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LiIon, LiPo, NiCd & NiMh General General Battery Support


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Old 03-16-2008, 04:04 PM   #1
mjr_larkin
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Default 3S1P 850mA 15C vs. 600mAh 25C - What's the difference?

Hi guys,

Right now i'm using a 600mAh 25C rated 3 cell lipo. I went to order more of the same kind and noticed they also have a 850mAh 15C rated 3 cell lipo. I know you do 850 * 15 = 12750 and 600 * 25= 1500 to figure out the capacity of both. So it appears the kind I have is greater capacity, however, the 850mAh is about $10 more and 15g lighter, so what's up with that guys? Since i'm still doing low elevation hovers I know i'm not drawing the full 25C so I could go with the 15C but I don't get why it's more money and less capacity.
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Old 03-16-2008, 04:16 PM   #2
wlfk
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The 'C' doesn't refer to the capacity, but to the maximum discharge rate.

A 15C 850mAh battery can safely give you up to 12 amps

A 25C 600mAh battery can safely provide up to 15 amps

This is assuming that both mfrs were honest in deciding the 'C' rating.

K
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Old 03-16-2008, 04:22 PM   #3
mjr_larkin
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Right. So why would a lower discharge rate battery be more expensive?

Also how can you tell how "long" it can sustain that discharge rate? amps is charge per second. If I want to get an idea of which pack will last longer how do I assess that?
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Old 03-16-2008, 04:38 PM   #4
wlfk
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Why would 2 batteries with identical capacities and 'C' rates be different prices? Who knows, but that's often the way it is.

Capacity = current x time

time = capacity/current

Say your helicopter draws an average of 10A, flight times would be:

(0.85/10)x60 minutes = 5 minutes

(0.6/10)x60 minutes = 3.6 minutes

The caveat to this is that power = voltage x amps. Now, your 2 liPos will be identical in terms of voltage output when they're sitting there doing nothing, but your 25C liPo will probably hold the output voltage better under load. So the advantage of the larger lipo might not be as evident in real life as it should be.

How warm do your current LiPos get? If they're pretty cool, I might be tempted to get the larger battery despite its slightly lower 'C' rating.

K
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