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I am using 8 brushless motors for an octocopter. Each motor can be run at maximum 30A. I use 4 batteries in parallel. How high C number is needed?

$$\frac{30*8}{4*2} = 30C$$

When running the motors at 100% load, it will draw 30C from each battery. Can a 25C with max 50C be used, or will it run hot?

Additionaly, how many ampere hours can be drawn from a 5000mAh battery before it's empty? Many 12V car batteries can only be drawn for 60% of their stated capacity before they need to be charged.

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Total current that can be drawn from a battery depends on discharge capacity (C) and ampere-hour (Ah) rating of a battery. Maximum continuous current that can be drawn from batter can be given as follows

Maximum Current (A) = discharge capacity (C) x ampere-hour(Ah)

For example, in your case if you are using 5000mAh, 25C battery, then you can draw maximum 125A continuously.

Generally discharge capacity of battery is mentioned as continuous discharge and burst discharge. Continuous discharge rating can be used to calculate maximum allowed continuous current. Burst discharge rating can be used to calculate maximum burst current that an be handled by battery. Usually burst current is can be drawn for few seconds (less than 10 seconds). For example, in your case if you are using 5000mAh, 25C-50C battery, they you can draw 125A continuous and 250A burst current.

So for application purpose, one should always use continuous discharge rating for calculations and keep in mind burst rating only as fail-safe.

If you are using batteries in parallel then make sure all have same discharge rating. Discharge rating and voltage of resultant battery will be same as individual battery and only Ah rating will add up. So in your case, resultant battery should be 9600mAh, 25C then only it will be able to supply 240A(8x30A) continuously. In case you are using 4 batteries in parallel then each battery should be at least 2400mAh.

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  • $\begingroup$ Thanks for your answer! So when I need 240A and divide that on 4 batteries, I need 60A from each battery. This means 60/5 = 12C. In theory I don't need more than 12C from each battery? So the 25C batteries will work fine? $\endgroup$ – Friend of Kim Mar 15 '14 at 13:28
  • $\begingroup$ And each battery can be drawn for 2400mAh, meaning roughly 48 % of its rating of 5000mAh? $\endgroup$ – Friend of Kim Mar 15 '14 at 13:31
  • $\begingroup$ Does the batteries need to have the same C-rating because this means an equal internal resistance, which means an equal draw when wired in parallel? $\endgroup$ – Friend of Kim Mar 15 '14 at 13:34
  • $\begingroup$ How are you getting 12C? 60A from each battery with 2400mAh and 25C will be (60/2.4 = 25). This is minimum Ah required given a 25C battery. Any higher Ah rating will work as well. Mostly good quality lipo battery gives you 80-90% of their stated Ah rating. $\endgroup$ – nikhil Mar 15 '14 at 14:19
  • $\begingroup$ batteries should have same Ah rating and same discharge rating while connecting in series or parallel. This ensures that all the batteries will attain low voltage at same time $\endgroup$ – nikhil Mar 15 '14 at 14:25

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