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Old Mar 26, 2012, 04:12 PM
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Capacity or discharge rate?

I'm just wondering which is the better option, a higher capacity battery on a lower C rating or a lower capacity with a higher C?

Just an example of what I'm thinking, 1 battery is say a 4000mah 30c, the other is a 3000mah 40c. Both are good for 120a constant (theoretically). Power in to the motor is something like 75-80a (or 90a if you like...)

I know the larger capacity will mean longer flight times, and both batteries are well within thier constant discharge rate, but will the lower C rated battery hold it's voltage as well as the higher C battery considering it's not really being pushed to it's limits? Or will the higher C battery just stay stonger to the end?
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Old Mar 26, 2012, 04:19 PM
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Only way to answer this question is to know the internal resistance and usable capacity of the two batteries in question. Writing on a label provides no empirical data so it's impossible to give a definitive answer without some measured results.

Mark
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Old Mar 26, 2012, 04:31 PM
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There's no easy way to tell. My guess is that since they both have the same theoretical maximum current rating they will both have approximately the same IR and so will maintain roughly the same voltage. But it's only a (semi-educated) guess .

Steve
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Old Mar 26, 2012, 05:09 PM
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Originally Posted by slipstick View Post
There's no easy way to tell. My guess is that since they both have the same theoretical maximum current rating they will both have approximately the same IR and so will maintain roughly the same voltage. But it's only a (semi-educated) guess .

Steve
Well a semi educated guess is better than what I can come up with

I was figuring that the lower C battery would hold a good voltage but probabaly not quite as strong as the higher C battery. I guess it would show the 40c better if the current draw was closer to the max constant discharge rate of 120a, but it's not (well not intended to be anyway...).

I was hoping that a larger capacity would give me slightly longer flight times while still maintaining voltage under load (at a 60-70% of constant discharge rate) I certainly hope it would anyway but that's why I'm asking. .
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Old Mar 26, 2012, 05:23 PM
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One factor which is relevant is that it is better to go for the lower C rating, higher capacity battery on the basis that the lower C is more likely to be genuine.
I have tested several lipos which are actually capable of 30C but have yet to test one which is genuinely capable of a continuous 40C discharge.
Some of these 30c capable packs were rated at 30C, one was rated at 25C and most were rated at 40C+.

Wayne
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Old Mar 26, 2012, 05:42 PM
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Good point about the C rating, I've read that a few times but seemed to forget about it while thinking about this

Most All of my setups run my packs to thier constant discharge limits, mainly small 2200 3s asking for 44-55a. I can't really test anything because I haven't got a larger than needed pack. If I did, I'd prop down a bit to see how they'd compare.

Seeing as I'm not after squeezing everthing I can from the pack, either for power or speed, it's 'Take your pick' time. I think I'm edging towards capacity and lower C.
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Old Mar 26, 2012, 05:45 PM
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The important thing to me would be what do I expect from the plane in terms of performance.
Choosing a pack for a Piper Cub and one for a Extra 300 are two different worlds IMO.

Am I going to be cruising around at partial throttle most of the time for 10+ minutes or am I looking at a lot of full throttle spurts with some critical demands on thrust output and maybe less than 5 min flights.


In the cruising case, most any battery would be OK as the draw on the pack would be in the 6C range. and critical power demands would be non-existant. In the Extra 300 case, I'd be more concerned about the pack holding voltage under load.

How long do you think you'll be using full throttle? How long are your usual flights?

90A draws from a 4000mah pack would be a 22.5C draw or if drawn continuously, a 2 min flight. The 3000mah pack would yield a 1 1/2min flight at full throttle.

FWIW, I prefer the larger battery because it will most likely give me more cycles.

I'm done rambling.
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Old Mar 26, 2012, 07:12 PM
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Well the planes a Funjet Ultra. There'll be 'some' throttle management. How much? I can't say.
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Old Mar 26, 2012, 08:00 PM
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Well the planes a Funjet Ultra. There'll be 'some' throttle management. How much? I can't say.
Looked up the Funjet Ultra, definitely not a CUB.....probably a lot of full throttle straight runs. The review recommends nothing less than a 3600mah if that helps ya. http://2bfly.com/reviews/multiplex-f...-ultra-review/
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Old Mar 26, 2012, 08:48 PM
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Cheers for the info. But I'm not going to use the recommended power set, I've got something else in mind.....

I didn't really want to get drawn into which plane, I know it has some relevance to throttle use, but it was more a q about how battery packs hold thier voltage when not being squeezed of their capacity and C rating.

The batteries quoted weren't specific to my intended setup, well, the 3000mah wouldn't be anyway, unless it was 4s and I'm watching the AUW, just generalised for ease of comparison with thier constant max discharge rate of 120a.
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