Automatic battery cut-off?

[tinman_72]

I bought a beater 1999 Mercury Mountaineer with V8 a while back. A previous owner had installed a huge electromagnetic battery cutoff (aka relay) that has a receiver that turns it on and off via a key fob. (Like this: Amazon link. ) As it turns out there is something draining the battery. After our last cold snap the relay quit working so I have to pop the hood and disconnect the battery manually.
I was going to just buy another for now, but upon researching I have learned that the device is not a latching relay. This means that it draws power from the battery to keep the power on. If I forget to turn it off, it will draw power from the battery and drain it.
I wonder if there is a way to use the signal from the ignition system and a momentary switch so that it will automatically cut power when the engine is cut off. Then I would have to hold the button while turning the key to start it. Something like this: (sorry for the crude drawing but this should get the message across.)

Is there an issue with the fact that I am essentially connecting a wire from the ignition straight to the battery when the button is held down? Will my schematic work as described? Do you have a better way? Eventually, I will fix the issue so I won't need the relay but I don't that much time to invest in this junk vehicle right now.

Thanks in advance.

 

[RonD]

First, the starter motor should have a separate 12volt cable, the big one, and no need to cut power to that
So you usually are just cutting power to the engine fuse box

Yes, your drawing is fine

Momentary button has 12v full time, and gives relay 12v to close it which powers the ignition switch so it would now hold relay closed until ignition switch power was cut off with key


I would just find the drain, lol

 

[Dirtman]

Yea seems easier to just fix the problem than to rig a complicated system to avoid it.

 

[91stranger]

I like the idea of a battery shut off that is as easy as flipping a switch.... o wait, they already exist. Fix the drain or just get an actual battery disconnect switch, yes you would have to pop the hood to disconnect the battery but that beats having a dead battery. Being a fairly newer vehicle it should be easy to find the power draw..

 

[Dirtman]

I have one of the automatic battery disconnects because my truck sits for long periods and I had a parasitic drain I couldn't trace. But I found it absolutely worthless because the cutoff voltage is too low. I.e. it disconnects at around 10 volts which isnt enough to start the truck.

I ended up just spending some more time tracing the drain.

 

[tinman_72]

Thanks Ron. The wiring is mostly there, I would only be adding in the momentary button. This "project" will take 5 to 10 minutes. Well, okay maybe a few more minutes to run a wire through the firewall.

Yea seems easier to just fix the problem than to rig a complicated system to avoid it.

How is it complicated? It is just a button and a relay.

I like the idea of a battery shut off that is as easy as flipping a switch.... o wait, they already exist...

Yes, they do exist. The previous owner installed one on this vehicle and I need to replace it. This would be even easier. No need to flip a switch.

...it disconnects at around 10 volts which isnt enough to start the truck.

I ended up just spending some more time tracing the drain.

I considered such a device and came to the same conclusion. As I said before, I have 20 minutes or so that I can spend on this beater vehicle. I work a split shift all day long so I don't have time to spend staring at wires and meters all day.

 

[RonD]

You would need TWO WIRES thru the firewall

The always Live 12v wire from battery to the button, put an inline fuse at the battery end, for sure.
And the return wire from button to the relay, which will also need to be spliced to an "ignition on" wire in the cab, or in the engine bay

 

[Snoranger]

$6... you pop the hood and turn the knob.

 

[Dirtman]

Those things suck sno... I had a few on plow trucks. They dont always disconnect fully.

You need a true on/off switch.

 

[tinman_72]

$6... you pop the hood and turn the knob.

Or I could go my route and not have to pop the hood... like I have to now. It is a pain in the icehole.

 

[91stranger]

Popping the hood is hard? Turning a switch is hard? Are you missing both arms or something? jk

 

[Snoranger]

Or I could go my route and not have to pop the hood... like I have to now. It is a pain in the icehole.

Its a beater that you have no time for. Cut a big hole in the hood, then you don’t have to open it to shut the battery off.

 

[tinman_72]

Ok. Done watching my own thread.

 

[RonD]

You guys crack me up

 

[Dirtman]

Ok. Done watching my own thread.

It's ok, I'll watch it for you and keep you updated!

FYI ron just said "you guys crack me up"

I'll email it to you.