battery cutoff on negative wire

Safe cutoff switch

All my trucks have the cut off isolator switches on the posative + power side of the battery (ies).
The logigical reasoning here is that. (a) On these trucks (not chevies dodges some fords etc WITH ECUs etc Which would fail "memory in the computer update running performance settings. RADIO memories cd s etc etc etc)
OUR trucks as is are strictly bare bones "as is" no frills and or electronics.

(b) With the isolater on the + ve side and CLOSE to the battery terminal there is no chance of an accessory short circuit. (with in my case) Items like siren kit, radio, lights, C.B.s Phone chargers etc etc. ALL 12 VOLT items running from the " last battery". (yes I know inverters..Mine use a LOT of current. (12volt winches in back for example)). ONLY for hauling Army moto bikes up.
Scenario. (simply explained). ANY of the 12 volt accessories which were grounded by accident would blow. Because THEY would be acting the -VE (negative for 24 VOLTS , closing the ground for the FIRST battery. For example. IF you forget to turn ON a Negative Isolator switch.." BOOM" the accesory FRIES! (goes up in smoke).
The posative on the first battery is closed to 24 volts through the second batteries power (accessory) to GROUND.
IF you have no 12volt accessories ok really. But it is GOOD principal to isolate the " power" +. then NO current flows. ((exept for the 12 Volt accessories IF used from the last battery))
To quallify, with an ISOLATOR switched OFF Via Negative TERMINALS current FLOWS through all wires till switched on at souce, ( switches for lights heater etc)and they EARTH to work via exit wires from those switches.
IT naturally follows that ANY wire accidently earthed (grounded) as when you might be carrying out maintnance or fitting something BEFORE the SWITCHES (that operate things) will be live and BURN. (current flowing through posatives looking for resistance WORK ( from switches to heaters radios whatever) to grounds)
long winded, but takes thinking and studying over if not a "sparky" (which I am Not)

9.2.2.3.1​

A switch for breaking the electrical circuits shall be
placed as close to the battery as practicable. If a
single pole switch is used it shall be placed in the​

supply lead and not in the earth lead.

Moderator DELETE this (2nd edited) now blank reply. As without CLICKING submit the post was accepted. then when actually clicked it appeared twice. and I can not Delete it . only modify it.

Yeah, but you guys also say, Anti-clockwise too~!

You do everything we do, backward.

Think of this, there are two sides to the electrical system. The + side (all contained in wires), and the negative side (every metal part of the entire truck). Which side is bigger?


Look at any industrial piece of equipment made in America. The standard has been to disconnect the negative battery cable.

Ha But!

YES, all that metal..and which "Fries" first? when a wire accidently touches earth, (the metal?) ..Yup..the wire.s. No positive no power no fry. lol.

Addendum

Or a P.S. I forgot to mention On all the trucks We also have a heavy duty "master switch"(es) on the posative side(s) of the switch(es) to the 12 volt accessories using the last battery.(ies).

dmetalmiki said:

All my trucks have the cut off isolator switches on the positive + power side of the battery (ies).
The logical reasoning here is that. (a) On these trucks (not chevies dodges some fords etc WITH ECUs etc Which would fail "memory in the computer update running performance settings. RADIO memories cd s etc etc etc)
OUR trucks as is are strictly bare bones "as is" no frills and or electronics.

(b) With the isolater on the + ve side and CLOSE to the battery terminal there is no chance of an accessory short circuit. (with in my case) Items like siren kit, radio, lights, C.B.s Phone chargers etc etc. ALL 12 VOLT items running from the " last battery". (yes I know inverters..Mine use a LOT of current. (12volt winches in back for example)). ONLY for hauling Army moto bikes up.
Scenario. (simply explained). ANY of the 12 volt accessories which were grounded by accident would blow. Because THEY would be acting the -VE (negative for 24 VOLTS , closing the ground for the FIRST battery. For example. IF you forget to turn ON a Negative Isolator switch.." BOOM" the accesory FRIES! (goes up in smoke).
The positive on the first battery is closed to 24 volts through the second batteries power (accessory) to GROUND.
IF you have no 12volt accessories OK really. But it is GOOD principal to isolate the " power" +. then NO current flows. ((exept for the 12 Volt accessories IF used from the last battery))
To quallify, with an ISOLATOR switched OFF Via Negative TERMINALS current FLOWS through all wires till switched on at souce, ( switches for lights heater etc)and they EARTH to work via exit wires from those switches.
IT naturally follows that ANY wire accidently earthed (grounded) as when you might be carrying out maintenance or fitting something BEFORE the SWITCHES (that operate things) will be live and BURN. (current flowing through positives looking for resistance WORK ( from switches to heaters radios whatever) to grounds)
long winded, but takes thinking and studying over if not a "sparky" (which I am Not)

9.2.2.3.1
A switch for breaking the electrical circuits shall be
placed as close to the battery as practicable. If a
single pole switch is used it shall be placed in the​

supply lead and not in the earth lead.

Click to expand...

Point taken. You are correct that if you forget to turn on a negative ground battery disconnect switch and the truck added equipment is in the on state, then hit the start button bad things can happen. The current the starter wants to draws will flow through any wire path (the turned on equipment) to the negative side of the battery. However all items in the truck including added equipment should be fuzzed. If they are the worst thing that will happen is you will fry the fuze (or if the equipment is off no problem). That is better than having something come in contact with an exposed non switched 24 volt wire or contact on a positive lead to the disconnect switch and then you will have major sparks and possibly a nice little fire to boot.

Everything is a trade off.

P.S. In the early 60s dmetalmiki I was down the road at Chicksands RAF base, USAFSS side to install the AN/FLR-9. Beautiful country. Wish I could afford to make a trip back.

http://www.forces-war-records.co.uk/564/RAF_Chicksands

P.S.

Yep I found ou the hard way..and tho' there were correct fuses in the units..they popped before I twigged what was happening. The p.s. Like you I was "down the road" too! lol..at 54...riverside...camp pendleton, also greenham and upper heyford amonst others. Had the bug for things green even before that. Cept for one RED one..and years later I actually own it! My M45. fire unit.

My M45 fire truck - YouTube

59apache said:

i can't prove right now - what i know is that many switches has different amperage at 12V or 24V. So i assume that the load in W is crucial.

so if Parlusk writes he has a switch that is rated 180A @ 12V
i dont believe it is rated 180A @ 24V .

100 Amp Battery Master Switch - Accessories, Battery Switches - Hella Marine

go to specs....

Click to expand...

Unless you have to break the circuit under load; doesn't matter, current is current.

Parlusk- great to see you back! On my trucks I put a Caterpillar 7N0718 battery cutoff switch below the passenger seat. Real easy to do, and that gives me the same key for both trucks. It is real heavy duty.

Caterpillar Equipment Battery Disconnect Switch 7N0718

I just drill a hole below the passenger seat, install the switch, hook up the neg cable to the switch and get a 2 foot cable to go from the switch to the battery. 40 minutes tops.

Will installing a switch between the connections the dogbone is normally connected to effectively "turn the battery off? I was thinking of doing mine that way.

Amram said:

Will installing a switch between the connections the dogbone is normally connected to effectively "turn the battery off? I was thinking of doing mine that way.

Click to expand...

That will work. I think there are potential safety advantages to switching the ground lead, but switching the center connection has the benefit of not needing any changes to the existing + and - leads.

nf6x said:

That will work. I think there are potential safety advantages to switching the ground lead, but switching the center connection has the benefit of not needing any changes to the existing + and - leads.

Click to expand...

Agree, if you put it in the negative connection, you won't run the risk of accidentally shorting something to ground when working around the batteries...

never install a disconnect between two batteries, never a good idea to have 2 different potential voltages (12 and 12)

either disconnect positive or ground both are acceptable methods for a 24v system, ground is usually the simplest method because there is usually only one connection (the frame) whereas the positive may have multiple connections.
the other reason for a ground disconnect is in the event that you are working on/near the batteries and accidentally touch any of the posts with something that is grounded/touching ground (wrench etc) there will be no arcing (the frame will temporarily become positive and no current will flow) however if the same scenario is applied to a positive disconnect and a short happens there will be arcing possibly leading to an explosion (worst case scenario).

When dealing with an dual voltage systems (two batteries with 12v being taken "between" the batteries) a dual circuit disconnect should be used (when i find the P/N ill post it) on both positive sides (12 and 24v)
this will prevent any damage to the equalizer or any other auxillary equipment.


A few other notes,

When charging batteries on the truck using a 12v charger as long as either the positive or ground cable is removed there is no danger of damage to the chrager as with no current path the batteries become isolated from each other and only the battery with the charger hooked up will chrage.

Never try hooking your batteries in parallel to charge 2 at once, you will damage something.

All of the above is assuming that the vehicles electrical system is negative ground (which 99% are)

peppy said:

When dealing with an dual voltage systems (two batteries with 12v being taken "between" the batteries) a dual circuit disconnect should be used (when i find the P/N ill post it) on both positive sides (12 and 24v)
this will prevent any damage to the equalizer or any other auxillary equipment.

Click to expand...

On that topic, I ordered up a few of the Pollak brand keyed disconnect switches that I generally use... and then discovered that my new M923 has loads connected to the 12V point. The M923 has four batteries in a series-parallel configuration. They're only paralleled at the ground and 24V points, and the 12V loads are just connected to 12V point of one of the battery pairs.

Pollak also makes dual-circuit switches, but the second circuit is just intended to disconnect the ignition lead from an alternator so that the switch can be turned off with the engine running without damaging the alternator. They rate the second circuit at 20 amps; I'll need to figure out if that's adequate for the 12V loads in my M923. For that matter, I need to study the diagrams more to figure out what the 12V loads are.

Edited to add: It might be that a single ground disconnect is adequate for my M923, but I'll need to study the circuit diagrams a lot more carefully to determine if that's the case. If anything is connected to both the 12V and 24V, either directly or via some path through other components, then it could still see a 12V potential difference with the ground removed. More research needed...

pa.rich said:

I am guessing that it doesnt matter, as long as you break the circuit. But anyway it has worked well on my five ton. Now the batterries are always up when I hop in after a month or two of sitting. If you freeze a set of dead batteries in the winter, they are shot. Ask me how I know. Now , next is the m211, that I always have to remove the bat cable or they go dead in a few days! Dont ask me why these mil trucks drain down, I havent heard a good answer to that yet! You would think once the ignition was off it would stop all battery supplied things, but obviously it does not.

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In marine applications you always put the disconnect on the positive circuit. This is to prevent a floating positive which can cause electrolysis and the next thing you know your components in contact with water start to corrode. This is especially true with automatic battery chargers that keep the batteries topped off when connected to shore power.
The only diadvantage I can see with doing it on the negative side of the circuit on trucks is if you turn off the switch while the engine is still running and the alternator is charging. This will cause the alternator to full field and blow the diodes in the alternator. As for hooking any disconnect to a battery circuit, all load wires hooked to the battery posts must be moved to the load side of the switch or the potential for battery drain is possible. The wires for permanently mounted chargers should be hooked to the battery side of the switch and not the battery post. This just makes for a cleaner installation.

Pa Rich....on your M211, disconnect your ground lead on your battery and put a test light in between the post and ground. If the test light glows bright you have a big draw and if its a 12 volt light the bulb will probably blow. If it glows very dimly its probably in your generator or if equipped with an alternator its in the diodes or regulator circuit. Disconnect one positive wire at a time until the light goes out. When it goes out then this is the component that is drawing and discharging your batteries. When doing this leave the wires you disconnect off until you find the one that is drawing current. Once this wire is found reconnect the wires you removed one wire at a time and check to see if the test light lights again. This will rule out multiple circuits/compnents drawing current. If your test lamp doesn't light when put in series to ground you might want to use a digital or analog amp meter because it is such a low current draw.
As for me....if I were to put a disconnect in the battery circuit it would be on the positive side of the batteries. This was the way I was taught. And ANY time you are working around batteriesyou should ALWAYS disconnect the ground. This is so you don't have to worry about shorting a battery post to ground.....and you also don't have to worry about blowing up a battery.....believe me....this is not pleasant....ask me how I know....

Some military Unimogs are set up with a negative side cut off.

A'ight- after reading through these posts I am a little confused. I was working on the truck the other day and earned myself a lightshow through my carelessness - arcing off my starter and 'welding' my exhaust pipe (using my socket wrench .) Ironically enough I thought about disconnecting my batteries before starting work, as I knew I would be in the vicinity of the starter, but decided not to since it was a quick, easy project and I was under a time constraing. LFMF.

Initially I thought to install a cutoff switch on the +'ve side of the battery to mitigate this, but after reading these posts I can't tell which side I should use to prevent accidental arcing while working on the truck. If I disconnect the -'ve side, won't the truck still ground to earth?

Seth_O said:

Initially I thought to install a cutoff switch on the +'ve side of the battery to mitigate this, but after reading these posts I can't tell which side I should use to prevent accidental arcing while working on the truck. If I disconnect the -'ve side, won't the truck still ground to earth?

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Interrupting the positive lead, the negative lead, or the between-batteries dogbone will all remove the batteries from the circuit (assuming there's nothing drawing power from the 12V tap at the dogbone cable). A switch in any of those three points would have prevented your accidental welding (assuming it was turned off, of course!).

I also use the cat switch on the negative side

Just a small addition to the discussion... I rewire vintage aircraft, mostly WWII and after, prop and jet. On every aircraft I've done, all had a battery or master disconnect. All had the disconnect in the + side. This included 24v and 12v aircraft. When I redo the Deuce I'm getting shortly, I'll be following the same logic as the aircraft design guys. There are good arguments here for both sides, but aircraft systems are highly engineered and breaking the positive has worked for many decades. Electrically, our MVs are very similar to aircraft. Just my 2 cents....