The team has built and tested new lithium-ion batteries which have extended the flight time of our UAV from 40 minutes to 1+ hour.
We were originally using standard Li-Polymer batteries. We were using 2 x 4S 5,000 mAh packs, this gave a flight time of 40 minutes. However the competition goes for 1 hour so we needed to extend the flight time. Each pack weighs 550g, so adding an extra pack would have considerably increased our total weight beyond what the airframe can safely handle.
We heard that the NCR18650B li-ion batteries could store more energy for the same weight. These are the same batteries that are used in laptops. We were keen to test these out to see if they could increase the flying time of our UAV without increasing our total weight.
We ordered 24 individual cells to make 2 battery packs, each consisting of 4 cells in series by 3 in parallel. Each cell is 3,400 mAh capacity, so that means each pack has an 10,200 mAh capacity.
The lithium-ion cells can be safely discharged to 3V (ie the total pack can be safely discharged to 12V). As can be seen the final voltage of our pack was 14V (ie 3.5V per cell), so we still had a bit of juice left. This is confirmed by the current meter which showed that we used 80% of the total capacity. (Note that voltage drops and fluctuates during flight compared to the open-circuit voltage due to the current being drawn).
Lithium-ion batteries are nearly double the price compared with the same weight lithium-polymer battery. Lithium-ion batteries must be ordered in cells and built rather than the ready made packs for lithium polymer. Lithium-ion batteries also have a much lower C-rating compared with the lithium-polymer, which means that they can’t be used quickly in shorter duration flights, but they are suited to 1+ hour flights. All these limitations are fine for us, as the main priority is achieving longer flight time within our total weight restriction.