Battery disconnect wiring

What are advantages/disadvantages of breaking ground (earth) vs + with a disconnect switch?


Tx, Steve C

Steve: Seems like that's the best way to do it. If you ground a ground, you just get a ground. But when you ground a hot wire, you get a dead short. Not good. Standard practice when disconnecting and connecting a battery is remove the ground first and connect it last. I'd like to heard advantages of placing the disconnect in the hot side.

I always thought you should disconnect the +ve amd not the-ve with battery kill switches. As to why, don't have a clue!

Most single seaters I have dealt with break the Earth ( ground ) wire with the kill switch, Frank

For a battery disconnect switch I would always put it on the -ve as close to the battery as possible.

But if you're talking about a motorsport emergency engine kill switch I believe it has to go on the +ve and must specifically disconnect the the ignition, fuel pump/s and alternator. The reason for this is that otherwise the alternator can continue to power up the system and the engine may keep running.

Not being an electrician I may have the reasoning wrong, but that is my understanding.

Cheers,

So far a 50:50 opinion here......

There is logic to what Russ says, i.e. alternator continuing to power the motor, pumps etc etc..

However, many electrically operated isolators (solid state or relay) also make/break the charging connection from the alternator to the battery, so they well and truly isolate them.

Re which wire to break, +v or -ve? well if the 'break' is right next to the battery you could argue that it makes little difference BUT if the isolation point is further away, then breaking the +ve will leave a large cable to the break point that could short to ANY earth point along its length in the case of an accident/impact.

If you break the negative, the section of negative cable from the battery to the break point would only be a problem IF is shorted specifically to another +ve cable and it should be possible to prevent this with careful placement of cabling.

So for my mind, breaking the earth (-ve) is safer...

One thing with these systems, always remember that they will make wiring for alarm and electric central locking systems more complicated esp if the earthing circuits are broken. So if yours is a road car as well as track, consider the requirements carefully...

p thompson said:

There is logic to what Russ says, i.e. alternator continuing to power the motor, pumps etc etc..

However, many electrically operated isolators (solid state or relay) also make/break the charging connection from the alternator to the battery, so they well and truly isolate them.

Re which wire to break, +v or -ve? well if the 'break' is right next to the battery you could argue that it makes little difference BUT if the isolation point is further away, then breaking the +ve will leave a large cable to the break point that could short to ANY earth point along its length in the case of an accident/impact.

If you break the negative, the section of negative cable from the battery to the break point would only be a problem IF is shorted specifically to another +ve cable and it should be possible to prevent this with careful placement of cabling.

So for my mind, breaking the earth (-ve) is safer...

One thing with these systems, always remember that they will make wiring for alarm and electric central locking systems more complicated esp if the earthing circuits are broken. So if yours is a road car as well as track, consider the requirements carefully...

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I agree with everything Paul says. The relay on the alternator supply is a good idea and would achieve the desired results.

Maybe the single seaters that Frank is referring to don't have alternators, or maybe they have a relay on the alternator wire?

I have the master switch in the +ve lead on my car.You can buy a switch with an auxiliary contact to cut the alternator's field circuit but I haven't found it necessary in the cars I've wired.The Clubman I've just finished has the switch in -ve battery lead. I agree the lead with the switch in it should be as short as possible. The advantage of having the master switch in the positive lead is that an alarm/light/fire ext bomb/tuner memory/clock or any other very low current circuit can be wired to the hot side of the switch and still have power when the switch is off.Not technically correct for the scrutineers who want to see total isolation, but much more practical for a road/track car.

Ross

Run both the + battery and the alternator output to the input of the switch, then run the output of the switch to the + distribution block or starter solenoid. When you throw the switch you kill everything. There is no practical way to do this by breaking the ground.

I've heard some alternators with isolated rectifier circuits (Lucas?) can be damaged if the lead from the rectifier to the battery is disconnected while the engine is running. I seem to remember something about the rectifier being in a bridge configuration and removing the battery + unbalances it causing excess current in two of the diodes, blowing them out. An electrical engineer out there can probably say whether this is true or an old wife's tale.

The battery acts as a large capacitor to the alternator. When it is fully charged, the voltage regulator either inside of or external to the alternator senses the lack of voltage differential and turns off the alternator output. As the battery begins to discharge, the voltage differential grows and the regulator resumes alternator output. When the battery is abruptly removed from the circuit, the regulator senses a huge voltage differential momentarily and the output of the alternator output spikes. This spike is strong enough to burn out the rectifier circuit and damage the regulator. Hooking the cutoff up the way I posted above, prevents the differential spike and damage. It is in effect, the same as turning off the key except all power is removed from other circuits.

I wonder what it says -ve versus +ve in the instructions/on the packet of a new kill switch? I checked my other cars that I had no hand in wiring and both are on the +ve side.

MReid said:

Run both the + battery and the alternator output to the input of the switch, then run the output of the switch to the + distribution block or starter solenoid. When you throw the switch you kill everything. There is no practical way to do this by breaking the ground.

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However, this approach leaves a high current capable live wire running direct to to the Alternator. Hopefully the builder has included a fusible link in the cable to provide some degree of current limit, but certainly would not qualify for track isolation requirements.

malb said:

However, this approach leaves a high current capable live wire running direct to to the Alternator. Hopefully the builder has included a fusible link in the cable to provide some degree of current limit, but certainly would not qualify for track isolation requirements.

Click to expand...

Would placing a capacitor in parallel with a resistor at the alternator terminal between the output and ground allow the battery to be disconnected from the alternator without damaging the rectifier? When disconnected, the capacitor would discharge through the resistor slowly, depending on the RC time constant, and the rectifier wouldn't see a sharp drop in voltage. Then you could use a two pole switch to cut power at the + distribution point and isolate the battery from the alternator.

Depends on your scrutineering requirements. Obviously different from place to place.

Here in NZ the regs specify "an ignition switch/circuit breaker positioned within easy reach of the driver capable of breaking all circuits that keep the engine running, i.e. the ignition, fuel pump and alternator". Closed dedicated race vehicles are required to also have an additional external one with ID markings "positioned at the lower windscreen mounting area". No requirement to totally isolate the battery.

In view of M Reid's last post, I would be inclined to have the kill switch/es wired as he suggests. And for peace of mind and total disconnect, to add a seperate battery isolator switch close to the battery in the earth wire.

To use the kill switch to totally disconnect the battery in a rear engine/rear battery configuration would mean running heavy wires all the way up to the front of the windscreen area, or for a non dedicated race car into the driver compartment. Not entirely desirable from many aspects IMHO.

Really the way you do it in different countries will depend on the regs that you have to comply with.

However where you don't need a motorsport kill switch then that simple battery isolator in the -ve is all that is required as it wouldn't normally be disconnected while the engine is running.

Cheers,

Russ, there is no requirement to run the leads & switch to that area- simply install a pull cable from that area to the switch- a sturdy hood release or manual choke cable usually does the trick, Mark it with the appropriate info ( pull/twist etc ) & decal. This also keeps the switch away from the elements- the contacts usually need a clean up on these if they are subject to moisture & then left sitting between meetings.

Or how about doing it the way an aircraft would?

Install a relay capable of carrying the entire elec. load, at the + terminal, either in or near the batt box. This can easily be controlled by a switch mounted on the dash. The power wire for the relay could be protected by a simple inline fuse.

Cheers,
Scott

This thread may also be of some interest, similar topic.

http://www.gt40s.com/forum/gt40-tec.../18542-battery-cut-off.html?highlight=battery

I run the alternator wire alway hot to the (+) of the battery and have the cut off close to the battery with an outside marked push/pull in the case of the mustang. I don't see much difference in running a long battery cable hot or running a hot alternator lead, both will be a problem if something bad happens. You see many cars with the battery switch in the cockpit or on the roll bar with a tangle of cables, seems to pass many tech for SCCA cars I have seen. So not sure many tech guys are aware of dangers other then everything must be covered with rubber boots...

I don't know if a small capacitor will save the alternator on disconnect a 10k resistor and 1uF cap are very small values and likely might not do much it likely needs more load to keep the regulator stable.

Bottom line is it really all that big a deal? If your normally driving, shut the car off and then flip the disconnect. If you upside down in a ditch with your fuel pumps full till, do you really care if you spike the diodes in your alternator? Oh yes, a better thing would be to install an inertia switch to cut the pumps and ign...

The main thing is to make sure your alternator does not keep things powered in the event the disconnect has to be done due to a crash or other catastrophic event.

In the RCR40 I have a master switch that controls a 100amp relay for everything except the starter battery main. Then will also have a master-master battery cut off as close to the bat as possible or like Mr. Mac says, will make a remote cable/rod actuator to keep the wire clutter down and all inside a sealed battery box.

Sandy

Not that keen on inertia switch's in race cars Sandy- most drivers have a bad habit of being able to create the specific amount of inertia reqd to activate the switch- problem is not one can give a plausible explanation of how or when-even after you have spent hours looking for the cause of the 'mystery' cut out or apparent fuel pump failure