How to positive trigger a switched ground battery disconnect solenoid?

Thread Starter


Joined Jan 18, 2023
battery disconnect diagram post 1.png
First I am not an expert but I do have a pretty good understanding of electricity and electronics. I am trying to use a 200A continuous duty solenoid to act as a battery disconnect on a vehicle using the ignition switch as the positive voltage trigger for the solenoid. Forgive my MS Paint diagram. The "bulbs" are not actually lights but just something I did quick to show a load in the circuit. I simplified the circuit but kept the hot at all times load leg to include a more complete circuit for the entire vehicle as simply as possible.

I currently tried to wire it like in the diagram. This works as long as the switch is on, the solenoid is triggered and connects the ground to the battery, if you remove the wire completely it also works as desired, the solenoid loses voltage and opens which disconnects the ground to the battery. My problem is that if you keep the solenoid positive trigger wire attached when you turn off the switch the solenoid opens and disconnects the ground which is desired, however the solenoid constantly clicks since the ground is disconnected because the ground side of the circuit reads system voltage but can't supply the solenoid with steady voltage since its on the ground side with ignition off. I would say it is clicking at around 180bpm or 3-4hz.

I need a way to "eat" this voltage when the ground is disconnected so the solenoid stops clicking but also doesn't interfere when the switch is turned on and goes back to normal system voltage I did try to put a resistor in series with the solenoid trigger on the positive side, the smallest I had on hand was 10ohm which still made it click but much slower and still functioned when you turned on the switch but at 20ohm it stopped clicking but the voltage drop was too much and didn't function when you turned on the switch. I had other things to be done in the shop so I went and did other jobs for the day to think on it. My other simple thought would be to move the resistor to the ground side of the solenoid so I don't get a voltage drop until after the solenoid coil but i'm thinking it will still click but easy enough to try, I currently have this set up with jumper wires to make it easy to alter until it functions properly.

Other than that I think I need to maybe use a transistor to isolate the positive trigger wire when the switch is off but even the transistor needs to be triggered on by the same wire which will have 12v when the ground is disconnected so i'm skeptical if that would work without other components. I would think if the voltage is enough to make the solenoid click it would be enough current to keep the transistor in an on state and the solenoid would still click.

Any advice would be greatly appreciated on how to get rid of this unwanted voltage when the ground is disconnected but also have it be there unimpeded when the switch is on and the ground is reconnected.


Thread Starter


Joined Jan 18, 2023
I'm sorry if the post is a bit confusing, was trying to explain an unusual situation.

The big difference between the diagram you showed vs mine is that I am disconnecting the negative not the positive with my solenoid. I am also not trying to add another switch and just use the existing ignition switch to trigger the solenoid so I need the ignition switch to remain connected to positive. so I can't disconnect the positive side otherwise I need to run an extra positive wire pre solenoid to power the ignition switch. The problem with doing that is if I run the extra positive wire to the ignition switch positive feed wire the voltage backfeeds and give voltage to the fuse boxes through the extra solenoid bypass wire with the switch off and puts me right back to having a parasitic draw, just through the extra wire instead of the main positive cable.

I am trying to prevent a parasitic draw from killing the battery, I have found the issue thats causing the draw but its a component I can't just leave disconnected (the gauge cluster ECU) and I can't get a replacement.

The unusual part of the situation is that since the ground is disconnected i'm reading positive voltage on the back side of the switch which under normal connections is 0v with the switch off and 12v with it on. but with the ground disconnected it reads 12v when I want it to be 0v.

I know the easiest solution would be to just add another switch to trigger the solenoid that is isolated from the rest of the vehicle circuitry but I really want to use the existing ignition switch so it just disconnects when you turn the key off and don't have to remember to flip the extra disconnect switch.

Like I said my diagram technically works but just causes the solenoid to constantly click because of the voltage that appears on the ground side when it disconnects the negative cable. other than that it works as intended but trying to figure out how to get rid of the unwanted voltage on the ground side when the negative is disconnected to stop the solenoid from constantly clicking.


Joined Jan 27, 2019
Welcome to AAC.

If you can disconnect the battery from everything then you can disconnect it from the ECU. Why not jut disconnect the ECU’s supply with the relay instead of everything?

It will eliminate the fatal embrace problem and the side effects of removing the battery from everything.


Joined Nov 6, 2012
My advice, on a factory-stock-car, is to never disconnect the ECU.
Every time You disconnect the ECU it has to "re-learn" a variety of functions
which may cause weird drivablity quirks.
For instance, there are certain Automatic-Transmission functions that are only
adjusted after a certain number of Ignition on-off-cycles,
and the the Engine "Fuel-Mixture-Trims" accumulate, and are applied only after
multiple Ignition on-off cycles.
These are not the only possibilities, they are simply common ones,
and they will depend upon the Year, Model, and Manufacturer.

You need a Battery-Maintainer.

I would say that disconnecting the Ground is always a poor idea,
and, depending on the particular car,
has the potential to create strange problems, or even damage in some cases.

You need a Battery-Maintainer.
It's cheaper that a big Solenoid,
and your Battery will thank You for it in the long run.


Joined Jan 23, 2018
What you need is a mechanically held contactor that has a pilot-duty switch to disconnect the "on" coil after the contacts close; Then the release coil operated off a separate switch that is only powered when the main contact is closed. That will save a lot of power and be more reliable.