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View Full Version : Lipo, voltage, etc...


VACaver
11-08-2005, 04:30 PM
Christmas is coming, so it's time to ask Santa for a couple of lipo's. Problem is, I know less than squat about them.

A friend suggested that I get a two-cell, 7.4v, 1320 mah. But my question is, if I'm currently using a 9.4v battery on my E-Starter...why would I go down to a 7.4v?

So, using a 400 motor on my plane, what lipo should I actually use?

Thanks.

hoppy
11-08-2005, 05:31 PM
You might be able to use either one BUT, you'll have to change the prop or gear ratio you are using in order to obtain the same watts you are now drawing from the pack.

For instance, with the 2s Lipo, you'll need a larger capacity cell and a larger/higher pitch prop because you'll have to draw more current to draw the same watts at a lower voltage. With the 3s pack, you'll need to downsize the prop in order to reduce the watts drawn by the motor.

Lets look at a 6V Speed 400 motor with a 2:1 gearbox and a 10x6 prop.
With a 9.4V 8cell NiMH pack it will draw 11.6A and 96W.
With a 2s Lipo it will draw 9.9A and 66W.
With a 3s lipo, it will draw 12.7A and 124W.

To maintain 96W with a 3s lipo, use a 8x4 (10A).
With the 2s lipo, I can't find a combo with motocalc that would give you 96W... all are less. For instance, a 9x6 on a 1.5:1 box gives 83W (11.5A).

Does that make sense to you?

These are all from MotoCalc and the actual figures may be different but I hope it gives you an idea of what's happening. In answer to your question, I'd go withthe 3s and downsize the prop or increase the gear ratio.

qban_flyer
11-08-2005, 06:45 PM
You might be able to use either one BUT, you'll have to change the prop or gear ratio you are using in order to obtain the same watts you are now drawing from the pack.

For instance, with the 2s Lipo, you'll need a larger capacity cell and a larger/higher pitch prop because you'll have to draw more current to draw the same watts at a lower voltage. With the 3s pack, you'll need to downsize the prop in order to reduce the watts drawn by the motor.

Lets look at a 6V Speed 400 motor with a 2:1 gearbox and a 10x6 prop.
With a 9.4V 8cell NiMH pack it will draw 11.6A and 96W.
With a 2s Lipo it will draw 9.9A and 66W.
With a 3s lipo, it will draw 12.7A and 124W.

To maintain 96W with a 3s lipo, use a 8x4 (10A).
With the 2s lipo, I can't find a combo with motocalc that would give you 96W... all are less. For instance, a 9x6 on a 1.5:1 box gives 83W (11.5A).

Does that make sense to you?

These are all from MotoCalc and the actual figures may be different but I hope it gives you an idea of what's happening. In answer to your question, I'd go withthe 3s and downsize the prop or increase the gear ratio.

Great post Hoppy. :)

VACaver
11-09-2005, 12:47 AM
Hoppy,

Thanks for the reply. I understand what you are saying...sort of.

Here's the actual situation...

My E-starter is heavy...about 7 ounces more than it should be. I've been flying it with a 350 "C" using the 7-cell, 8.4v Nimh batteries and a 10x8 prop. It's sluggish as hell with the current setup.

I do have a 400 motor with a "D" drive and will install it as soon as my new speed controller arrives.

A 3-cell lipo will not fit into the battery compartment without cutting the opening bigger...something I'd rather not do.

I'm grateful for the explanation you gave about maintaining the draw, but I'm electronics stupid.

So, basically, will a 2-cell, 7.4v, 1320 mah lipo work with the 400 motor? I'll drop down the prop size as needed.

Thanks for all your help.

hoppy
11-09-2005, 07:12 PM
7 oz? That's what? 30% over weight? You need a loadmaster on that plane.:)

"It's sluggish as hell with the current setup."
My guess is it's the excess weight. What does the plane weigh?

"So, basically, will a 2-cell, 7.4v, 1320 mah lipo work with the 400 motor? I'll drop down the prop size as needed."
No, with a lower voltage battery, the prop size needs to be increased. Looks like that plane doesn't have a gearbox...is that correct?

hoppy
11-09-2005, 11:41 PM
As to the speed 400 motor
Which gearbox ratio?
Which prop?

VACaver
11-10-2005, 01:15 AM
Yes, it's geared, but I don't know the ratio. I'm out of town testing parachutes at the moment, so I don't have access to the airplane. However, the gearing is the GWS "D".

And yes, it could use a good Loadmaster onboard. When we were out of CG limits or too heavy, the first thing recommended to get rid of was the Navigator ;)

I appreciate your help and will probably go with the 3-cell lipos, and will adjust the prop accordingly so I don't burn out the motor.

hoppy
11-10-2005, 03:23 AM
I checked the GWS literature_
Your current setup is using the wrong prop according to GWS. I like the performance shown by the 10x4.7 (1047). http://www.gws.com.tw/english/product/powersystem/eps350c.htm

With a 8.4V battery, it power is shown as 86W
With a 9.6V battery, this increases to 122W by their charts.

The GWS Speed 400 info is here:
http://www.gws.com.tw/english/product/powersystem/eps400c.htm
It shows a 10x4.7 or 10x8 to be the best choices.

If you compare those two charts, you'll not find much difference between the power of the 350 or 400.

I'd try the 1047 with a 9.6V pack with your speed 350.