Pack Design
Our primary power concerns and limitations are from our NGM-SCM150 motors and WaveSculptor 20 motor controllers. According to their respective datasheets, the NGM-SCM150 has a peak power consumption of 7.5kW and the WaveSculptor 20 has a continuous voltage maximum of 160V. Aiming for a maximum pack voltage of 150V gives us some buffer room and results in a peak current draw of around 50A per motor from the motor controllers.
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Note: To clarify the nomenclature we're using here, 36s denotes that there are 36 individual units (modules) connected in Series, while 36p denotes that there are 36 individual units (18650 cells) in parallel.
Module Design
We define each set of parallel cells as a module. Our goal is to design the modules such that they will be relatively easy to manufacture, modular, and easy to replace. We have two major concerns—airflow and electrical connections.
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To reduce labor, we are considering a purely mechanical pack. This approach uses a dimpled copper grid in place of the nickel grid, relying on compression through foam (and possibly magnets) to ensure an electrical connection. Even pressure will applied by running threaded rod through a number of modules in series with backing plates between each module. To achieve our series connection, we plan on extending the copper grid vertically and adding holes so that adjacent packs can be bolted together. We will likely use a copper block as a spacer, which can serve a dual purpose as our voltage sense tap. For safety, we plan on covering all exposed copper with insulated caps.
Inspiration
Original solderless pack design
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Layout
An approximately 60kg pack will be difficult to lift. With additional electronics and the added weight of connectors, brackets, and the box itself, we expect the pack to be between 70-80kg total. To make this easier to manage, we are considering splitting the pack into two boxes. Note that we have approximately 40" x 47" x 9" of space to work with.
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Both packs will also need an HV relay. The main box will also need to contain our auxiliary battery.
Follow-up Discussions
2017-03-01 Battery Pack Design Review
Decisions
Ultimately we decided to go with a Spot-Welded Pack Design for MSXII.