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JimClark
11-25-2005, 10:17 PM
I have a question in reguards to figuring out how much might be left in a Lipo pack after one run. Here are the spec's can someone help me with the math?
the lipo is a 4S1P 14.8V 20C continious battery.
45amp discharge rate
2.25 min's runtime
how can I predict what would be left in the the battery?

I am being told in testing the battery that it is dropping 1V and only using 1250mah and I think thise are bogus numbers but I am not sure how to figure it out.

Jim

timocharis
11-25-2005, 11:27 PM
The easiest way is to recharge the battery on a charger that tells you how much it put back in. After a few runs you can plot your own curve. I'm not sure how you could do it otherwise -- batteries vary in their MAHused/voltage ratio.

Let's just say I'd find another answer interesting also!


Dave North

Fred Marks
11-26-2005, 01:33 AM
See

(http://www.fmadirect.com/tech_data/techdocs/)

Section III, Page 18.

fred

JimClark
11-27-2005, 02:16 AM
Maybe I should have written this better. Here is what I am asking.

If a plane was running 12 sub C gp 3300 batteries and after one session it used up 2500mah of the sub c pack. Now if you put an approiate sized Lipo in the samp plane and run it under the same conditions wouldn't you expect to be using aproxamately the same amount of MAH in the Lipo?


Here is what is happening. I run RC hydroplanes and our club is goign to allow a Lipo 4s1P 4200mah 20C battery for next year. I come to you guy's because you know way more about lipos than others in RC. In testing the same boats that when running 12 Sub C GP3300 batteries would use about 2500 mah of the battery. The members who are testing the lipo battery claim that the same boats are using only 1250mah and dropping only 1 volt in one heat of racing. Something just seems bogus here and I cant figure it out. Granted the lipo would be lighter than the sub c's but I doubt that would account for the difference in mah used.

Jim

hoppy
11-27-2005, 04:22 AM
Interesting - You're right, something is not adding up.

Now, about the 20C lipo.... without knowing the brand, no comments are possible because they differ so much under real world testing.

Tell us the LiPo brand and what the amp draw of the boat is and we'll go from there.

JimClark
11-27-2005, 05:06 AM
I cant tell the brand all I know right now is that it is made in China in a oem factory that makes batteries for sharp and phillips. Amp draw is 45amps
Thanks

timocharis
11-27-2005, 05:40 AM
Jim,

They might be exaggerating a bit, but maybe not by much. There are three basic reasons.

A 4s battery will pump a little more voltage to the motor than the 12-cell NiMH (I assume NiMHs; even more so for NiCd of course). More volts will deliver the same power with less amps.

Next, the weight probably does make a significant difference. With less draft you'll have less drag (and that's a _lot_ less weight). Plus, of course, cheaper acceleration.

Third, the internal resistance of the NiMHs expresses itself as heat, and they're running under a higher demand (3300 vs 4200). So you're going to have a loss to resistance.

Add it all up and I believe the 1v drop -- I'm not sure the mah numbers "feel" right but I can't say they aren't.


Dave

slipstick
11-27-2005, 09:51 AM
Another interesting question to ask how they are measuring the 1250mAh for Lipos and the 2500mAh for NiMH. If they're measuring what a charger puts back into the packs then that would account for some of the difference too. LiPo charging is more efficient than NiMH charging so if you USED exactly the same amount of charge you'll need to put more back in the NiMH to get it fully charged than you do into the LiPo.

It's not something that matters to most people ;).

Steve

Matt Kirsch
11-27-2005, 02:36 PM
Unless they run their boats through a heat with a Whattmeter inline with the battery pack... That's the only way to MEASURE the mAh used, and the only way to ensure consistency from one battery type to the next.

I'm a little worried that allowing a 4S LiPoly in a 12-cell class is giving the people that can afford LiPolys an unfair advantage that will lead to trouble down the road. Between reduced weight, higher voltage, and greater capacity, they ought to be able to run circles around the "old-fashioned" NiMH boats.

hoppy
11-27-2005, 05:28 PM
I cant tell the brand all I know right now is that it is made in China in a oem factory that makes batteries for sharp and phillips. Amp draw is 45amps
Thanks

At 45A, the GP3300's have an average V of ~1.1V/cell or 13.2V
13.2V x 45A = 594W
3200mah/45A= 4.26 min NiMH run time.

Is 4.26min about the right heat time?



The 4s Lipo might have an average voltage of 3.6V/cell or 14.4V
14.4V x 45A(?-probably higher) = 648W
1250mah/45A = 1.62min Lipo run time.

Somethings wrong is right!

Did they both have about the same heat times?

JimClark
11-27-2005, 07:05 PM
Both heat times shoult be the same and that is just slightly over 2 mins so lets just say 130 seconds.

Thanks for all the info guy's

At 45A, the GP3300's have an average V of ~1.1V/cell or 13.2V
13.2V x 45A = 594W
3200mah/45A= 4.26 min NiMH run time.

Is 4.26min about the right heat time?



The 4s Lipo might have an average voltage of 3.6V/cell or 14.4V
14.4V x 45A(?-probably higher) = 648W
1250mah/45A = 1.62min Lipo run time.

Somethings wrong is right!

Did they both have about the same heat times?

hoppy
11-27-2005, 07:26 PM
3200mah in 2 min = 96A draw

Was that a 2 heat draw for the NiMH and a 1 heat draw for the Lipo? That would make sense (almost).

JimClark
11-27-2005, 07:41 PM
Everythign is one heat worth of racing. I have been told the boats pull 40 amps in the straights and can pull up to 60 amps in the turns. i may be off on the mah number ffor the NIMH GP3300's but the 3300's basically need a full charge after one heat. For refrence here some pictures I have taken of what we race at http://photobucket.com/albums/v604/JimClark/E-RCU%202005/
Jim

hoppy
11-27-2005, 08:48 PM
http://www.e-rcu.org/Newsletters/october%2005.pdf
Found your newletter and the LiPo info. $30 for a 4s4000mah pack? That is REALLY cheap. A Thunderpower 4s2p 4000mah is in the $180 range.

JimClark
11-28-2005, 01:36 AM
The news letter is wrong. They are quite a bit more expensive than the 25.00 originally quoted and they are going to be 4200mah 22C 4s1P.
Jim

http://www.e-rcu.org/Newsletters/october%2005.pdf
Found your newletter and the LiPo info. $30 for a 4s4000mah pack? That is REALLY cheap. A Thunderpower 4s2p 4000mah is in the $180 range.