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Old 03-22-2009, 12:50 PM   #1
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Default Lipo capacity .

Hi ,

I want to buy my first lipo , people at my club use it, its 25C but only 3600mah .
Is a 3600 mah lipo = to a 3600 mah NIMH ?
I will be running it with a 6.5BL motor .
How long can i run my 3600 pack on a flowing track before it slozs down ?

Thanks in advance.
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Old 03-22-2009, 03:29 PM   #2
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no a 3600 lipo is > 3600 nimh. while it may have the same mah rating, lipos have a MUCH MUCH lower weight, thus requirng less power to get up to speed = more runtime. Also, whereas 6 cell nimh was 7.2 volt (1.2 per cell), lipos have a slightly higer nominal voltage of 7.4 volts, because the celles only come in 3.6 volts, so that is as close as u can get to 7.2 volts, 2 cells.
dont know about mod, but I can run my 17.5 for like 8 minutes on a 3800, so u should be fine. In regards to slowing down, lopos have a very different discharge curve compared to nimh. as u no a nimh can slow down gradually over time, but a lipo holds its voltage for a long time until it gets to the end of its capacity, at which point the voltage drops off super sharply (telling you it is time to stop and charge, lipos dont like being over discharged), thus giving you nearly 100% power until the very end. A 3600 25c should be fine for your first lipo
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Old 03-23-2009, 01:00 AM   #3
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Originally Posted by Little_Mike View Post
no a 3600 lipo is > 3600 nimh. while it may have the same mah rating, lipos have a MUCH MUCH lower weight, thus requirng less power to get up to speed = more runtime. Also, whereas 6 cell nimh was 7.2 volt (1.2 per cell), lipos have a slightly higer nominal voltage of 7.4 volts, because the celles only come in 3.6 volts, so that is as close as u can get to 7.2 volts, 2 cells.
dont know about mod, but I can run my 17.5 for like 8 minutes on a 3800, so u should be fine. In regards to slowing down, lopos have a very different discharge curve compared to nimh. as u no a nimh can slow down gradually over time, but a lipo holds its voltage for a long time until it gets to the end of its capacity, at which point the voltage drops off super sharply (telling you it is time to stop and charge, lipos dont like being over discharged), thus giving you nearly 100% power until the very end. A 3600 25c should be fine for your first lipo
Wow , great help Mike it clears alot of things , but as a 6.5 is alot more powerfull then a 17.5 ( at leats twice , ) we could divide 8 mins in 2 and it makes 4 min .
I dont know if i should get this 3600 lipo wich is 39 or a 5000 mah 35C lipo for 75 ?
I will just need the pack for practise , i wont use it often .
Wich pack would you recomend ?
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Old 03-23-2009, 08:47 AM   #4
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Originally Posted by cyclone speed View Post
Hi ,

I want to buy my first lipo , people at my club use it, its 25C but only 3600mah .
Is a 3600 mah lipo = to a 3600 mah NIMH ?
I will be running it with a 6.5BL motor .
How long can i run my 3600 pack on a flowing track before it slozs down ?

Thanks in advance.
The capacity is actually the same. But a LiPo battery delivers the power more efficiently so you will see more actual run time from the LiPo. I would expect 20%-30% more run time from a LiPo of the same capacity as a NiMH.

The weight of a LiPo of the same capacity is also much less. But this does not figure into the run time significantly.
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Old 03-23-2009, 10:28 AM   #5
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Originally Posted by Rick Hohwart View Post
The capacity is actually the same. But a LiPo battery delivers the power more efficiently so you will see more actual run time from the LiPo. I would expect 20%-30% more run time from a LiPo of the same capacity as a NiMH.

The weight of a LiPo of the same capacity is also much less. But this does not figure into the run time significantly.
HA - finally someone knowledgeable who has a clear opinion on this Maybe we can stop the hearsay now

Rick, would you care to enlighten us and explain where this increase efficiency is happening? I used to buy into the "less weight hence less energy needed to get up to speed" too as the longer runtime's key driver... I'm more than interested in hearing your take on this. The only other one I can think of is less IR ie less energy lost to global warming.

Thanks for sharing the knowledge

Paul
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Old 03-23-2009, 10:36 AM   #6
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Since you mentioned you will be club racing, the lighter lipo won't matter because you still have minimum vehicle weight requirements to make...just get the highest capacity pack you can fit in your chassis and go race!

-rocky b
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Old 03-23-2009, 10:40 AM   #7
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Originally Posted by Lonestar View Post
HA - finally someone knowledgeable who has a clear opinion on this Maybe we can stop the hearsay now

Rick, would you care to enlighten us and explain where this increase efficiency is happening? I used to buy into the "less weight hence less energy needed to get up to speed" too as the longer runtime's key driver... I'm more than interested in hearing your take on this. The only other one I can think of is less IR ie less energy lost to global warming.

Thanks for sharing the knowledge

Paul
The IR of a LiPo is less than that of a NiMH so you lose a lot less energy as heat. Unfortunately, I can't emplain why one is more efficient than the other. But they are quite a bit different.

Weight will only be a factor if the weight of the entire vehicle is reduced. In TC racing for example, using a LiPo places the car way under the minumum weight. Ballast is added to bring the car to the minimum so both batteries end up moving the same weight. Even so, the runtime increase over NiMh is substantial.
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Old 03-23-2009, 10:54 AM   #8
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Ok guys , thanks for the help , i will get the 5000 one , thanks for the input .
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Old 03-24-2009, 05:01 PM   #9
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An informal discussion of run time is posted here:

http://www.batteryuniversity.com/parttwo-31.htm

where you can see a number of factors involved with run time performance of different chemistries.

Vs - open circuit source voltage
Rs - internal source resistance
DoD - depth of discharge (0 - 100%)

On a per volt basis, I think the LiPO actually has greater internal resistance for a single cell.

This how you compare:

Stack up 3 NiMH Cells in a Series String:

Vs = 3*Vcell = 3*1.4 = 4.2 Volts (fully charged)

Compare to 1 LiPO:

Vs = 1*4.2 Volts (fully charged)

In this comparision the LiPO may have a little more internal resistance Rs than a stack of Sub-C NiMH cells. However the Rs in NiMH begins to rise pretty fast after 50-60% depth of discharge, and in high performance racing, this kills the peak power output from the motor, because it cuts down the amps that can be drawn when you apply full throttle. The LiPO will have a more constant Rs until maybe 80-90% depth of discharge, so you get more of the stored energy out of the pack (extended run time). These are BALLPARK figures for illustration.

If the amp draw is low in an application (not as in racing) then both batteries might provide just about the rated capacity. Amp draw is high in racing.

Now for better performance the Manufacturer (don't try this at home) can string two LiPO cells in parallel, essentially making the cell "fatter." This doubles the energy stored at 4.2{V} and cuts internal resistance Rs in half. This LiPO will substantially outperform NiMH in time delivered and peak power, without adding weight.

Power dissipated as heat in the battery is armature current squared times source resistance, Ploss = Ia^2*Rs. Since LiPO can be built with less Rs and it stays lower for a greater depth of discharge, LiPO is more powerful and more efficient.

A last minute search to confirm my logic turns up this great link:

http://www.rchobbies.org/lithium_bat...eakthrough.htm
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Old 03-24-2009, 05:03 PM   #10
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I ran a 40c Thunder Power 3200 at the nationals, and it last 6 minutes plus, so i think you would be fine.
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Old 03-25-2009, 12:22 PM   #11
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Quote:
Originally Posted by SystemTheory View Post
The LiPO will have a more constant Rs until maybe 80-90% depth of discharge, so you get more of the stored energy out of the pack (extended run time).
I may be in error about that 80-90% figure, it may be constant to 100% of the discharge capacity.

This page shows a graphical comparison of NiMH to LiPO from a few years ago, where the technology is improved but the method of comparison is sound.

http://www.slkelectronics.com/ecalc/gen4a.htm

According to this source a little warmth in the LiPO actually reduces internal resistance Rs, and it stays relatively constant up until the cut-off voltage. Thus LiPO has a dual advantage of higher energy density and lower internal resistance per nominal volt.
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