EV Conversions

What is a Contactor?

In the most basic sense, a Contactor can be thought of as a high power relay. Both a relay and a Contactor refer to an electromechanical switching device where a small current is used to create a magnetic force that closes an electric contact. Contactors differ from relays in that they are designed to switch currents of over 1000 amps and operate at voltages up to 800 volts.

In the EV Industry, the term Contactor is used to refer to the electro-mechanical device used anywhere to switch high voltage and current on and off. Typically they are found at both the positive and negative sides of the main HV battery, the pre-charge circuitry, the on board charger connection and any devices that have access to switched high voltage power.

Gigavac is the go-to for contactors, especially the GX Series units with Aux feedback signals. The AEM EV VCUs will use this Aux feedback for confirmation that the contactor has closed. We primarily use this on the negative contactor as the positive state is inferred through the model logic.

I want to convert my vehicle to electric, what should I be considering?

  • High Voltage or Low Voltage? Before you do anything you need to decide what base voltage you are going to use. Low voltage systems (~80v to 130v) have been used for years, are generally lower cost, lower power systems and mostly DC motors (with some exceptions). High voltage systems (300v-400v) are what the OEMs have used for the last decade and are typically higher power and use AC motors w/inverters. If you want to use any components from OEM donor vehicles like Tesla, Leaf, Volt, Bolt etc. then you will likely do a high voltage setup.
  • Transmission or Direct Drive? Some EV conversions may perform better with multiple gears while high power, high RPM motors may not need them. Some installations will almost mandate their use for mounting reasons (think rear engine Porsche) so do not automatically dismiss a transmission.
  • Drive layout considerations. Are you converting a RWD and want to retain a traditional longitudinal driveline with a live axle? Converting a FWD? Want to convert a RWD car by putting a complete motor and differential in a new fabricated rear subframe? The physical layout of the driveline needs some thought and you may find it is not a simple as you think. Even with FWD conversions, make sure the rotation of the EV motor will work and that the physical configuration fits. An example here is the Tesla front motor has the motor mounted behind the differential and won’t physically fit in most FWD engine bays since ICE installations typically have the engine mounted in front of the differential (you can mod the Tesla motor to spin backwards but that is another subject).
  • Suitable Motor and Inverter. Assuming you choose to do an AC motor then you need to determine the power you require and see what is available. The Inverter needs to be compatible with the system voltage you choose.
  • Emergency brake and/or parking pawl. This is a big issue and it is almost never mentioned. EV’s almost never have a parking pawl and you can’t leave them “in gear” so a quality E-Brake (preferably automatic activation) is an absolute must.
  • Cooling system for the motor and inverter. The motor and Inverter will make heat and need cooling. It won’t be anywhere near as much as an IC engine makes but it still needs to be dealt with or the inverter will start de-rating the power to protect itself. The cooling system usually includes a cooling loop with pump, radiator and fan. In some systems you may have separate loops for the motor and inverter.
  • Battery with BMS. A suitable high voltage battery will likely be the single largest expense of the build. It needs to be sized properly both in total energy capacity as well as instantaneous current capability to support the power you want to make.
  • How big a battery do I need? While it is impossible to say with any degree of confidence given the near limitless options for EV conversions, here are some very rough estimates for how many miles you can expect to travel for each useable kw/h of capacity your battery has.

Economy driving = 3.2 mi/kWh

Normal driving = 3 mi/kWh

Spirited driving = 2.5 mi/kWh

Racing = 1-2 mi/kWh

There are so many variables that affect this that you should take those numbers with a massive grain of salt.

  • Thermal conditioning system for the battery. The temperature of the battery is important and it needs to be monitored and kept in a relatively tight window. If it is too cold, it needs to be heated before charging or driving. If it is too hot it needs to be cooled. If you want to extract the absolute maximum performance from the battery it needs to be preconditioned to an exact temperature before use. This requires a fluid heater, a pump, a radiator, a fan, diversion valves, and a VCU to determine what to do and when to do it.
  • On-Board Charger. Unless you are building a dedicated race vehicle that never strays far from the pits, it will need to have an On Board Charger. These modules allow you to connect to standard Electric Vehicle charging points either at home or in public areas.
  • DC/DC Converter. Unless you are building a dedicated race vehicle that runs for very short periods and has a large enough 12v battery to not need to be charged during use, your EV will need to have a DC/DC converter to keep the 12V battery charged.
  • Some method of providing cabin heat for the occupants. Unless you are building a race car with no heat or are in a warm climate, you are going to need to provide some method to generate cabin heat since there is no IC engine generating waste heat that can be used to keep the cabin comfortable.
  • Some method of providing air conditioning for the occupants. Unless you are building a race car with no A/C or live in a cool climate, you may want to provide A/C for the cabin. EV’s don’t have a spinning engine to spin an A/C compressor so you have to install a compressor specifically designed for an EV.
  • Some method of providing power steering. If you want to have power steering you need to think about this. There is no spinning engine to drive a hydraulic pump so accommodations need to be made.
  • Some method of providing power brakes, Typical power brakes are vacuum operated but there is no IC engine source of vacuum.

What is a DC/DC Converter and does my EV need one?

A DC/DC converter is basically an EV’s alternator that generates the charging voltage for the 12v battery. EV’s still have a normal 12v battery and needs a charging system since almost all the electronics in the vehicle (headlights, radio, dash display etc.) are 12v. A DC/DC converter is a module that generates the low voltage charging current using your EV’s main HV Battery as the source and can output as much as 100 amps continuously.

Some DC/DC converters are stand-alone devices, requiring only an ignition switch input to activate the charging output, while others allow control over CAN. These allow the user to activate it, set the output voltage and max current as well as receive updates with the system voltage, output current and diagnostics over the CAN bus. AEM VCUs support both types.

Almost all EV’s will need a DC/DC converter with the only exceptions being a few specialty vehicles like ¼ mile drag cars that wouldn’t have run an alternator either.

What is a Motor Inverter and does my EV need one?

If you have an AC drive motor, you need an inverter. The inverter takes the high voltage, high current DC power that is provided by the battery and converts it to the alternating current electricity that is required to spin an AC motor. The inverter also controls the frequency of the power supplied to the motor to control the rotation speed and torque output of the motor. The design and operation of the inverter is closely tied to the motor being driven and in most cases the motors that any specific inverter can drive is determined by the inverter manufacturer. In some cases the inverter and motor are combined into a single unit and supplied as a matched set. In other cases inverter manufacturers publish a list of motors that can be run by their inverters and the configuration settings needed to do so.

What is a Battery Management System (BMS) and does my EV need one?

A BMS is considered by many to be the “brain” of a battery pack. It closely monitors the battery condition down to individual cell levels and, working with the Vehicle Control Unit (VCU), tailors the energy flow in the vehicle to protect the battery cells under a wide range of operating conditions. It also balances the energy available within the individual cells allowing the most efficient use of a batteries capacity and identifying problem cells before irreversible damage is done.

A BMS provides individual cell protection by monitoring each cells voltage as well as the current flowing into or out of them. This is used along with the cells characteristics to determine the maximum safe current limits in its present condition which it communicates to the VCU to set limits on available power as well as the allowable regeneration energy from the brake by wire function. Having a BMS allows the maximum power to be safely extracted from a battery module at any given time.

The cell balancing feature of a BMS extends the life of the battery by continuously matching the individual cells state of charge with each other. Batteries packs consists of many individual cells that have to work together. To fully use the available energy capacity from the battery all the cells in a pack need to be kept at the same state of charge as the worst cell in a pack determines the current limits for the entire pack and an imbalance in an individual cells state of charge results in an overall loss of effective battery capacity.

What is an Electric Vehicle Supply Equipment (EVSE) and do I need one?

The EVSE is the interface between your EV and the electrical grid and its primary mission is to keep the charging process safe. The EVSE could take the form of a commercial charging station or a simple home based charging cable that came with your vehicle.

A basic EV charging system has 2 main components, some sort of EVSE and a vehicle mounted On-Board-Charger (OBC). The EVSE communicates with the car and determines the correct charging based on its own limitations and the maximum current the car can receive. It also sets safety limitations and monitors the connection status.

Once connected, the EV’s on board electronics provide most of the control decisions regarding charging with the EVSE simply providing power and some safety monitoring.

There are a few different charging standards provided by EVSE’s and used by vehicles and they much match for charging to be possible. See the Charging Standards Explained FAQ for an explanation of the different types of charging standards that exist currently.

It is very common for a user to have a home EVSE to recharge their EV after use. Many users use this exclusively and almost never use a public charging station but of course range and driving habits will dictate this. Home units can be inexpensive and low power units can even plug into basic 110v outlets.

What is an On Board Charger (OBC) and does my EV need one?

A basic EV charging system has two main components, some sort of Electric Vehicle Supply Equipment (EVSE) like a home-based charger or a public recharge station and a vehicle mounted On-Board-Charger (OBC). The OBC provides the vehicle half of the means to recharge the battery from the AC mains. Basic EVSE equipment uses a standard J1772 connector and relies on the vehicles OBC to tailor the power delivery to the specific vehicle requirements.

The OBC is a smart device that is tailored for your specific vehicles battery configuration and tailors the generic EVSE power into something usable by your vehicle. In many cases the supplied EVSE power is simply switched mains power with some safety interfacing taking place. The OBC takes the power (AC 110 – 265) delivered via the charging connector and converts it to the DC voltage and current suitable for the vehicles battery condition to be charged.

In some cases, race vehicles may choose to not have an OBC in the vehicle itself in an effort to save weight, instead keeping it in the pits to recharge the battery as needed. This would almost never be done with a street vehicle as it eliminates the ability to recharge the vehicle except at a predetermined location.

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