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Goals
- Design an enclosure that will house:
- batteries
- Plutus
- Chaos
- Design an enclosure that provides adequate cooling for the battery pack
- Determine how to create modular battery strings that are individually removable from the pack
- Determine how to create strings and electrical connections that maintain these electrical connections between individual cells within a module, and are resistant to vibration
- Design and/or evaluate mechanical battery holders that will hold 18650 cells
- Determine how these battery strings will be mounted to the battery box, and resistant to vibration and sliding
Background and strategic fit
A cell is the smallest individual unit in the battery pack. These are comprised of 18650 Li-ion cells, and will be connected in parallel to form strings. The string will be our smallest removable unit from our battery pack, as designing the pack to allow for individual cells to be removable is probably overkill, especially since we will be replacing our battery pack each race. Multiple strings will be connected in series to form the battery pack that provided the required voltage to our electrical systems.
"Battery Box"
In MSXI, the battery box only contained the battery pack, which was actually not in compliance with the American Solar Challenge 2016 regulations. For MSXII, its new "battery box" will contain:
- batteries
- the Battery Protection System: "plutus"
- Power Distribution: "chaos"
- relays
- solar array relays
- battery relays
Assumptions
Requirements
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Designs being Evaluated
18650 Cylindrical Battery Holder
These are sold by various sellers on AliExpress to hold 18650 cells together, are fairly cheap, and interlock together. In addition, there are guides for nickel strips (which makes spot-welding much easier). Since this solution involves spot-welding, it results in the module as the smallest individual unit.
If we were to use these, we would just buy them in bulk off AliExpress, and not bother manufacturing them ourselves. They're cheap and will save us lots of time.
Battery Blocs
These are an interesting concept that allow us to hold 18650 cells together, and allow us to reconfigure the pack as needed. This design also incorporates mounting points into the pack.
Buying BatteryBlocs for the entire pack would be rather expensive, so we would probably look at designs that draw inspiration from these, and whether we can build something similar for cheaper.
Questions
Below is a list of questions to be addressed as a result of this requirements document:
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Battery Box Mount
The battery boxes will be mounted in the chassis just behind the seats. They will slide in and out on aluminum extrusions bolted to the steel frame. When the rear panel is raised a person will be able to walk into the rear of the vehicle and slide the battery boxes out of their mounting fixtures. The size of the battery box is defined by the size of the mounting fixture in CAD.
Enclosure Material and Structure
The battery box will be made out of Teklam panels bonded with an adhesive and reinforced with 90 degree plastic extrusions (PVC or other). The top panel of the box will be able to slide to allow access to the internals while the box is mounted in the vehicle. This panel should be able to be locked in position once the battery is certified at the race.
Module Structure
The battery box will be made up of several modules (groups of 36 cells held together between two plastic plates.
Electrical Connections
The screenshot above shows the approximate make up of each module however we will not be using mechanical connections or flexures to make the electrical contact. Instead we will copper tabs bolted to busbars running along the top of the box.
Cooling
The modules will all be mounted in the same orientation such that all the cooling pass throughs are aligned. Fans will be mounted inside the box and air will flow along the length of the box.
Additional Components
The boxes will be come in two configurations: master and slave. The following additional components will also be housed in each configuration
| Master | Slave | |
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| Number of modules | ||
| Additional components |
HV Conductors
The HV conductors will connect to either the front panel or inside panel of the box.
Handles
Handles will be required on the exterior of the box to slide the box in and out of the vehicle and lift it.