White Paper - The Ignition Switch conumdrum

Where to wire an ignition switch in an EV with a Li-Ion traction pack

Standard car

In a standard car, the ignition switch is in series with a 12 V lead-acid battery. Some loads are powered through the ignition switch, some are powered directly. If you leave the car parked a long time, the loads that are powered directly will eventually discharge the 12 V battery completely. That's bad, but not terrible, as you can still jump start the car.

Standard car ignition.

That approach may or may not work in an EV powered by a Li-Ion traction battery.

Use a 12 V Lead-Acid battery

That same approach works if a 12 V lead-acid battery is used, though with some disadvantages.

• A 12 V charger powered by the traction pack keeps the 12 V battery charged
• The charger must have a low-voltage cut-off, to keep the traction battery from being fully discharged if the EV is parked for a long period (Li-Ion cells are damaged when their voltage drops below 2.0 V)
• The low voltage cut-off will save the battery, but the battery will be needlessly discharged

Using a 12 V Lead-Acid battery.

Place the ignition switch between the Li-Ion pack and the DC-DC converter

Alternatively, if a 12 V DC-DC converter is used (instead of a 12 V lead acid battery), the ignition switch can be installed between the traction battery end the DC-DC converter. That does have a couple of limitations:

• The ignition switch may not be rated for operation at high voltage
• The ignition switch will not be able to power the 12 V accessories

Ignition between pack and converter.

Leave the DC-DC converter always on

Your next approach may be to power the DC-DC converter directly from the traction battery, so that 12 V is always available

• The DC-DC converter must have a low-voltage cut-off, to keep the traction battery to be fully discharged after a long period of inactivity (Li-Ion cells are damaged when their voltage drops below 2.0 V)
• The low voltage cut-off will save the battery, but the battery will be needlessly discharged

Converter always on.

Power the DC-DC converter through a relay

If it is not acceptable to operate the ignition switch at high voltage, you may be tempted to drive a relay that powers the DC-DC converter from a 12 V ignition switch. However, that will not work: you will be able to turn off the EV, but you will not be able to turn it back on:

• The battery voltage goes through the relay, to the DC-DC converter; the output of the DC-DC converter goes through the ignition switch, to power the relay
• That forms a loop, which, once broken, cannot be closed again: once the ignition switch is turned off, there is no longer voltage on it to turn the power back on

Relay between pack and converter.

A solution

Here is a circuit that will overcome all of these issues.

• When the vehicle is on or plugged in, the 12 V charges-up a supercap
• When the vehicle is off, a diode keeps the supercap from discharging
• That supercap is always available to power the ignition switch
• Should the supercap be discharged, plugging in the EV will restore its charge

Using a supercap; charger w/o 12V output (top) and with (bottom).

"State Of Charge estimate with Li-Ion batteries" by Davide Andrea is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Permissions beyond the scope of this license may be available by contacting the author.

Davide Andrea, Elithion, 4/17/09