SHIFT EV is converting this VW bug to electric power for a couple in Washington state.
The basic design goals are:
- Minimum Range: 150-200 miles from fully charged.
- Acceleration: Similar or better than stock gas configuration
- Instrumentation: Stock speedometer, fuel gauge to display correct State Of Charge (SOC), and inconspicuously placed "current limit" and "error" LED's
- Charging: SAEJ1772 Level 1&2 (110VAC and 220VAC)
- Heat: PTC (electric) heater, Operated by stock heater controls
Evaluating battery type and capacity we have to consider the energy consumption rate for typical driving. Many bugs have been converted in the past with efficiency claims between 200 and 300 Watt-hour per mile (Wh/mi). Many variables contribute to this efficiency such as weight, tire type, motor efficiency, speed, etc. To determine battery capacity we'll take the upper and lower end of our range goal, and multiply them by 250Wh/mile.
(250Wh/mi) x (150 miles) = capacity of 37,500Wh, or 37.5kWh
(250Wh/mi) x (200 miles) = capacity of 50,000Wh, or 50kWh
With target battery capacity understood, we then look at available motor and controller/inverter solutions that provide good efficiency and power per dollar for this application. We narrowed it down to either an HPEVS ACXX or one of the NetGain Hyper9's. Ultimately settling on the Hyper9. That locks us into a maximum and minimum voltage range that our batteries must provide.
After reviewing a few types of batteries in different scenarios, the Tesla battery modules used in the Tesla Model S and X are a good option for this build in several ways.
- Slightly used Tesla S and X battery modules are 5.2kWh (depending on some model variants). 5.2kWh x 10 modules = 52kWh of capacity. That's 2kWh more than we'll need.
- They have a high energy density, meaning they're more compact relative to many other batteries.
- They're capable of more than enough power that our system will draw 750A.
- As long as healthy modules are purchased, they're a good value at around $230/kWh on today's market.
- We can accomplish our drive systems maximum and minimum voltage range
- With 5 modules wired in series, the full and empty voltages fall within the operating range of our motor controller. To use all 10, we'll run two strings of 5 modules this way, in parallel. During discharge, one string will its + and (-) outputs in parallel with the other string. To avoid hazards is critical to have a BMS that properly monitors and manages the two strings and decides whether each string is allowed to connect the DC output bus that is used by the motor controller or any other loads. For more on this surprisingly complex topic, start with this document by Orion BMS.
With the high-level system decisions finished, we move into how we're going to properly and elegantly fit and wire these parts in the space we have. Beginning with the significant battery pack spaces.
Mockup of proposed battery box design, to confirm fit of the complex battery box shape.
3D Scans of the car
Test assembly of the 5 Module battery box for under the hood. Those are dead modules we use for mock-ups like this.
Front box in place. Fitting the stock sized spare tire is a bonus.
5 Module battery box for the rear.
5 Module battery box in place, allmost. It will move rearward a bit more on final assebmly.
More text and images content to come...
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