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.
According to the Panasonic NCR18650B datasheet, each cell is rated for 3.2Ah. Assuming a target of around 1C per cell, we should be safe pulling 3.2A per cell. If each module consists of 36 cells in parallel, it would safely support around 115.2A, which provides us with some buffer space. With a nominal voltage of 3.6V and safe range between 2.5V and 4.20V per module, we can achieve a maximum pack voltage of 151.2V with 36 modules in series. This results in a nominal pack voltage of 129.6V and minimum pack voltage of 90V. This is an approximately 15kWh pack.
Thus, with a 36s36p configuration, our total pack would require 1296 cells. At 48.5g each, we would have a total battery mass of 62.9kg. If we wanted to aim for a 60kg pack, we could build a 36s34p pack, resulting in 1224 cells at 59.4kg. By our target of 1C, this would still safely support 108.8A.
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.
Our current design takes advantage of 8s6p 18650 brackets available from China. To maximize airflow without compromising total volume required, we have decided on the cell arrangement shown below, where the x's mark the location of cells within a particular module. This allows for an air channel for each row of cells, while still packing the cells reasonably closely. Each module would then be approximately 7cm x 12cm x 16cm. With airflow from the side, this should adequately cool the pack. Modules would be arranged with the two air channels are running parallel to the box bed.
| x | x | x | x | x | x |
| x | x | x | x | x | x |
| x | x | x | x | x | x |
| x | x | x | x | x | x |
| x | x | x | x | x | x |
| x | x | x | x | x | x |
We originally planned on spot-welding a grid of nickel strips to the cells. This is standard procedure for building 18650-based packs, and is much safer and more reliable than soldering. We would then solder 6 or more 10 AWG wires to the grid, terminating the wires in a blade connector. However, we have identified a number of concerns with this approach. The available connectors have a low lifespan of 25 mating cycles and require enough slack in the wire and clearance to actually connect and disconnect them. In addition, spot-welding all of our cells and soldering all the wire would be a labor-intensive task. Spot-welding requires trained operators and a calibrated machine, which are major bottlenecks considering around 3000 welds would need to be done.
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
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.
The AFEs that we are considering support up to 12 modules each, so with 36 modules, we will most likely put 24 modules in one box and 12 in the other. The pack with fewer cells will also contain the power distribution and BMS systems. If necessary, we can split the boxes differently, but this would require an additional AFE and processing of an incomplete AFE in the middle of our daisy-chain.
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