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Another lipo question

PostPosted: Thu May 17, 2012 6:47 am
by quarry44
I'm looking at two lipo packs.One is 3000Mah-40c,the other 4000mah-30c.
Am i correct in thinking they can both supply 120A burst,or am i overlooking some basic fact here?
All this electrickery is making my head hurt :?
At the moment I run 2200mah 3cell-20c packs in my axn.I've noticed that the no.2 cell drops well below the 3.3v level.Surely that can't be good for it?A balance charge brings it back on an even keel,but it still worries me a bit.
Is the axn asking to much of a 20c battery pack?Is the 6x4 prop too much of a good thing?
Or is there a different reason for the no.2 cell dropping so far?
Both packs have only had 2 or 3 flights on them.

Re: Another lipo question

PostPosted: Thu May 17, 2012 7:58 am
by RCModelReviews
It could be that your 2200mAH packs don't have very evenly matched cells -- or it might be that you've over-discharged the middle cell before and now it's permanently down on capacity. Strange thing is that when a 3S lipo gets damaged, it's nearly always the middle cell -- probably because that's the one that gets no cooling because it's insulated by the other two cells.

As for your first question -- yes, in theory, both packs can do 120A -- that's if the C-ratings are honest (which they aren't always). I've found that most manufacturers tend to be a little optimistic and, once the cell starts to get old, the ability to deliver current starts to drop off pretty badly in most packs.

The AXN should work fine with a 20C battery. Even with a 6x4 prop it's hardly drawing 20A which is just 9C anyway.

Cooling is a big factor though -- if you've got no way for air to flow around the battery it can still get too warm, even when operated below its rated C current. It's easy to end up blocking off the cooling holes with an AXN if you're not careful.

Re: Another lipo question

PostPosted: Thu May 17, 2012 8:22 am
by quarry44
Thanks,Bruce,I check the pack after every flight,and it's never been warmer than room temperature.
Myabe I just got a dud pack,at hk's prices it wouldn't be impossible.That's why i got 4 3000 mah packs on the way.

Re: Another lipo question

PostPosted: Fri May 18, 2012 2:48 am
by pldb64
Quarry/Bruce,

I thought it might be worth explaining some of the maths you're both using for those that haven't got their heads around it yet (i know it took me ages to figure this part out).

The 120A calculation for 3000mAh-40C battery is obtained by multiplying the 1C battery capacity (3000mAh or 3Amps) x the maximum C rating (40C). Hence 3Amps x 40C = 120Amps is the manufacturers stated maximum current draw that the battery can supply.

Likewise Bruce's calculation of the AXN drawing 9C is obtained by taking the observed current drawn from the model setup at WOT (wide open throttle). In this case with a 6x4 prop, the stock motor pulls about 20A. The battery being used has a 1C rating of 2.2 Amps (2200mAh). Hence 20Amps / 2.2Amps = 9C. Hence the maximum draw of the system is well within the rated maximum C rating of the battery (20C).

So simply put, a 20C battery should do quite happily for an AXN with a stock setup or even with the more aggressive 6x4 prop setup. No need to pay extra for a 45C rated battery - you'll never get close to needing it.

Re: Another lipo question

PostPosted: Fri May 18, 2012 2:55 am
by pldb64
Quarry,

Let us know how you go fitting the 3000mah packs into the AXN! ... I figure if i were to opt for that size battery, i'd be able to keep the AXN in the air for at least 30 mins :-). You've got me thinking though!

What battery have you selected, i'd like to assess how it compares against my existing 2200's from the point of view of dimensions and weight!

Thanks
Peter

Re: Another lipo question

PostPosted: Fri May 18, 2012 4:09 am
by quarry44
Sorry,Peter,the 3000's are for my canadair twin.I thought I'd give that one a bit extra,seeing as there are two to feed :mrgreen: These are the ones I got:http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8851.They are thinner,but longer,and of course heavier.
The 2200 seems just right for the axn,as I didn't need to balance anything.I get 6-8 minutes out of the 2200,which is just enough for me.Hans.