 3D with a Hacker B20-15L motor. Many guys saw me fly this
>airplane at SEFF. Full throttle current draw with a 9x6 prop on the 3s
>packs was 23 amps - needless to say with 250 watts the vertical on this
>airplane was unlimited! With throttle managment, flights averaged 20
>minutes...so no, I did not abuse them at 23 amps for a large part of the
>flight - only in vertical. Still, the performance boost over 10 cell
>HeCells was amazing!

The 20 minute flight time for acrobatics is frequently reported in Li-Poly
discussions. This is about a 3C average discharge. What is the cycle life
at 3C average discharge? Most acrobatic routines with NiCd and NiMH
batteries consume about 8C to 12C average current, and result in battery
life of 100 to 200 cycles.

Note, I work at University of California San Diego (UCSD), and I am doing
Li-Poly battery testing with a student for wireless sensor networks.
Another colleague is doing Li-Poly battery testing for a small electric
UAV. Most of my testing is with high rate pulsed discharge with a goal of
optimizing charge recovery effects.

Thank you for your information.

Don Kimball

> Jason
>
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