So I have battery cells with a rated capacity of 3.5Ah and a nominal voltage of 3.5v Now, say I have a system that requires 12A for a period of 10 seconds. For this battery it is advised not to discharge beyond 2C or the efficiency hit becomes unreasonable. From my understanding, I can increase the amount of batteries in parallel to increase the capacity, but cannot increase the available current. Correct? Will this cell be unable to meet the 12A requirement? I think I'm missing a concept here. At 1C this battery can discharge 3.5A for 1hour. So then can it discharge 14A for 15 minutes then? If that's the case, then can I not use a much lower c rate to achieve this requirement? at C/10 I discharge .35A for one hour. Or it could be more if I decrease the time, possibly getting me my 12A for 10 seconds. What is the limit on this?
For this battery it is advised not to discharge beyond 2C
A 2C discharge rate for a 3.5 Ah battery would be 7A. So, the manufacturer is recommending that you do not draw more than 7A from a single instance of this battery.
From my understanding, I can increase the amount of batteries in parallel to increase the capacity, but cannot increase the available current.
This is partially correct. By placing multiple batteries in parallel, you do increase the capacity, and you CAN increase the available current. In fact, most battery packs have multiple cells both in series, to increase the available voltage, as well as in parallel, to increase the available current.
With two of your 3.5Ah batteries in parallel, you'd have 7Ah of capacity, and your 2C discharge limit would be 14A. Two batteries in parallel should be able to handle your 12A load safely.
PS: Just be careful when trying to recharge these batteries, especially if they are Lithium Polymer. LiPos require careful cell balancing in order to be charged safely.
If you put two 3.5Ah batteries in parallel, you now have a 7Ah battery, and a 2C discharge current is 14A, meeting your 12A requirement.
A rated capacity of 3.5Ah probably means the 10 hour rate, which is 350mA for 10 hours. Unfortunately, as the discharge rate goes up, the actual battery capacity falls. You might get as much as 3Ah over 1 hour at 3A, but it could be less. It will be less over 15 minutes. There's a reason that manufacturers rate their batteries at low rates!
If you have the datasheet for the battery, see if the manufacturer is upfront about the capacity at higher discharge rates.