How Kill Switch Location Impacts Current Control

In motorsports a kill switch is a safety requirement, it can be used as a tool to mitigate battery drain, and if used improperly can fry electrical components.  A kill switch can see spike current of 1,000 amps during start up, 150 amps steady state in some applications, and has a voltage source on both sides of the switch (battery and alternator). Comparing a kill switch to the up and down stream impact of opening flood gates on a dam is overkill but thinking a kill switch is as simple as a light switch can get you in trouble. This article aims to define the purpose of a kill switch and how to properly implement one.

Storage

Turning off the battery cutoff switch while the car is parked eliminates battery drains and accidental shorts while the car is being serviced.  If the switch is near the battery, there is only a short length of unprotected wire.

Starting

While starting the engine, all of the electrical current required comes from the battery.  The largest portion of this goes to the starter.  This is typically in the hundreds of Amps.

Warmup

During the first minute of engine operation, the alternator is supplying the current needed to run the engine’s electronics.  The alternator is also putting as much current as it can into the battery to recharge it.

On Track

Once the engine is warmed up and the battery is recharged, most of the alternator’s current is likely to be used for component cooling.  A low level of current is also maintaining the battery.

During a Typical Accident

If the battery cutoff switch is turned off while the engine is running, the alternator can typically supply enough current to keep the engine running.  Imagine the car nose first into a ditch and the drive wheels are off the ground, the throttle is depressed and the corner worker has turned off the battery cutoff switch!

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During a Protected Accident

If a true kill switch system is turned off while the engine is running, the battery and the engine are both disabled.  When the engine stops, every electrical circuit in the car goes dark.  And, safe!

Alternator Storage

If the battery cutoff switch is turned off while the engine is running, the alternator can typically supply enough current to keep the engine running.  Some people address this issue by wiring the starter and alternator straight to the battery.  This ensures that the engine will stop and most circuits will go dark when the kill switch is turned off.  However, it leaves a high current wire to short in the engine compartment!

3 Lug Kill Switch in an Accident

If a true kill switch system is turned off while the engine is running, the battery and the engine are both disabled.  A relatively simple way to achieve this is to use a 3 post kill switch.  This separates the car’s electronics from both of the potential electricity sources. A 3 post kill switch can be purchased directly from our website.

Reading Further

MoTeC has a similar ideology to Electron Speed and has information on how to integrate current protection in to a PDM. If you would like to know more about how to do this or have Electron Speed build this functionality in to your car let us know! A PDM can be purchased directly from our website.

If you want to kill some time and see the effects of power switching on a 500kV system watch this!