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EV Automotive Electronics

This page covers some of the work I did in repairing, relocating, and modifying the original electrical system of my Toyota MR2 EV.

Still present in the car is most of the original wiring. This controls the interior and exterior lighting, powers most of the gauges, the radio, and the power windows and door locks. It is powered by a 12-volt garden tractor battery (an automotive starting battery is no longer necessary as there will never be any engine-cranking loads). The battery is recharged by the DC-DC converter, which takes the place of an alternator, converting power from the Traction Battery to recharge the smaller 12-volt battery. It is possible to do without a 12-volt battery at all in an EV, just relying on the DC-DC converter, but it is a good idea to have a redundant power source in case the converter dies.

Re-tasked components

This table shows my planned "re-tasking" of some of the indicators, gauges, and other original electronic components in the car. I'll get the basic things working to start (idiot lights) and get more sophisticated things light the tachometer working after the car is running.

ComponentNew Function
Water Temperature GaugeController Temperature
Oil Pressure GaugeMotor Temperature
Check engine lightMotor Temperature idiot light
Fan Warning LightController over temperature
Brake Warning LightLow Vacuum Pressure + original functions
Tachometer GaugeMotor RPM
Clutch switchReverse "gear" interlock
Cruise control vacuum switchEV Braking System low vacuum warning switch

The water temperature and oil pressure gauges are both actuated by simple varying-resistance senders. I will have to find similar ones in order to use these gauges for the purposes I want. My Advanced DC motor has a built in over-temperature switch, so I will wire it to the check engine light.

All the indicator lights need to be operated by simple switches that turn "on" (close contacts) when the condition that they check for becomes true.

For the controller temperature idiot light, I used an SPDT water heater thermostat which switches at an adjustable temperature setting from about 100 through 190 degrees farenheit. I'll set it to about 150 degrees, and hook it to the engine cooling fan warning light.

I will use the cruise control vacuum switch for EV Braking System low vacuum switch. It turns "on" below about 8 inches of mercury, which is about half of level of vacuum that the vacuum pump in the EV Braking System should deliver. The remaining brake warning conditions (low fluid and parking brake on) are wired in parallel with the low vacuum switch, so any one of these conditions will light up this warning light.

I hope to use the tachometer as is, by installing a new sender on the motor tailshaft. Tachometer senders are available that can be mounted on a motor. I am naively hoping that the signal that these output is compatible with the original tachometer gauge. If not, I'll build some kind of cirucuit to translate the signal.

Removed circuits

There are a variety of electrical systems in the car that I no longer need. I will be removing or disconnecting these (whichever is easier). This will simplify the remainder of the wiring harness, reduce the likelyhood of a problem occuring, and maybe save a pound of two of weight. These include:

  • Cruise Control Circuitry
  • Air Conditioning Control Circuits
  • EFI (engine computer) circuits
  • Fuel Level Gauge and warning light (just disconnected, could be useful later)
  • Engine starting circuit

Relocated Components

I moved the engine compartment fuse box up to the front compartment of the car because there is no longer room for it in the rear compartment, and because it is more logical for it to be up front, given that that is where the DC-to-DC converter (which provides power to the entire automotive electrical system through this fuse box) is located. I removed unnecessary fuses and relays from this the fuse block, and installed a new terminal block to make hooking up the battery and DC-to-DC converter easier.

DC-to-DC converter

The DC-to-DC converter takes the place of the alternator in a regular car. It powers the automotive electronics and charges the 12-volt garden tractor battery. My Converter is an InteliPower 9100 which is capable of converting input voltages of approximately 120 volts AC or DC into 12 volts DC at 60A, which is more than enough to keep the electrical system in the car functioning.

This unit is actually an onboard battery charger that is designed to be installed in RV's. It is really meant to be run off of 110VAC, but it works just fine when powered off the traction battery pack as well. It weighs less than five pounds and is fairly compact at about 3.5" high by 8" square. This brand-new unit was sold to me by a fellow EV enthusiast for the (very good) price of $150. Normal retail seems to be more like $250 or so.

The DC-to-DC converter should power up as soon as the "ignition" is turned on in the car. If the main contactor is wired such that it opens up when there is zero throttle (such as in my setup), then there must be a second, smaller contactor to switch on and off the power to the converter which stays on at all times when the "ignition" is on.

Physically, I originally planned on mounting the converter in the tunnel area underneath the car, where the gas tank used to go. However, that would make accessing it difficult, so I did not really like the idea. After playing around with different placement options for a while, I decided that I could weld some brackets onto the front strut bar that would hold the converter (as well as some other things) in a vertical position in the space behind the front EV Battery Racks. This is a much more accessible spot for it, not to mention being much better protected from the elements than it would have been if mounted under the car (even with the undercarriage cowlings in place).

I did notice a way to slightly improve the efficiency of the DC-DC converter. Since it is really a battery charger designed to run off AC power, it incorporates a bridge rectifier on the power input side. This rectifier is unnecessary in the EV application. I could remove it, and save two diode drops worth of voltage loss (about 1.5 volts) That's about 1.5 watts of power savings for every 10 amps the converter outputs at 12 volts, or almost 10 watts at full load. That's about the amount of power it takes to run two or three marker light bulbs. Not a huge improvement, but maybe worth doing if I ever need to open up the converter for any reason.

New Functions

The 12-volt system in the car needs to do a few new things in my EV. These are:

  • Energize the main contactor, kill switch contactor, and reversing contactors, incorporating appropriate safety measures and interlocks.
  • Power the vacuum pump in the EV Braking System
  • Power the heater contactor circuit

These additions are covered on the EV Control Electronics page.