Introduction

One of the main tasks of Power Systems is to design and manufacture the main high-voltage battery for the solar car each year. At a minimum, the goal of the battery is to be able to supply power to the motor and be big enough to have a constant, full, power flow of power from the solar array.

Motor

The motor we use, unless we decide to change it this year (highly unlikely) will be the Mitsuba M2096D-III 1WD which is a motor built specifically for powering a solar car.

Requirements:

• • Nominal voltage: 96 V
  • Nominal power: 2.0 kW
  • Nominal current: ~21 A (2000 W/96 V)

Battery Pack Configuration

Individual Cells

• • 288 cells total
  • Each cell is rated for 5 Ah @ 3.7 V
  • Each cell weighs ~6.82 g (~19.6416 kg total) which allows us to stay under regulation weight (20 kg)

Modules

• • 32 modules total, 9 cells/module
  • Each module is rated for 42 Ah @ 3.7 V
  • The modules are connected in parallel to provide more current

Segments

• • Three segments total:
    • 2x 11 Module Segments
    • 1x 10 Module Segments
  • The segments are connected in series to provide higher voltage

In total, the battery pack has a nominal voltage of 118.4 V and a total capacity of 44.27 Ah.

Early Cell Manufacturing

When we receive the cells from the manufacturer, they normally come bare. This means for us to handle them and use them as batteries safely, it is better to wrap them. See “Battery Cell Wrapping Guide” for information on that process. During this, we assign each cell a number and start the documentation. 

Testing

Once we have gotten the cells safe to handle, we start charging, balancing, and documenting cells. The main thing we document is the impedance which varies from cell to cell. It is important to match cells with a similar impedance to increase a single module’s efficiency for more information on that check this out. 

Manufacturing

This is the majority of what Power Systems does with batteries, it is a lot of soldering and a lot of spot welding. After we have sourced cells and materials; notably nickel plate, some sort of connectors, braided cable, and some sort of cell array holder, we are good to continue. For the most part, this depends on the battery pack design, but I will delve into what we did this year with our battery. We started by assembling the connectors that would be soldered onto the nickel plates to connect each module in series. For most of our battery designs, we have used Bullet Connectors, something like these. They are useful because of their quick-disconnect capabilities in case of a problem. We solder them in line with a snippet of braided cable. That bit of braided cable also must be a specific length to minimize power loss but still allow enough room to comfortably unplug the modules. After soldering the connector to the braided cable, we put a layer of heat-shrink tubing on the connector to limit the possibility of the leads shorting to something, that might be catastrophic. When doing this it is important to leave enough space at the other end of the braided cable to have enough room to have a very sturdy connection when soldering to the nickel plate. For this year’s battery we have three total between each module for redundancy, just in case one of them breaks. So, when three of the same type (male/female) are done, you can move to soldering them on the nickel plate. Nickel plate is quite difficult to solder onto, so scratching the surface, or applying solder to the plate beforehand helps. Once you have soldered the three connectors, you will next need to spot weld it onto a soon-to-be module. Make sure when doing this to select cells that have similar impedance and follow the documentation. Once your cell holder is assembled, make sure all the connections are pointing the correct way, as noted here in step 15, put in your cells making sure that they are all facing the same way. After that, place your nickel plate on top and begin spot welding. For spot welding, please read the safety documentation regarding it, and after that read over step 18. Once you have finished spot welding each cell to the nickel plate, you have one more nickel plate to go, and make sure it is the opposite connector type. After that, you have completed a single module! For this year’s design, you only need 31 more!

Post-Module Manufacturing

After all the modules are completed and ready to be connected, multiple things are required before that happens. We need to add all of the BPS (Battery Protection System) wiring, which includes temperature sensors and voltage taps. We will also need to fabricate the longer cables that will connect each battery segment. At this point, the battery is ready to hand over to electromechanical where they will be routing and planning connections for the wiring in and outside of the battery box. We will be tasked with helping them with this, but the majority of the battery-related aspects are complete. Once you have gotten to this point making spare modules is important in the occasion of a module on the car breaking in some way.