An EV battery isn’t like a typical car battery. It’s a collection of individual cells connected in a way to meet specific power requirements. These cells are the building blocks of a battery, and they’re connected in parallel or serial (or some combination of both) to meet your needs.
Ev battery vs car battery
There’s many resources to help you build your EV battery.. The one I used (and recommend) is Micah Toll’s DIY Lithium Batteries. He explains things very well and gives examples and problems to solve. By the time you finish you’ll have a good understanding of cell selection and configuration.
An EV Battery has several components:
- Battery Management System
- Cell Interconnects
- Environmental accommodations (heating or cooling)
Let’s look at each one individually:
I chose Lithium Iron Phosphate (LiFePo4) cells because they don’t explode (like Lithium Ion cells) and were available in the US at an attractive price point. I could have salvaged cells from a wrecked Tesla or Nissan Leaf, but that would have been another project. Maybe next time…
Battery Management System
A Battery Management System (“BMS”) keeps all of the cells at the same voltage during charging and discharging. Think of it as an insurance policy to maintain the life of your battery. It maintains a balance by monitoring each cell with a balance wire. I can then monitor the status of the battery with a bluetooth app on my phone:
I used ¼” thick aluminum that I cut on my CNC to make my interconnects. Aluminum is a little more than half as conductive as copper (which is half as conductive as silver) and a good choice since I got it for free! I used Noalox anti-oxidant paste on all connections to discourage corrosion and galvanic reactions. The cells are connected positive to negative until I get to a total of 72 volts. LiFePo4 cells have a 3.2 nominal voltage, so I used 24 cells connected in series. The interconnects have slotted holes to provide some flexibility when the car goes over rough surfaces.
Cells generate heat as they discharge, and the heat must be dissipated so the cells don’t get damaged. I had no way of determining the heat dissipation required so I build in some passive ventilation channels with air intake holes at the bottom of the case, a 1.5” headspace “plenum” and air exhaust holes on the top cover.
The case serves several functions:
- Keeps the cells tight and immovable so they don’t bulge during charging and discharging
- Keeps the cells dry
- Supplies a method to connect the cells into the vehicle.
I’m a woodworker so I used wood for the case. Wood is nonconductive when dry, but wet wood can conduct electricity. I used waterproof and boil proof Okume plywood from Boulter Plywood in Medford, MA. This plywood is constructed in such as way that all voids that could attract moisture are filled with non-conductive adhesive. To make sure I was using a safe material I soaked some overnight and then tested the continuity with a multimeter. The case construction features dadoed and rabbeted construction to ensure strength, hold the cells tightly and accommodate slight bits of movement when driving. Then I applied 4 coats of marine polyurethane and sealed the joints with silicon caulking. There’s a lot of pieces!