Lithium Ion Battery 21700 3s2p

####UPDATED VERSION FOUND AT https://www.thingiverse.com/thing:7190304
This is a 3-in-series, 2-in-parallel (3s2p) lithium ion battery using 21700 cells. The materials, tools, build steps, and further technical details are presented below.

####Materials

• 2mm diameter rods, quantity: 4
• PETG filament spool, quantity: 1
• 21700 cells, quantity: 6
• Red 10 AWG wire, 10 [cm] length
• Black 10 AWG wire, 30 [cm] length
• Red 22 AWG wire, 10 [cm] length
• Black 22 AWG wire, 50 [cm] length
• 4 pin JST-XH connector, quantity: 1 (this is the balance connector)
• XT60 connector, quantity: 1 (this is the power connector)
• Shrink tube kit, quantity: 1
• 10 [mm] wide Nickel strip, 1 meter length should be more than enough

Tools

• Spot welder (I use the Sequre SQ-1)
• Soldering iron
• Needle nose pliers

####Build Steps

1. 3D print three cell holders using the provided STL files
2. Put two 21700 cells into each of the cell holders
3. Spot weld the nickel strips onto the cell terminals: it helps to do the parallel connections first, then the series connections
4. Insert the four 2 [mm] diameter rods into the corners of the cell holders, with each rod piercing each of the three cell holders: this enforces the prismatic shape
5. Strip and solder the 10 AWG power wires to the XT60 connector, and strip and solder the 22 AWG balance wires to the 4-pin JST-XH connector
6. Solder the power wires (10 AWG) and balance wires (22 AWG) to the cell terminals: red 10 AWG wire to the positive terminal, black 10 AWG wire to the negative terminal, and the four balance wires at each terminal in series (order is important!)
7. Glue the power and balance wires onto the body, and cover exposed conductors with kapton tape.

####Technical details

• This battery was specifically designed to fit on the inexpensive and well-designed F450 quadcopter frame (see image)
• The cell-to-pack mass fraction for this battery is around 78% (which is independent of cell chemistry since most 21700 cells have a mass of around 70 [g])
• The specific energy of the battery using Molicel P42B cells is 134 [Wh/kg], while the specific energy of the battery using Samsung 50S cells is 153.5 [Wh/kg]; the takeaway is that in order to get the longest flight time you should use the cells with the highest charge capacity which meet your max current draw constraint: here the Molicel P42B cells have a charge capacity of 4.2 [Ah] and continuous discharge rating of 45 [A], while the Samsung 50S cells have a charge capacity of 5 [Ah] and a continuous discharge rating of 25 [A]. Given that when cells are in parallel their charge capacities and continuous discharge ratings are additive, we have for the Molicel P42B battery a theoretical charge capacity of 8.4 [Ah] (via 4.2 [Ah] * 2 in series) and a continuous discharge rating of 90 [A] (via 45 [A] * 2 in series), while for the Samsung 50S battery we have a theoretical charge capacity of 10 [Ah] (via 5 [Ah] * 2 in series) and a continuous discharge rating of 50 [A] (via 25 [A] * 2 in series). Since the quadcopter draws ~6 Amps in hover, we should use the battery with the higher charge capacity but lower discharge rating to get maximum flight time, since it's discharge rating of 50 [A] is more than sufficient for our needs.
• At 10 [A] discharge, the measured charge capacity of the Molicel P42B battery is 6.514 [Ah], which is 78% of the theoretical charge capacity (via 6.514/8.4*100)
• At 10 [A] discharge, the measured charge capacity of the Samsung 50S battery is 7.929 [Ah], which is 79% of the theoretical charge capacity (via 7.929/10*100)