Electric Vehicle Conversion/Control and interlocks

Note the presence of hazardous materials and conditions that must be approached with proper precautions and procedures to avoid damaging, injurious, or even fatal consequences.

It is possible to build an EV conversion without the safeties shown below. These safeties are intended to assist and protect the casual driver - one who may not be informed of the limitations or quirks of the conversion or a driver with long experience and ingrained habits. The goal of these safeties is to reduce the chance of improper operation by a driver and should always be included in any EV conversion.

System ready indicator
Some visual indication of system readyness should be provided, since there will be no noise or vibration from an idling engine. Often a converter will simply ensure that the low battery light will be illuminated whenever accessory power is available. More sophisticated systems will illuminate a ready light when the run circuit is active. If provided, this run light may be configured to illuminate at a lower brightness when running at night. The run circuit may also be routed to a dashboard voltmeter. By providing a push-button that switches this voltmeter to a constant battery circuit (e. g. the clock power), this meter may also be used to partially diagnose non-starting conditions.

Vacuum control
For vehicles with vacuum operated power brakes it is necessary to include a vacuum pump and vacuum reservoir, check valve, pump, pump control, and pump power. Vacuum pumps are powered from the twelve volt vehicle service, usually provided by an auxiliary battery. The circuit illustrated is complex but of low cost. A single vacuum sense unit may be obtained from specialty suppliers which has separate on and off adjustable setpoints, eliminating the need for several relays.

Vacuum alarm
A separate vacuum sensing switch should be used to alert the driver of low vacuum. This should activate just below the low limit of the vacuum pump control turn-on setting. This can be connected to an existing unused warning light such the parking brake/dual brake differential alarm light or some other available indicator such as the low oil warning light (provided when no oil pressure gauge is provided and not needed in an EV) or may be directed to a newly installed indicator. Most indicator lights in modern vehicles provide power to one side of the lamp at the instrument cluster and route the other side to ground the indicator circuit at the distant sensor(s) to illuminate the warning. The addition of a low vacuum acoustic alarm enabled after the initial start pump-up is also appropriate. A vacuum gauge is essential for setting the vacuum switch control points and may be included permanently as a dashboard or underhood instrument.

Startup circuit
The goal is to make the vehicle operate safely and to be started using the Start keyswitch position - similar to a conventional ICE vehicle. The continuation of the run state here requires that the vehicle's ICE ignition circuit be active to operate, enabling the keyswitch to be used to reliably shut off the system. The start-run circuit should control application of pack voltage throughout the system by activating the main contactors and by applying pack voltage to the twelve volt converter. This will also control the operation of the vacuum pump, which from a cold start (zero vacuum) also offers the effect of an acoustic signal that the vehicle has started, but is not yet ready for service until the low vacuum warning is off.

The startup circuit is disabled by the charge-door-open sense, AC power-applied relay (if installed), and the off-throttle detector switch, and the clutch-depressed or transmission-neutral switch. With appropriate configuration of the startup circuit any of these error conditions should cause an alarm condition and acoustic alert if a start is attempted. Note that once running, any open start interlock will not cause the system to halt.

Charge door open sense
The goal of this sensor is to reduce the likelihood that the vehicle will be driven off while connected to the charging source. For connectors that are locking (such as twist lock or paddle connectors) this can cause severe damage to the equipment or the structure to which it is attached. Note that if the access door circuitry is damaged in a collision it may then not be possible to move the vehicle. For this reason the door sense switch wiring should include a bypass disconnect to be used in case of emergency (this is not a switch but rather a pair of properly polarized wire connectors). This door sensor is made a part of the start-run circuit in series with other interlocks.. A magnetic reed switch (normally closed when close to an appropriate magnet) can offer simplicity in mounting. A simple, though unreliable, technique for mounting magnets and switches is to use the sticky outdoor tape used for adhering house numbers (e.g. 3M outdoor tape), backed up by a plastic "zip tie" through small drilled holes. The tape holds the positioning of the device and the zip tie ensures against tape failure or mechanical disturbance. This method is considerably faster and simpler than using screws or nuts and bolts. If mounting places are not conveniently located in the vehicle then a soft aluminum strip of 1/16 to 1/8 inch thickness can be formed and attached to the vehicle to make a mounting for the switch or magnet.

AC power applied relay
Some vendors sell an AC relay, intended to prevent operation while AC power is applied to the on-board charger. Note that if the charging station is turned off, or the power has failed or ground faulted that this device will be completely ineffective. For this reason the charge door sense mechanism described above should be also included.

An AC voltage relay is more productively applied to the charging circuit as shown in another chapter, for it can both ensure the removal of AC ground faulting potential from the motor via the controller pre-charge resistors (a problem in older motors in damp conditions from the presence of slightly conductive brush dust) and may also be configured to remove pack voltage from the charger when not charging and so enhance underhood safety.

Off throttle detection switch
With an EV it is possible that it be left in gear with the clutch out while the system is activated. As it is a common habit among ICE vehicle drivers to open the throttle during start (a procedure to release the choke in older vehicles), the vehicle may accelerate. To avoid this an "open throttle" sense switch is included in series with the access door sense as part of the start-run circuit. This open throttle sense is not necessary if a neutral lock out switch is included (this is part of an automatic transmission and may be found on some manual transmissions that have been imported directly from Japan) but is recommended for additional safety.

Clutch depressed switch
It is possible that the throttle could be pumped by an inexperienced driver (again, a habit developed from starting older vehicles in which the accelerator depression releases the choke and operates the accelerator pump in the carburetor to "prime" the engine). By including a switch in the start-run circuit that ensures that the clutch is depressed an additional level of safety is provided. (Some vehicles built in the 1940's used full depression of the clutch to operate a floor mounted starter switch, so this is not a new idea.)

Neutral safety switch
All automatic transmissions incorporate a neutral safety switch. More rarely this switch may be found in a manual transmission, or in some cases, a provision for its installation may be incorporated, but no switch is installed (this may be found in some non-us made vehicles). If available, it should be incorporated instead of the clutch depressed switch. This switch will be closed when the transmission is in neutral so that it may be included in series with the off throttle and charge door switches.

Service brake depressed
The brake-light pressure switch may be used for additional startup security. In combination with the clutch depression switch this ensures that the driver's foot is likely not on the throttle pedal.

Backup circuit
Electric vehicles can operate at very high starting torque. Most transmissions (especially truck transmissions) use a very low gear ratio in reverse, lower than low gear. To obtain this low ratio they use a very small diameter driving gear, subject to breakage if over torqued. A throttle restrictor should be used when reverse is engaged. This may be done by electronically limiting the effective throttle dashpot range with relays and resistors or mechanically by using a door unlock motor or a solenoid to limit the throttle motion when reverse gear is engaged. (Note that light truck conversions with four or five speed transmissions are usually operated starting in second gear, not using first gear except for uphill starts.)

Shutdown circuit with lights on and other alarms
The diagram illustrates a headlight alarm and a headlight reminder. If the system is shut down and the headlights are on an alarm will sound. If the wipers are on and the headlights are off a reminder chime will sound.

Electric vehicles using manual transmissions require that the parking brake be applied - they are unforgiving if this is neglected as there is no compression of an ICE motor to restrict vehicle motion if it is left in gear. For this reason an additional circuit is added to the start-run latch circuit. When the vehicle is turned off with the key an alarm is sounded if the parking brake is not engaged. Power for this circuit should be on at all times so the circuit providing power to the clock and/or dome light should be tapped. While it is easy to detect "off stop" engagement using the existing parking brake alarm switch it is better to add a separate circuit that detects a substantial operation of the parking brake - to the point where it is effective in stopping the vehicle. (Late model vehicles using automatic transmissions will typically prevent key removal unless the transmission control is in the "park" position.)

It is also a good idea to curb the wheels where a curb is available (in any vehicle) and to use a wheel block in other cases where no parking pawl is used.