Pack Design

Our goal is to create a 150V pack that safely supports a max load of around 100A. This is approximately 7.5kW per motor at peak, which is the peak power consumption of primary power concerns and limitations are from our NGM-SCM150 motors . Note that the 150V target is with a fully charged pack, not nominal voltage. Our WaveSculptor 20s have 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, so we never want to exceed that with our pack. 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 maximum nominal voltage of 3.6V and safe range between 2.5V and 4.20V each cell, 

Module Dimensions

We have decided to use 36 modules each containing 36 cells. As of now each module will be 7 cm wide, 12 cm long and  16 cm high with each module being 8 cells high (two empty columns to optimize air flow) and 6 cells long.

Battery Box layout 

As of now we will be using 2 separate battery boxes, one containing 12 modules and the relays and the second containing 24 modulesper 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.

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 arrangement shown below, where the x's mark the location of cells. Each module would then be approximately 7cm x 12cm x 16cm. With airflow from the side, this should adequately cool the pack.

To connect the cells electrically, we plan on spot-welding a grid of nickel strips to the cells. This is a standard procedure for building 18650-based packs, and is much safer, more reliable, and easier than soldering or a purely mechanical solution. In order to carry the current, we plan on running 10 awg copper wire in the gaps between the blocks of cells which then run to blade connectors. To support our peak current draw of 100A, we should have 6 or more 10 awg wires connecting each module. Ideally, these should be evenly distributed for current sharing.

To reinforce the copper wire, we can punch holes in the nickel strip grid where the wire will be run and source thin copper discs to be placed where a cell would normally go. Then, we can solder the wire directly to the copper disc, providing some support from the 18650 brackets and reducing stress on the nickel strips. The purpose of punching holes in the nickel strip is to increase the copper to copper surface area. The goal is that during this, the nickel and copper would also be joined. This operation could be done before spot welding to reduce heat spread to our cells.

Note that this design is still in development and is subject to change. We have yet to determine how we'll be mounting these modules to the box itself and how it's supposed to withstand 20g's of force. We also need to determine which connector best meets our design requirements.

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.

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.