Sorry Sky high, the big ‘C’ stands for Capacity and when we talk about the ability of a set of Lithium Polymer batteries to deliver power we use the C rating and add a multiplier.
So if you are (for example) looking at getting a set of cells for a motor that is capable of up to 400 watts that wattage could be supplied by 12.6 volts at 30 amps. (leaving a little safety margin).
(Lithium Polymer cells are supplied at a median voltage of 3.6 or 3.7 volts, but you need to think fully charged voltage for the launch which is 4.2 volts per cell.)
But since we’re looking at Capacity and considering the cells’ ability to supply sufficient current and voltage the 30A could (just) be supplied by a set of 3 cells (referred to as 3s – for 3 in series) of 3000 mah (3ah) capacity that were only capable of delivering 10 times their capacity (10C = 30A).
Alternatively it would be possible to get the required current from a 3s set of 20C cells of only 1500mah.
Or 40C cells of 750mah.
All these would be just capable of doing the job but obviously the last two would not be able to do so for as long a flight as the first set, which has the greatest capacity.
Ducted fan models are not very efficient – so much so that they typically fly for only about half the time of a prop driven aircraft on the same battery pack. They are current-hungry and can drag the voltage down to a level where flight is either sluggish of impossible. It pays therefore to ensure that the current delivery of your power pack is sufficient to deliver the required current without dropping the voltage significantly.
I still use the old technology 10C cells for several of my models – including the Q6 in my avatar, but am steadily converting to Loong Max cells which are capable of delivering 20C – 35C.
There are however some even newer cells that are capable of 40C and of being charged (and look at the caveats before embarking on that course) at 5C, which in the case of our 3000 mah cells would be 3 x 5 = 15A.
Personally I don’t plan to go that route until the Pathfinders have been there!
I hope I have explained that adequately – and I apologise if I have insulted your intelligence!
So yes, whatever model you choose – and I would suggest a prop driven job for a start – you will need to choose a set of cells to power it. The manufacturer will always make his reccommendation, but just don’t skimp on it – it ain’t worth the hassle.
Since cost is an issue have a look at http://www.giantcod.co.uk – it’s the cheapest site I’ve found so far for cells and other bits and pieces.
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