The AV3 battery pack was originally picked during the 2009 capstone project.
Battery Pack Requirements
- SHOULD be a 4-series cell Li battery
- Battery SHOULD cost <500$
- Pack SHOULD have energy-mass density (100,,) W.hr/kg
- Pack energy-volume density SHOULD be (200,,) W.hr/l
- Battery capacity SHOULD be (4,8,) AHr
- SHOULD have dimensions less than (,3.0,3.5) inches in the cross sectional plane of the airframe, length (,,10)inches
- Total battery "unplugged" leakage MUST be < C/(1,5,) year rate (e.g. ~ 100 uA for 4 AHr)
Chosen Battery Cells
We're using a 4S2P pack of "AA Portable Power Corp" PL-5467100-2C (2.75,3.7,4.2) V at 4.250 AHr Li-polymer cell.
- This gives us a (11.0,14.8,16.8)V at 8.5 AHr battery pack.
- We'd like to be able charge this at 1C, which means an 8.5 A charge rate.
- Each cell is 101 x 69.2 x 5.7 mm thick so the pack is roughly 101 x 70 x 46 mm
- Each cells weighs 85 g so the pack weight is 680 g (1.5 lbs).
- Although the pack has no protection circuitry, the battery sensor board does.
- The charge cutoff voltage is 4.20 +/- .03 V
Battery pack enclosure
- Must be stiff enough to prevent accidental punctures.
- Must be soft enough to not be able to puncture batteries if sharp.
- Should be as thin and as lightweight as possible.
- Insulation is a bonus, but not necessary (to protect against cold temperatures at high altitudes) because of the rocket's and battery's thermal mass.
The pack should be held together by double-sticky adhesive-only (thin) tape. It can also be held in the battery box this way, but it will make it essentially impossible to take out if we go this route.
The thermisters for the battery sensor board need to fit in the pack; I'm thinking it's a thin SMT part that gets inserted between cells 2 and 3 and between cells 6 and 7, probably in between two strips of tape.
Battery cell boards
In order strain relieve the battery tabs, we're going to solder them down the tabs to small, thin PCBs that will live on the small thin shelf of each cell, like a battery protection circuit. The shelf area is 65 mm wide, 4-5 mm deep, and about 4 mm high.
What we want the PCB to be < 60 mm x 4 mm, and have an area to solder the tabs, and an area to solder two different wires for each tab. Thus we'll have four wires coming off each cell, two for B+ and two for B-.