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Old Feb 09, 2010, 03:24 AM
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Cleveland, Oh
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Lower C rated batteries and how to size battery to application...

Many times we are looking to save weight and for many models and when using an efficient motor the amp draw is very conservative.

The problem is I don't see many options anymore for lower C batteries to save weight. It seems a shame that for many models we may be using a vastly overqualified battery. My guess is this is just a matter of economics and since people mostly want or think they need bigger and better the lower C products wouldn't sell as well and therefore aren't offered.

Are there any lower C batteries still offered?

What are the alternative options as far as sizing goes? I'm not quite sure what calculations I need to do. I know how to figure out how many amps a battery will take, but my knowledge beyond that is basic.

Does amp draw directly correlate to run time or does mah only matter there? I assume it does matter.

What are the guidelines for choosing the right pack besides amp draw? How would I figure my flight time if I size down mah to match the lower amp draw since I can't seem to get lower C batteries that seems like the approach to take.

Thanks
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Old Feb 09, 2010, 04:56 AM
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Staffs, UK
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Run time = Capacity (mAh/1000) * 60 / current (amp draw) minutes

So 1500mAh, 10A draw = 1500/1000 * 60 = 90 then 90 / 10A draw = 9 minutes.

So run time is directly related to mAh (more mAh more time) and inversely related to amp draw (more amps less run time).

You'll nearly always find that the weight (and cost) saving of buying small capacity batteries isn't worth losing all that run time. Anyway having batteries that will easily deliver all the current you need or more is not a bad thing...the batteries tend to last a lot longer that way.

Steve
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Old Feb 09, 2010, 05:40 AM
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Thanks for the explanation. I figured more amps would be related to run time, I didn't know that it would be the direct inverse though.

Yes, I was still going to overrate the battery for my application. It's generally 80% of the C rating right? Do they mean the constant or burst when they mention that?

If you are within those limits I don't see why it wouldn't be appropriate to have a lower C rated battery to save weight? Also, if the battery is well matched to the situation it seems like people get plenty of run time on smaller mah ratings. If your getting the runtime you want and it can handle the amps safely why not go with the smaller and lighter battery?
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Old Feb 09, 2010, 08:13 AM
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Originally Posted by aman_74 View Post
If your getting the runtime you want and it can handle the amps safely why not go with the smaller and lighter battery?
If you're getting the runtime you want (while using less than 80% of the total capacity) and the battery can handle the current safely (i.e. the actual MEASURED current is less than 80% of the calculated maximum the battery can handle) then that IS the right battery .

BTW ignore "burst" ratings, there's no general agreement on what they actually mean. Everything should be based in the constant/continuous ratings.

Steve
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Old Feb 09, 2010, 08:31 AM
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Originally Posted by slipstick View Post
If you're getting the runtime you want (while using less than 80% of the total capacity) and the battery can handle the current safely (i.e. the actual MEASURED current is less than 80% of the calculated maximum the battery can handle) then that IS the right battery .
Umm, no...I was replying to you saying it wasn't worth cost or weight savings when going with an overrated battery. You were saying to just stick with the overrated stuff. I'm saying if the lower weight battery is giving the run time and meeting the 80% rule for amp draw then why wouldn't you take the lower weight/cost battery?
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Old Feb 09, 2010, 01:52 PM
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I give up .

Steve
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Old Feb 09, 2010, 02:56 PM
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I give up .

Steve
Ok man. I know you're trying to help and I appreciate it. I feel either you or wrong or not expressing yourself clearly.

If a battery sufficiently meets amp draw requirements with room to spare and satisfies your runtime requirements as well why would you possibly want a heavier battery? I'm no expert that's for sure, that's why I'm asking, but I can't see any logical reason.
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Old Feb 09, 2010, 05:42 PM
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OK I think I know what you are asking. Let me see if I have it right.

For instance lets say you have a 2000mah 20c battery. At 80% that battery would provide 32 amps. But your plane only needs 16 amps.
So you could use a 2000mah 10C battery. At 80% that battery would provide 16 amps.
The 10C battery would be cheaper and probably a bit lighter. So you would save a little money and some weight.

Is that where you were trying to go?

Glen
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Old Feb 10, 2010, 07:06 AM
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Yes.
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