Structure and Assembly
The battery stack consists of three PCBs (top, middle, bottom) and two 850mAh batteries which are soldered and mechanically attached to the middle PCB. The batteries are connected in parallel, resulting in a 2P battery pack with 1700mAh of charge capacity at 4.2V. The top and bottom boards primarily provide electrical connections and also function to keep the battery compressed for operation in a vacuum.
Before connecting the batteries in parallel, it is necessary to make sure they are at the same voltage to avoid large voltage differences (and therefore current draws) between the batteries.
- Cut and strip the battery leads, keeping them fairly long (a couple inches) and ensuring that they don't touch.
- Set a benchtop power supply to output 4.2V and 0.425A.
- Connect a battery to the benchtop power supply (GND to GND, + to +), taping down the alligator clips so they don't touch.
- Assuming the battery is not fully charged (let's say it's at 3.7V), the power supply will go into constant current (CC) mode and output 0.425A at 3.7V. Once the charge voltage is reached (4.2V), the power supply will go into constant voltage mode and the current will start decreasing.
- When the charge current is below 0.01A, the battery is fully charged. For this assembly, it's probably fine to stop at 0.05A.
- Repeat for the other battery.
The power and ground wires of the batteries need to be soldered to the middle PCB.
- Cut the battery leads to ~1cm.
- Strip the battery leads 3-4mm.
- Lay the battery on the bench and align the leads with the appropriate (Black to GND, Red to +) pads on the middle board.
- Solder.
- Repeat for the other battery.
For ease of assembly, we use double sided polyimide (Kapton) tape to attach the batteries. A flight assembly should use epoxy to additionally secure the battery to the middle board after assembly.
