5S on 1700KV is a great set-up. Just check this out if you're worried about space:
A Hyperion VX 3300mAh 5S pack (61.05Wh) is 136.3x43x36mm
A Hyperion VX 5500mAh 3S pack (61.05Wh) is 158x46.8x30.7mm
Overall volume of the 5S: 210992.4mm^3
Overall volume of the 3S: 227008.08mm^3
Cost of 5S: £104.95
Cost of 3S: £104.95
The 3S has higher volume, is wider and longer than the 5S. From my experience, the higher voltage packs are just as easy to fit as the same power lower voltage packs.
In fairness, 3S is a crap voltage to run on. It is very inefficient, even when compared to 4S. The extra current produces a lot of extra heat - a sure sign of inefficiency. 4S or above is a far better option. 5S even better.
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