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Old Nov 09, 2003, 12:20 AM
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gbruce's Avatar
Canada, AB, Airdrie
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EDF Design changes for LIPO batteries

Sooner or later I WILL be going lipo tech...

...And due to the lighter weight of these packs I would imagine an EDF will need to be redesigned to allow the motor to be closer the the C of G. (ie. moved forward)What effect will this have on our thrust tubes and efficiency?? Anyone experimented with this stuff before?

Any other design issues that we need to bring up for Lipo usage??

Bruce
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Old Nov 09, 2003, 01:26 AM
smug in granny panties
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Never thought about it that way, I mean as batteries get better and lighter it's gonna have to have an effect isn't it? Higher pitches maybe, or more blades? Or planes we wouldnt have thought of modeling before that lend themselves to shorter ducting or more compliant CG.

Barry
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Old Nov 09, 2003, 07:33 AM
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Really the size of the pack will most likely be more of an issue than the weight. Mostly people are pushing the nimh/nicd hard enough that you can't make huge drops in the pack weight without really stressing the cells or spending lots of money - (using a million 340HD cells).

For example my EJF F-18 dropped from 42 oz to 38 oz going to TP packs, however if I had been using CP1300 instead of CP1700 I wouldn't have seen any change.
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Old Nov 09, 2003, 11:32 AM
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I don't think exhaust tube length changes have as much impact as inlet ducting design, at least that's my gut feeling based on reading info here and in other places over the years.

Weight change is a consideration though. Hopefully you can shift components around or the pack itself to make the changes, but in some instances moving the fan forward might be the best option. That might make design changes necessary so shifting the pack is the best option.
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Old Nov 09, 2003, 05:07 PM
EDF rules... :)
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Joined Nov 1999
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I agree with Ed, once youhave gotten the air thru the fan there is little that can be done to help it except to not be too long.

Cheers,

Eric B
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Old Nov 09, 2003, 05:18 PM
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To get 50A out of LiPo your battery is ending up at about the same as a NiCd/NiHm pack, for instance 15xCP1700's = 4s4p TP7800 - same weight but 5x capacity.

You need the large capacity in order to extract the current you need. Atleast in some appsbut not all
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Old Nov 09, 2003, 09:33 PM
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Thomas Nelson's Avatar
Calgary, Alberta, Canada
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Quote:
Originally posted by Haldor
To get 50A out of LiPo your battery is ending up at about the same as a NiCd/NiHm pack, for instance 15xCP1700's = 4s4p TP7800 - same weight but 5x capacity.

You need the large capacity in order to extract the current you need. Atleast in some appsbut not all

... Here's a different approach to 750W (est 15v X 50A)

Why not use 6S1P 8000mah LiPoly cells? At 5C (40A) and probably 21v under load you'll have 840W. Pack weight is 8.25 ounces. Pack price is around 120 USD. And you'll have 4x the capacity of the 1700s.

Of course, you'll need to use a lower kv motor. For a flying example, check out this EDF Kyosho F-16 on a 5-turn! Mega by sguty:

http://www.rcgroups.com/forums/showt...highlight=6s1p

Naturally, this doesn't help CG issues!

References from http://thunderpower-batteries.com

TP8000-2S4P "Dynamic Power" LiPo Electric Flight Pack
8000 mAh, 2S4P cells (7.4V), 13 gauge wires
Rating: 5C Max Avg. Discharge
Output:7.4V Nominal, 8200mAh
Applications: Sub C replacement for 3D aircraft and helicopters
Dimension: 50mm x 125mm x 28mm
(313gr)
$159.95 NOW SHIPPING
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Old Nov 09, 2003, 09:47 PM
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Haldor's Avatar
Stavanger, Norway
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Quote:
Originally posted by Thomas Nelson
... Here's a different approach to 750W (est 15v X 50A)

Why not use 6S1P 8000mah LiPoly cells? At 5C (40A) and probably 21v under load you'll have 840W. Pack weight is 8.25 ounces. Pack price is around 120 USD. And you'll have 4x the capacity of the 1700s.

Higher voltage/less current is a very viable option. But your data is incorrect. the 6s1p TP pack you refer to do not hold 8000mAh capacity, thats only true for a 4p pack. 1p = 2000mAh pack, 17% more capacity than the CP1700. The other downside is that as the 1p pack at 5C yield only 10A (as its in fact a 2Ah cell)

To get 50A out of a LiPo pack today and assume 6C as a safe discharge ratio you need 8Ah array of cells. Then there is only a marginal weight gain, but a substantial capacity gain in favour of the LiPo option.

I hope my math is correct, its way later here and I'm overdue for a nap....
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Old Nov 09, 2003, 10:21 PM
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Quote:
Originally posted by Haldor
I hope my math is correct, its way later here and I'm overdue for a nap....
Looks about right to me.

Lipos give the builder/designer the option of either trading lighter weight for about the same duration, or (and my favorite) about the same weight for longer duration.

I tested my new TP 8A (not to be confused with the earlier 8.2A from TP) on my MiG and took one 7 minute flight which drew an average of about 33A for the duration of the flight for a total of just under 3500 mAh out of the pack. I normally try to keep the flights to about 6 minutes each. This gives me a 2-3 minute reserve of power at the end of the second flight.

I could go with lighter batteries, but I like having the 540W of WEP that I get at full throttle.

I did get one comment today from one of the guys who'd never seen a lipo-powered jet and joked about thinking it was nuclear-powered due to the duration!

Daren
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Last edited by Daren; Nov 09, 2003 at 10:24 PM.
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Old Nov 09, 2003, 10:47 PM
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Redmond, WA, USA
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Hi all!
To clarify, my setup that Thomas Nelson referred to is a 6s TP2100 pack, so the 8000maH figure is, unfortunately, not available as a 1p, <2 oz/cell configuration with current technology ()
However, the amp draw capabiltity from the TP2100s is well in excess of 5C--I've been pulling 14A (static) (without dropping the voltage below 19.6V) initially, with wattage stabilizing at 230-240W for the Mega 16/15/5 and Minifan 480 drive system. I put in a 5 minute flight this afternoon and only 590 MaH back into the pack.
I'm expecting even better performance from a 7s pack, but need to await the arrival of an ESC that can handle the voltage--the CC Phoenix 35 works fine with 6s but balks at 7s.

Steve G.
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Last edited by sguty; Nov 09, 2003 at 10:52 PM.
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Old Nov 10, 2003, 12:29 AM
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Aha ... Epiphany!

I DID notice that 'doing the math' yielded surprisingly light per cell weights. Should have thought a little more about that ... obviously too light. So a 8000 mah "cell" is made up of 4P 2000mah cells then. (I wonder why they wouldn't just parallel 2100s?)

And the lower "C" values for the higher capacity packs (5C versus 6C) are due to the close proximity of all those cells to each other and thermal issues - is that it?

Thanks for the gentle correction, guys. Hmm ... now what do I do about that errant post?? (If it seems too good to be true ... )
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Old Nov 10, 2003, 12:54 AM
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Redmond, WA, USA
Joined Oct 2000
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Hi Thomas--
As best as I can figure, TP went through three very rapid technology advances in well less than a year--1950, 2050, and 2100 cells with different dimensions and current capabilities--there's a sort of "read between the lines" message if you look at the offerings from their distributor, Esprit Models (http://www.espritmodel.com)--e.g., the 7800 and 8200 maH packs. Some of these are clearly channel inventory that likely will be price-adjusted to move.
The 2100s are their cutting edge, but nothing is standing still here--if Kokam can scale up their 20C 340s, the frontier has moved forward a lot!
I've not seen a high correlation between cell proximity and heat buildup, but haven't really pushed cells _too_ far past limits.
I did talk Zb into creating a 3s2p 4200 pack for my Firecat which can handle 36A bursts and awesome vertical, but that probably belongs in 3D.

Steve G.
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Last edited by sguty; Nov 10, 2003 at 12:57 AM.
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Old Nov 10, 2003, 12:57 AM
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6S1P 2100 - Should weight about 9 oz - 240 watts/9 oz = 27 watts/oz

4S4P 8000 - Estimated weight about 23 oz - 750 watts/23 oz = 33 watts/oz - this more of a burst wattage , running closer to the continous rating = 27 watts/oz

3S3P 6000 - weight 12 oz - 320 watts/12oz - 27 watts/oz

Although some people seem to think that putting more cells in series is some type of magic, the cells really don't care if they are parallel or series - the watts/oz that you can get out of the pack will be about same. Running at higher amps will cause more losses in the wire and ESC, but I don't think the loss are enough justify changing a power system to run on higher voltage unless you are starting new on the system. Even then staying at 3S or less so you can using a built-in BEC maybe a better option sometimes.
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Old Nov 10, 2003, 01:14 AM
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Redmond, WA, USA
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Hi Scott--
Understand where you're coming from, and agree that there is no magic in cells for the sake of cells, but there is somewhat of an art to matching current capability to motors--and when we're talking $300 battery packs, matching them to $80 motors is getting the cart and horse mis-aligned, IMHO.
Worst case would be going 20% over the continuous limit of a 1p configuration and having to (buy &) carry around a 2p pack--double the money, double the weight, why not consider a lower Kv motor to stay 1p, or a higher one to take full advantage of the max current capabilty? Point is, LiPos re-define the criteria, no?
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Old Nov 10, 2003, 06:08 AM
The blade numbers go up to 11
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Sydney, Australia
Joined Aug 2002
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Well, given that the power lost in the esc, wires, connectors etc is proportional to the square of the current, and that people are already cooking stuff due to longer flight times with Lipoly, I think we'll find that high volts/low amps is the way to go, even if it means adding half an ounce worth of extra caps to the esc to suppress the voltage spikes. So far I've found that you can push them quite a bit harder than recommended for a few seconds without damaging them - the harder you push, the shorter the time. Don't forget, in a few seconds a jet can go a long way. The new Irate and Tanic cells seem to be alot wider but thinner than others, and seem to be outperforming the rest so I guess this means for us we will have to midify our batt compartment to suit. Also, given the fragility of the cells, some sort of crash protection is in order so that in a mishap the cells won't be damaged and become dangerous. Pretty soon I'll be trying a 9S1P pack in a minifan with a B4019L in my F104, which I guess is the opposite end of the spectrum to what we're used to, but should give me heaps of power at less than 20A.

Stu Maxwell
StumaxAircraft
stumax@hotmail.com
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