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Old Feb 03, 2013, 06:41 AM
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Joined Feb 2013
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Question
Understanding battery ...

Hi

Dont worry i will not ask you to explain all.
I have made my own researches on websites and forums
I gathered infos and I think I understood but I have some doubts

Can u confirm or correct the facts ?


Example : 3S 2100mAh 25C 2C

So what does this say ?:

1) How many amps maximum can be drawn : Amp*C*P
Amp = Capacity A (2,1A)
C = Discharge Rate (25)
P = number of parallele block ( 3S1P = 3S )
2.1*25*1 = 52.5 A
52.5 Amp maximum can be produced by the battery

Why is it useful?
We can check if the motors can be well supplied
ex : motor drawing 5A
For a quadricopter : 5*4 = 20 A
The battery can deliver 52.5 A, which is enough ( 52.5A > 20A)

2) The flying time when the battery discharges max
1 hour = 2.1A
we need 20A to supply 4 motors (5*4)
20/2.1 = 9.5 times more than the capacity per hour
meaning it consumes 9.5 times faster
60/(rmotors Amp / battery Ah) = 60/(20/2.1) = 6.30 min
the batttery will be empty after 6.30 min

is it correct ?? coz I have read another method which gives a different result
mAh : 2100 / 60 = 35mA per minute
35mA*C = 35*25 = 875
2100/875 = 2.4 min
the battery will be empty after 2.4 min

this is the original text :

Quote:
"... a 20C rating on a 2000 mAh battery? 20 x 2000 = 40,000 milliamps or 40 amps. Time wise, a 40 amp draw on this pack would exhaust it in about 3 minutes (2000/60= 33.3 multiplied by 20c = 666 mAh per minute - divided into the packs capacity of 2000 mA = 3 minutes)
I understand that actually to calculate the flying time, it depends on Watts and weight , so it's more complicated ...


3) we have not to empty the battery at 100%, it would damage it
100% = 2100mAh , 80% = 1680mAh

Question bonus : if we have a battery : 3S2P 2100mAh 25C 2C
2P means parallele connexion = double capacity
so it means we multiplicate mAh by 2 ?? (4200mAh)

thanks for your help
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Old Feb 03, 2013, 06:52 AM
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Ken Myers's Avatar
Commerce Township, MI
Joined Aug 2001
4,364 Posts
"52.5 Amp maximum can be produced by the battery "

The manufacturer/supplier claims that up to 52.5 amps can be delivered, not produced by the battery.
Manufacturer and supplier claims are not to be trusted.
25C is 2.4 minutes, which you figured out but simpler formula is 60 minutes / 25C = 2.4 minutes
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Old Feb 03, 2013, 07:10 AM
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Joined Feb 2013
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Quote:
Originally Posted by Ken Myers View Post
"52.5 Amp maximum can be produced by the battery "

The manufacturer/supplier claims that up to 52.5 amps can be delivered, not produced by the battery.
Manufacturer and supplier claims are not to be trusted.
25C is 2.4 minutes, which you figured out but simpler formula is 60 minutes / 25C = 2.4 minutes
thanks Ken for you help

well, i am french, maybe i used the wrong english word
I understand what u mean.

About flight time, my method which for me was logic is not good then
it's the other formula that I have read which is the right one.

thanks
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Old Feb 03, 2013, 08:05 AM
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Staffs, UK
Joined Nov 2003
10,270 Posts
The first method you used for flight time is correct. When you say 20/2.1 is 9.5 times that's the same as saying the actual discharge rate you're using is 9.5C (not the full 25C that the battery COULD do).

The second one is working out the time if you discharge at the full 25C. If you notice, the real current you are using (20A) doesn't appear anywhere in the calculation which can't possibly make sense .

Steve
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Old Feb 03, 2013, 08:46 AM
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Quote:
Originally Posted by slipstick View Post
The first method you used for flight time is correct. When you say 20/2.1 is 9.5 times that's the same as saying the actual discharge rate you're using is 9.5C (not the full 25C that the battery COULD do).

The second one is working out the time if you discharge at the full 25C. If you notice, the real current you are using (20A) doesn't appear anywhere in the calculation which can't possibly make sense .

Steve
Oh ok ! thanks
I guess it's not very smart actually to calculate with these formulas . As Ken said, if we can't trust the manufacturers about the discharge rate and that the autonomy of the drone depends how we use it

I have not a quadricopter yet, so i can't really realize with real experience
but it seems it's often less then 15 min
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