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Battery Isolators, Kill Switches and Voltage Spikes

January 05, 2018

This is a problem we’ve seen many times over the years; not only with our products but with our customers using other products as well. One of our customers blew up a brand new Power Distribution Module because of this problem so please take note.

How (Almost) Everyone Wires them the First Time

Most battery isolator switches / kill switches, especially cheap ones, have only 2 terminals. Most people (the first time) tend to just connect it in series with the battery positive terminal:



The problem with this configuration is that if the isolator switch is opened while the engine is running, one of two undesirable consequences can occur:

  1. The alternator continues to provide current, so that the car’s electrical system is still powered up, and the engine keeps running.
  2. The alternator has previously been providing current to charge the battery as well as to supply the electrical system of the car. Even though the alternator’s output is regulated, if the load of the battery is suddenly removed, the voltage output of the alternator will spike. This could be a very high voltage spike, which can then damage electronics powered by the 12V supply (including ECUs, ignition systems, race dashes or power distribution modules).


How an Isolator should be Wired

The isolator must be wired so that the alternator doesn’t power up the car with the alternator disconnected. Here is the most basic way that this can be done:



In the above circuit, the alternator is not isolated from the battery. It avoids the two problems mentioned above, and allows the whole purpose of a kill switch (which is to kill the engine). However some people want to be able to hit the isolator / kill switch to ensure that all the wiring in the car is powered down, so that the car can be worked on without fear of dropping a ratchet onto the back of an alternator or starter motor and shorting out the battery. In that case, there are two options that we’ll present here (of course there are always more solutions):



This is the way that the Adaptronic race car is wired up; in general when we isolate the engine, we just switch off the isolator switch. However if we do want to power everything down for maintenance or work on the engine, we can just flick off the 120A circuit breaker. It also saves complexity compared to mounting a 100A fusible link.


Another technique, is to use a 4-pin isolator switch, if you can find one, which actually has two switches inside which are activated at the same time.