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 Post subject: Which battery
PostPosted: Sat Jun 15, 2013 7:44 am 
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Joined: Mon Sep 03, 2012 3:53 am
Posts: 21
Location: Oxford
Hi all some more help needed, I have a 2.6 Geronimo which I have just purchased a new motor for ( jeti 45-3 ) and wondering what battery to use it has a 11x7 prop and a os 170 speed controller. at present I have some 4c 2200 25c.

Is the battery ok
Do I need bigger batt
Smaller prop

Any help would be great


 
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 Post subject: Re: Which battery
PostPosted: Wed Jul 10, 2013 2:06 pm 
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Joined: Sun Oct 10, 2010 8:15 pm
Posts: 217
Location: Stokenchurch
Hi Smudge,

Sorry for the slow reply – just catching up on some threads!

Others may have other ways of working out the battery required but this is how I’d go about it.

First thing would be to have some idea of the current / power the motor will use / generate. Do you have a watt meter?

One starting point is to work out how much current the battery can deliver. To do this multiply the capacity by the ‘C’ rating – for the battery mentioned 2200 x 25 = 55000mA or 55Amps. This would be the (theoretical) Maximum current draw the battery could provide. Although it is probably advisable not to push batteries to their limit!

If you could then measure the current and power the motor is pulling then this will give you an indication as to whether the battery could provide enough current. Key will be the Discharge rate of the pack.

Found these links useful as a starting point when I started getting in to electric flying (I’m sure there are other sources of info as well):
http://www.4-max.co.uk/pdf/What%20the%20numbers%20mean%20with%20regards%20to%20LiPo%20cells.pdf
http://www.4-max.co.uk/pdf/Working%20out%20the%20power%20train%20for%20electric%20aeroplanes%20-%20Version%201.0.pdf

and a thread I started when I was looking for small capacity packs with high discharge rates
http://www.esoaring.net/forum/viewtopic.php?f=3&t=2363

Hope of some help

Guy


 
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 Post subject: Re: Which battery
PostPosted: Wed Jul 10, 2013 3:08 pm 
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Joined: Fri Jan 12, 2007 12:55 pm
Posts: 342
Not noticed this post before myself, so sorry Smudge for delay in reply.

In addition to previous advice, a quick check with MotorCalc predicts that your settup is all OK

Assuming your model weighs around 70oz your motor will pull about 33Amp on the supplied prop which will give a clime rate of about 1100ft.min.
If you want better performance you can change the prop to a bigger one, up to about 15 x 7

This size prop would give a predicted clime of around 2000ft.min but would pull about 58Amp which is getting too much current for your battery to be safe.

Best thing is get a Watt meter to check current, but first fly the plane as is and see if the settup is what you want.

Hope that helps


 
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