My original intent with this project was to just use 3 3000mAh 3S LiPo batteries from HobbyKing. After coming across a post regarding a special set of high energy density Lithium Ion cells on diydrones though, I thought “why not” and opted to buy these cells – Panasonic NCR18650B’s to be exact. They cost me $10 apeice on eBay, spec at 3250mAh of capacity and weigh only 47.5g per cell. This is almost half of the weight of equivalent LiPo cells that I was looking at.
While my tests thus far have verified that the SkyEye will be capable of carrying the 9000mAh load of LiPos required for the 30 mile flight, nearly halving the weight is an interesting proposition that opens up extending the flight range even further.
The cells I purchased on eBay don’t have the charge protection nor did they have battery tabs. Luckily there is a retail chain called “Batteries Plus” that will weld battery tabs onto raw cells for you. Cost me an additional $2 per cell, so the 3 packs will end up costing me a little more than $100, a bit more expensive then their LiPo equivalents. Here is a picture of 6 of the cells with the tabs welded on, ready for assembly:
This was my first time assembling a battery pack with a lithium-type cell. No big surprises but I was a tad nervous. Here is a picture of the three packs once they were assembled – they are also attached to the wiring harness and placed on the plane where they will ultimately end up.
All of the packs and the wiring harness weighs in at 555g, much less than the 720g I was anticipating.
After building the packs, I did some tests on their discharge behavior and real capacity. Pictured to the side is my test set-up. Not very professional but it got the job done. I discharged a total of 7600mAh from the 3 packs in my last test with no problem. One problem with these 18650 cells is they do not have a high discharge potential. Each pack is rated at a max output of 2c or about 6Amps. This gives me only 18Amps total to work with between the packs. Since cruise rests at around 10Amps this should be fine. Near the end of the discharge, the packs are not capable of giving much more than 3Amps apeice. I will need to verify this is sufficient for level flight.
Next up is some real flight testing on these packs. Should be really boring!