Battery Cut Off Switch Ideas

[Fishcop]

Hi Guys

I'm planning on a hidden battery cut-off switch whilst I have opportunity (full wiring strip-out). This is for security/anti-theft, so I'm just using the Hella two-pole switch (engine won't be running). Is it okay to run a cable off the negative terminal to the switch and then ground it at a suitable location near where-ever () I place the switch, rather than back at the earth post near the battery?

Thanks for any advice

Cheers
John

[304065]

John,

Most people put them on the positive side, the theory being that the car is negative ground, so by interrupting the positive side you make it impossible for a short circuit to continue anywhere in the chassis. Given that our batteries are often mounted in locations where, in a bad wreck, one could end up with the negative squeezed against the body sheetmetal, the preference is for the positive side.

Here's another reason: the switches are all on the positive side of the battery- the only connection between the negative and the body is the ground strap. If you interrupt the ground, you're creating another failure point, where corrosion, vibration or mechanical problems can result in failure of the circuit. It's not tough to diagnose a big copper strap sitting there, but where a switch is concerned, it gets (slightly) more complicated.

Of course you know the reason why we don't use the switch when the engine is running, tends to blow the alternator and any sensitive electronics.

All that said, from an electrical standpoint it doesn't make a difference if you break the negative or positive, as long as you open the circuit from the battery, no current will flow.

[mike curnow]

As far as blowing the alternator or other things, why is disconnecting the battery different than switching off the ignition? How about in a double plug engine with twin CD boxes and switches to control them?

[M_deJong]

Picture the alternator as a giant water pump, the battery as a reservoir and all the other electrical loads on the system as little (variable size) holes in the pipe connecting them. The pump puts out a constant pressure (voltage) to accomodate the little leaks (loads). As leaks come and go, the battery takes the overflow as the alternator doesn't respond instantly to changing loads. Also the battery is itself a load under charging conditions. Now the two scenarios:

1. turn off ignition. Loads go to zero (no leaks) when the switch is turned off but the battery takes the excess flow from the alternator until alternator slowly stops pumping.

2a. disconnect battery while running (battery fully charged). Loads unchanged, alternator maintaining steady voltage, no place for excess flow to go but maybe ok if everything stays steady state, engine speed included. Any reduction in load results in a voltage increase until the regulator can adjust output.

2b. disconnect battery while running (battery charging). A big load removed and at the same time the reservoir for excess flow is removed. Big voltage spike results. Bad things happen.

Hope that makes sense!

[JohnT]

2a and 2b

Add Corollary - 1

disconnecting the battery with the ignition on permits the engine to continue to run using the alternator voltage. i.e. it doesn't kill the engine This kinda exacerbates all the bad $hit in your examples. (er, I think)

johnt

[edmayo]

John, I think that only works with a generator, not an alternator.

[Fishcop]

Cheers John.

My thought on using the negative pole was that running a pair of fat 12v cables to a switch somewhere away from the battery could lead to to trouble over time. Whereas a failure of a single negative cable won't be catastrophic. Maybe I'll just go with a hidden fuel pump switch - carbed car, so I should find it within a few blocks

Mike, great explanation.

Cheers

[jim amato]

I am empolyed as a service tech for caterpillar ,all of the disconnects that come from the factory are close to the battery and the negative is opened and closed with the switch, be certain that the switch you select can handle the amperage during cranking, "starter draw", my $.02 jim amato

[mhhm]

John
My car is retro-fitted with a Battery cut-out Master Switch/isolator ( sorry no pic.)

You are probably familiar with it. ie. fitted in-line in positive cable from battery to starter/engine bay / Heavy duty terminals / Tee-handled red plastic key with locking barbs (like a light bulb) which is inserted against a spring-loaded base and turned about 90 deg. to engage the switch.

These things are used on camper vans, boats etc and readily available.
Key is removable and can be hidden in car...difficult to see the small socket with key removed.
Also gives you a chance to isolate battery (from within drivers cabin) in an emergency/fire etc?

Fitted up high,in footwell side panel/under dash, near A-pillar ...same side of car as battery location ( ie passenger side under glove box on RHD)

This can isolate ignition/starter motor and /or everything... but maybe a good idea to run a separate fused line to retain clock setting or any alarm install?

Cheers
Bob

[obrut]

Short the coil and save yourself the walk!