Tech Info: Other Components
A contactor is basically an electronically controlled switch, designed
to carry a large amount of power. For those familiar with electronic
relays, it's the same principle on a larger scale.
They consist of an coil (electromagnet) which is energised from
a low voltage (typically 12V for EV use). When energised, the
coil pulls contacts closed allowing current to flow through the
In EVs, they are used to isolate the battery pack when the key
is turned off - the equivalent of ignition in an ICE vehicle.
For typical EVs, contactors rated to around 200 amps continuous
are recommended. Contactors up to 600 amps continuous are available
for high-performance EV applications.
Most people are familiar with the function of fuses, and their
use in EVs is no different - to protect the electronics from overloads
and short circuits. Internally they have a conductive section
which will break if a certain electrical current threshold is
exceeded, stopping the flow.
For safety reasons, it is recommended that each group of batteries
in an EV includes a fuse very close to the positive terminal.
Circuit breakers perform a similar function to fuses - protecting
the electronics from overload and short circuit - with the added
advantage that they are resettable. Circuit breakers can also
be tripped manually using a switch.
Circuit breakers are more expensive than fuses but are a good investment
in the long run. For typical EV projects, circuit breakers around
200 amps continuous are recommended.
Petrol vehicles use a cable between the accelerator pedal and the
engine's throttle. In EVs, the throttle end is replaced with a
connection to a pot box, which is basically an electronic throttle
connected to your motor speed controller.
Most pot boxes are simply an industrial variable resistor (5Kohm
is the standard) in a robust packaging. Some newer motor speed
controllers are using digital pot boxes with optical encoders,
which tend to be more reliable.
Most vehicles (electric or petrol powered) have a variety of electronic
systems which run on 12 volts - such as lights, stereo, electric
windows, alarm, etc. Petrol vehicles use an alternator running
off the engine to power these systems.
Electric vehicles typically use a DC-DC converter, which just converts
the high battery voltage into a 12 volt DC output for these auxilliary
systems. Typically, the DC-DC converter will be rated to about
300-500 watts, although this does vary greatly depending on requirements.
As with petrol vehicles, a conventional 12V lead acid car battery
is typically used in conjunction with and charged by the DC-DC
The most important piece of instrumentation in an electric vehicle is a state-of-charge meter, to tell you how much charge remains in the battery pack. It is basically the same function as the fuel gauge in a petrol-powered vehicle. Such devices usually monitor a current sensor or shunt, and integrate current flow to add up the power flow in and out of the battery. Some will even act like a trip computer, telling you how much
range you have remaining.
Two other common gauges you may see in an EV are a voltmeter
to monitor your battery pack voltage, and an ammeter which measures
either the current coming out of the battery pack or the current
flowing through the motor.
Often all of the above functionality (plus more) is integrated into a single monitoring device. The TBS Expert Pro is a popular example of this.
Vehicles with a good battery management system may also have a
display showing the voltage levels of all batteries in
the pack. This is a great way to monitor the health of individual
cells, and to make sure none get over-discharged.