Battery Cut-Off Switches: Break the Ground Side or the Hot Side??? (Page 1/1)
imacflier JUN 20, 01:55 PM
Good Afternoon, All,

Since I am replacing battery cables anyway, I thought installing a battery shut-off might save me some long term time and wear and tear on my battery terminals. Yeah, it is classic 'Mission Creep', but sometimes it is still worthwhile!

In looking at the switches on Amazon, I discovered that SOME of the manufacturers recommend installing the switch on the ground side rather than the hot side.

It is not obvious to me why breaking the grounds rather than the hot side is better.

Can one of you experts out there (theogre?) please explain to me why one is better than the other??

TIA,

Larry
Dennis LaGrua JUN 20, 03:50 PM
It may be easier on the switch. I always put the switch on the ground - side. During summer storage. I also insert a 20A fuse in a holder that I put across the terminals so that the PCM and clock settings are not lost. It will blow if you try to start the engine making it part of an anti-theft component.

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"THE COLUSSUS"
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[This message has been edited by Dennis LaGrua (edited 06-20-2020).]

theogre JUN 20, 04:17 PM
Most I've seen is Hot side install for cutting battery power completely for "race use."
Is often required this way to pass rules for "track"/association so Safety people can kill all power in a wreck etc.

Normal car use? Why?
Example: You think will store the car easier? Not really. More so if stored w/o heat in winter. Battery will self discharge over time and low battery for any reason loses freeze protection. Store car wherever but pull battery and keep inside. Pull battery on charger a couple times during the winter. Don't let any "battery tender" and "trickle chargers" on battery for months. Many claim they are "smart chargers" but battery and power bill doesn't need to have charging connected 24/7 for months. Just charge ~ 45-60 days or anytime battery below 12.1v. Unconnected Good battery will stay "full charged" for several weeks to months.

Anti-theft or otherwise disconnecting often causes ECM/PCM, radio, and more to lose backup power and very annoying at minimum to most people.
ECM/PCM w/o backup power often make the engine to idle bad and more until you drive above ~ 35mph so can reset IAC etc.

Many battery "switches" are made cheap causing problems too. Some times have problems even tho you don't use them much.
Why?
Every connector/contact adds resistance to a given circuit. Might not notice this to run a car but Starter Motor will suffer having many voltage drops because Full Charge Battery may putout 12.6 to 13v but starter only gets ~ 11v or less going thru a battery switch, iffy cables, etc. and quickly dies.
See my Cave, Electric Motors

Note: Storing cars have other problems beside a dead battery.
Brake Cooling and more parts often hate being parked for weeks, worse parked for months. This is why I drive my cars every couple weeks for ½ hour minimum even if don't need to go to stores during CCPvirus panic. This keep brake fluid film on piston so calipers etc work right, same w/ coolant film for WP shaft seal plus keeps coolant mixed up to protect from "rust" inside the system. (Funny... While I write this, CVS just text me for a refill script so have reason to drive now... Other car was driven a few days ago.)

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Blacktree JUN 20, 04:23 PM
It depends on the intended purpose of the switch. There are two main types of battery disconnect.

If you want something to eliminate battery drain while the car is parked, you can get one of the cheap and simple battery disconnects. It's usually easier to install on the negative battery terminal. However, you shouldn't use it to cut power to a running vehicle, because it can damage electronics. It should ONLY be used when the engine is off.

If you're going to race the car, safety regulations may require a switch to cut power to the vehicle (in case of a fire or crash). For that application, it's customary to cut power on the positive side. Cutting power on the positive side can be more complicated, because you also need to isolate the alternator, and possibly the ignition system. That type of cutoff switch can be used to cut power to a running vehicle without frying electronics.

[This message has been edited by Blacktree (edited 06-20-2020).]

imacflier JUN 20, 04:35 PM
Thanks to all who answered.

Sometimes what I hope are simple straightforward questions get the darnedest answers!

In this case, I will accept the wear and tear on my battery terminals (AND on my wallet) and just let it be!

Thanks, again,

Larry
hunter29 JUN 20, 06:50 PM
imacflier check your PM
theogre JUN 20, 07:56 PM
Many put battery "switches" on Ground side only because often have 1 or 2 cables/wires there.
Other just put there because don't need to insulate the cable ends.
In Many cars, the + side terms for battery have many wires besides big cable to the starter. Many of those aren't easy to move them just to replace a dead battery. Cables/wires aren't really flexible as well so you don't want to move them often.

Electrically where a battery disconnect is doesn't matter. Starter etc sees same Ω causing voltage drops anywhere in a circuit.
Example: You check V-drop not w/ an Ω meter because of wire size and others are to big to measure Ω w/ standard equipment.
So connect Volt meter between starter and battery + term (Ideally connected to battery lead not other hardware.) then start the car often easily see 1v or more drop just having a iffy + cable.
Starter Ground path is often worse because goes thru starter case, trans bell and engine, then likely has iffy ground cable too.

Disco the battery cutting + or Ground on any running car can fry E-parts. Is why the Bogus Test for alt to run the car w/ battery disco was a bad test even before cars had radios and more. Most early Generators and Alternators Regulators fried when people tried that test.

"Race Tracks" et al Do Not care about frying E-parts when battery is shut off by a switch.
Driver or Safety Staff if driver in disabled can shut off a motor....
If they use a Fire Hose or Extinguisher, the Engine Dies very quickly if still running. Dry Chem and other types of FE can destroy a running engine.

But Battery can dump 200+ amps now or soon after a crash w/o Warning. That's Why most Race Org's require battery disconnects that are easy to find. Many require them on the body in specific locations so safety people don't go into the cabin, engine bay, or whatever. (Many are on the rear near right taillight for this reason.)

Many "Junk Yards" and "Insurance yards" have fires cause by batteries left connected. Shorts can happen hours to weeks later when wires are pinch and pressure slowly displace the insulation. (I-yards are for "Totaled" vehicles and often Not open to the public.)
sourmash JUN 21, 09:59 AM
This'll start an argument, but I'm right, so there.

When you hook battery jumper cables up, the NEG goes on last to complete the circuit. So I'd put the disconnect on the Neg cable.
hunter29 JUN 21, 12:43 PM
Power flows from negative to positive , thats why you put the switch on the negative side, there are exceptions.
cvxjet JUN 21, 02:20 PM
I put mine on the negative side- because when I moved the battery up front I didn't like a AWG2 wire (always Energized) running next to the gas tank, so on the positive wire I installed a Ford-style separate starter solenoid that only energizes the big cable during starting, then ran a #8 wire to actually power the car- and that has a fuse so that if it shorts during a (High-center) accident, it will hopefully shut off the power before gasoline gets to it.....



[This message has been edited by cvxjet (edited 06-21-2020).]