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If I have two identical batteries (capacity:300mAh Voltage=7.4v C rating=25C) is I connect those two batteries in parallel or in series does this effect the C rating of the combination of the 2 batteries

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    \$\begingroup\$ Does this answer your question? Batteries connected both in series and in parallel \$\endgroup\$ – Marcus Müller Sep 22 '20 at 13:49
  • \$\begingroup\$ when connecting the 2 batteries in parallel it's equivalence to offering a higher capacity battery for the same voltage the C rating is the maximum current the battery can source without a series damage to it's performance with respect to it's capacity so 300mah battery can source 300 milliamps of current for an hour but it can source a current of up to 300mah * 25 = 7.5 amps continuously for around 2 minutes Now when connecting 2 of them in parallel you are doubling the c rating for the combination or the equivalent battery that is formed. \$\endgroup\$ – Mahmoud Salah Sep 22 '20 at 13:58
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    \$\begingroup\$ @MahmoudSalah See my answer - notice the need to specify what the C rating is relative to. \$\endgroup\$ – Russell McMahon Sep 23 '20 at 2:09
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    \$\begingroup\$ @MarcusMüller This is a trickier question than it looks and the cited answer addresses mAh ratings and largely NOT C ratings. For identical cells. If both have eg 25C capacity then in parallel they are 50C rated compared to ONE CELL. BUT if each is 300 mAh then in parallel capacity is 600 mAh and C rating is 25 x 600 mAh Ie you need to specify if the new C is relative to the per cell capacity or the new combined capacity. Roughly: Paralleling adds C ratings relative to one cell. Series gives C rating of lowest Ah cell. \$\endgroup\$ – Russell McMahon Sep 23 '20 at 2:15
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There is a "trap" in understanding this question.
It is NOT the same as asking about mAh ratings.

This is a trickier question than it looks and the cited answer addresses mAh ratings and largely NOT C ratings. For identical cells:

For parallel cells:

If both have eg 25C capacity then in parallel they are 50C rated compared to ONE CELL.

BUT if each is 300 mAh then in parallel capacity is 600 mAh and C rating is 25 x 600 mAh
ie the C rating relative to the new combined mAh capacity is the same as the C rating of each cell individually.
But the mAh capacity of the battery is the sum of the mAh rating of the two cells.

ie When specifying battery C rating you need to specify if the new C is relative to the per cell capacity or the new combined capacity.

For series cells the mAh capacity of the battery is the same as each cell and the C rating does not change.

Roughly: Paralleling adds C ratings. Series gives C rating of lowest Ah cell.

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It is presumed that the batteries are identical (7.4 V, 300 mAh, C 25), new, good and fully charged.

Case No.1 - Batteries in series

The rating of the combination would be 14.8 V, 300 mAh, C 25

Case No.2 - Batteries in parallel

The rating of the combination would be 7.4 V, 600 mAh, C 25.

The yardstick is the same irrespective of the batteries being single or combined.

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  • \$\begingroup\$ Your case No.1 new mAh is incorrect. If one cell is 300 mAh then two cells gives 600 mAh. <- Do you agree? ||| " This battery then has C = 25 relative to 600 mAh or C=50 relative to 300 mAh. See my answer. \$\endgroup\$ – Russell McMahon Sep 24 '20 at 10:49
  • \$\begingroup\$ No Russel, it's correct. Energy / battery / 25 h = 7.4 * 0.3 = 2.22 Wh . Energy / 2 batteries in series or parallel / 25 h = (14.8 * 0.3) = (7.4 * 0.6) = 4.44 Wh . It figures. \$\endgroup\$ – vu2nan Sep 24 '20 at 14:36
  • \$\begingroup\$ Trying again: Please follow this carefully - it's important. Really. || No BECAUSE. C rate is NOT proportional to energy - it is proportional to mAh capacity. When you double the cells - I agree that in parallel and series you double the energy. BUT: In parallel you double the mAh but do not change the voltage. In series you double the voltage but do not change the mAh. In BOTH cases V x mAh = energy = the same. But in parallel the C rate doubles. \$\endgroup\$ – Russell McMahon Sep 25 '20 at 2:24
  • \$\begingroup\$ Hi Russel, Case 1: A 300 mAh C25 battery will continuously deliver 300/25 = 12 mA for 25 hours. Case 2: A 600 mAh C25 battery will continuously deliver 600/25 = 24 mA for 25 hours. Case 3: Two 300 mAh C25 batteries, connected in parallel, will continuously deliver (12 + 12) = 24 mA for 25 hours. Since Case 2 & Case 3 behave identically, would it not be right to conclude that the rating of the parallel combination is 600 mAh C25? \$\endgroup\$ – vu2nan Sep 25 '20 at 10:31
  • \$\begingroup\$ I agree with you. My 1st comment has a typo (or brain fade) and should have referred to your case 2. || What you have written is correct in both cases, but it may not be obvious to some (even though in plain sight) that your C25 in parallel is relative to the new 2 cell 600 mAh battery. It's C25 wrt battery and C50 wrt one cell. ie even though the C25 figure is unchanged the C25 current has doubled. So I agree that what you have written is correct. What I wrote is also correct (apart from the typo :-). || I suggest that adding a comment that C25 is now double may avoid misleading a newcomer/ \$\endgroup\$ – Russell McMahon Sep 25 '20 at 10:56

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