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 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 theneed 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-.
We have 22 awg high quality wire (it's pre-tinned, high-temp insulation, 32 strands). After some experimentation, we decided 0.35 inch hole works for this wire.
Battery Prep Instructions
- CAREFULLY remove the masking tape from the cells. One of the tabs is sandwiched between the tape, so you have to be really careful.
- Tin the bottom of the battery tabs.
- Clean the bottom of the tabs with acetone and put on lots of flux.
- Put a bunch of solder on the tabs, spreading it out with the iron.
- Suck off the excess solder with the vacuum desoldering tool.
- Flip the battery right-side up, and put the PCB under the tabs. With about 2mm of gap, mark off the tabs.
- Cut the tabs with the CERAMIC scissors. Did I mention CERAMIC? DO NOT USE METAL SCISSORS for the love of god.
- Now solder down the tabs to the PCB, centering the PCB on the battery (not the tabs necessarily).
- Cut 4x 5 cm long 20 awg wires (the brown/red wire is best).
- Strip about 4 mm of insulation off two of them, and tack solder them (with lots of solder) down to the PCB facing off the end.
- Do the same on the other side.
- Using 1.5 cm mylar tape, put strips of tape over the board and wires.
- Flip the battery over, and bend the leads around the narrow end of the PCB.
- Tape that wrapped-around end to strain relieve it.
- Bend the wires at 90 degrees on the underside of the battery towards the back of the battery.
- Tape everything down.
- CAREFULLY and SLOWLY bend the tabs so that the PCB flips around and lies in the "battery protection circuit" area in the front of the battery.
- Tape the hell out of it to protect the leads and the PCB.