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White Paper - The Ignition Switch conumdrum

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

Standard car anchor

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
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 anchor

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
Using a 12 V Lead-Acid battery.
Place the ignition switch between the Li-Ion pack and the DC-DC converter anchor

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
Ignition between pack and converter.
Leave the DC-DC converter always on anchor

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
Converter always on.
Power the DC-DC converter through a relay anchor

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
Relay between pack and converter.
A solution anchor

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

 

Creative Commons License "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

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