Battery disconnect for dual 6V

[audio1der]

I've searched, Googled, Bing'd and Youtubed but can't seem to find a definitive answer; what ios the proper way to add a battery disconect to a dual 6v battery setup?
Between the + of the first batt and the power to the trailer?
TIA

[SteveC7010]

Originally Posted by audio1der

I've searched, Googled, Bing'd and Youtubed but can't seem to find a definitive answer; what is the proper way to add a battery disconnect to a dual 6v battery setup?
Between the + of the first batt and the power to the trailer?
TIA

I've seen them done on the positive side and also on the ground side. Nobody seems to have a truly definitive answer.

For what it's worth, every ambulance and fire truck I've ever had the opportunity to examine up close or to work on has always had the battery cut-off on the positive side. Last week the Chief and I replaced the main module power solenoid on our ambulance. It was on the positive leads between the batteries and the rest of the unit.

[BullDog33]

For +12 volt systems you should have the disconnect on the positive (red line) out from the battery to the trailer. This provides you with a disconnect closest to the source, keeping the majority of the system from being charged and provides the greatest level of protection.

Think of it this way, if you have a short on the wire you want it to be completely without power. If you put the disconnect on the negative site for a +12volt system then the entire line will be able to carry a charge to the point where it is shorted and that short will drain your batteries. If you put the disconnect close to the battery, the short won't drain anything.

[raytronx]

If you put the disconnect on the negative site for a +12volt system then the entire line will be able to carry a charge to the point where it is shorted and that short will drain your batteries.

How would that short develop and drain if there is no return path to the battery with the negative disconnected?

I have mine on the Positive side mainly because it's also a battery switcher between 2 banks.

[JRTJH]

We have discussed this a number of times and it always seems to boil down to personal preference. From an electrical standpoint, I can understand both sides of the argument. Steve indicates that on ambulances, the kill switch is installed on the positive side of the battery. This would certainly be indicated in any vehicle with an alternator since if it were on the negative side, and were used to "kill" a running vehicle, the alternator would continue to provide a source of power to continue operation since the alternator would provide its own source of ground and the battery is not necessary to operate the systems in the vehicle. So, in vehicles, to "kill the vehicle" you'd have to install the switch on the positive terminal.

However, conversely, if you attempt to remove the positive connection on a battery first, you have an increased risk of shorting the battery to ground causing injury or damage.

In a travel trailer, if you disconnect the positive terminal, you can still, acidentally short out the positive terminal and damage the battery or the trailer systems if you happen to drop something conductive anywhere from the battery to the "hot terminal" of the cutoff switch. While this "shouldn't happen" there is a possibility of shorting the switch or the battery accidentally.

If the switch is installed on the negative side of the battery, there is less potential to short the battery since there is no return circuit in the system, it has been disconnected at the negative terminal.

Anyone familiar with electron flow in a battery circuit will remember that electrons flow from the negative terminal of the battery to the positive terminal. What that means is that current actually flows out of the negative terminal into the trailer frame (ground) through the appliances and it returns to the battery via the positive terminal.

Some have indicated that in newer cars, mechanics are instructed to disconnect the positive terminal before removing the negative terminal from the battery. Apparently this is to maintain a ground on delicate electronic computer equipment in the car. I've not yet been able to find that in any factory documentation. I'm not saying it doesn't exist, I just can't find it.

In aircraft maintenance, if the battery is to be removed, the aircraft is grounded, the negative terminal is removed, then the positive terminal is removed, and the battery is finally removed. I'm inclined to believe that sequence should be followed even if it is not completed. In other words, if the battery is to be disconnected, but not removed, follow the steps up to battery removal. Then when the sequence is reversed, the steps are followed again in reverse order. That would mean to disconnect the battery at the negative terminal. So putting a switch in the system, that is where it would be placed. (At least in my understanding of electricity)

As I said, I can find no "do it this way only" reasons for installing the switch on the positive side nor can I find reasons to install it on the negative side. I'm inclined to believe that either side "effectively disconnects" the battery and serves the purpose we're looking for in an RV, which is to stop discharging batteries while in storage. And since there's really no "explosion potential" or "electrical catostrophy" likely to occur, it's pretty much to each his own......

[BullDog33]

Originally Posted by raytronx

How would that short develop and drain if there is no return path to the battery with the negative disconnected?

I have mine on the Positive side mainly because it's also a battery switcher between 2 banks.

A short to the chassis of the trailer can close the circuit and cause the electricity to flow. There are quite a few components in the trailer that are grounded to the chassis and even without the battery negative terminal hooked up.



As for the statement about personal preference, I don't necessarily agree with that. Electrically, it is always better to have the disconnect closest to the source as possible. It is just good electrical engineering.

[BullDog33]

Originally Posted by JRTJH

Some have indicated that in newer cars, mechanics are instructed to disconnect the positive terminal before removing the negative terminal from the battery. Apparently this is to maintain a ground on delicate electronic computer equipment in the car. I've not yet been able to find that in any factory documentation. I'm not saying it doesn't exist, I just can't find it.

EPROMS (Erasable Programmable Read Only Memory) chips are highly susceptable to grounding issues. The reason everyone is starting to change their standards of disconnecting batteries with the ground last is to make sure that any latent voltage is dispersed through the system. When you disconnect the ground first, the system effectively stays energized and can lead to increased possibility of discharge. Discharge potential to EPROMS is exponentially higher in this case and could lead to erasure of the provisioning and possible destruction of the chip. These chips are starting to be seen everywhere.

I've been working in telecom for over 20 years in operations with my team mostly working in on DC systems. A lot of those are -48 VDC systems where the current flows from negative to positive as opposed to positive to negative on +12 volt systems in trailers. Most of my experience comes from DC systems and we engineer them all with the switches closest to the current source as opposed to the load.

[gepaine]

BullDog33,

I disagree with your argument. If the negative terminal of the battery is disconnected, it is impossible for electons to flow thru the battery.

[raytronx]

A short to the chassis of the trailer can close the circuit and cause the electricity to flow.

Where is the electricity going to flow to, only one terminal of the battery would be hooked up. The wire from the chassis to the negative battery terminal is disconnected. An open circuit.

[B&T]

Originally Posted by BullDog33

A short to the chassis of the trailer can close the circuit and cause the electricity to flow.

I disagree, it will not happen with either lead of the battery disconnected.

There are quite a few components in the trailer that are grounded to the chassis and even without the battery negative terminal hooked up.

True, but whats your point?

As for the statement about personal preference, I don't necessarily agree with that.

It certainly is personal preference! The effect is the same, disconnect (-) no current will flow, disconnect (+) and no current will flow.

Electrically, it is always better to have the disconnect closest to the source as possible. It is just good electrical engineering.

This is true but it still makes no difference whether you disconnect the (+) or the (-) side.

[JRTJH]

I got busy this morning looking for answers from the "big three" regarding this topic.

Here is the statement from the 2011 F150 owner's manual, page 383:

"It is recommended that the negative battery cable terminal be
disconnected from the battery if you plan to store your vehicle for an
extended period of time. This will minimize the discharge of your battery
during storage."

Here is the statement from the 2013 Chevrolet Silverado 2500HD owner's manual section 10-28:

"Infrequent Usage: Remove the black, negative (−) cable from the battery to keep the battery from running down.
Extended Storage: Remove the black, negative (−) cable from the battery or use a battery trickle charger."

Here is the statement from the 2013 Dodge Ram 2500/3500 owner's manual page 667:

"If you are storing your vehicle for more than 21 days, we recommend that you take the following steps to minimize the drain on your vehicle’s battery:
• Disconnect the negative cable from battery."

I also went to a number of sites that sell battery disconnect switches, some upwards of $100 each) looking for installation instructions and here's what I found:

Granger Automotive: http://www.grainger.com/Grainger/FLA...t-Switch-5LPK5

FLAMING RIVER Battery Disconnect Switch, Heavy Duty
Comments: "Battery Disconnect Switches Use to quickly disconnect batteries to prevent battery drain, theft, tampering, and unauthorized use; help improve the safety of stored vehicles. Mount on the negative terminal to reduce drain from ground connections."

WIRTHCO Automotive:

Compact Battery Disconnect Switch: http://www.wirthco.com/battery-disco..._170-l-en.html

"All our Compact Battery Switches feature:
•Easy to install on negative battery post.
•Battery is disconnected by unscrewing the knob a few turns, and reconnected by tightening the knob.
•Knob can be removed completely to deter theft."

I could list more, but I don't think it is necessary.

I would also add that NHRA (National HotRod Association) regulations require that vehicles raced in sactioned races must be equipped with a battery cutoff switch that is installed in the POSITIVE side of the battery circuit. The reason stated is so that the alternator can be disabled in the event of an emergency situation thereby cutting all power to the vehicle. (There is not alternator in a travel trailer.)

So, from looking at information from the "big three" automakers, they all recommend removal of the NEGATIVE battery cable for storage, multiple battery switch manufacturers installation instructions refer to installation in the NEGATIVE battery side of the circuit and NHRA mandates the POSITIVE side for race cars (with an alternator)

Again, I don't think there's a "right way" or a "wrong way" to install in an RV. The ultimate goal is simply to remove the battery from parasitic drains during storage and either installation position does that.

It comes down to a matter of personal preference. Either way works quite well for the intended purpose. I wouldn't recommend that anyone go out and change the current installation of their switch. Now if you're going to race your fifth wheel at an NHRA sponsored strip, you might want to check for more specific instructions.

[raytronx]

I'm going to install mine between 2 series 6 volts! That way I can be on a positive and a negative post!

[Festus2]

Another reason that is often given for disconnecting the negative side first is one of safety. While using a wrench to loosen the nut, it is always possible to inadvertently touch "ground" with it, causing sparking and personal injury IF you were to remove the positive side first.

[JRTJH]

Festus2, that is so true. During my surgery days, we had to repair or amputate a number of ring fingers due to electrical burns from patients gripping a wrench and shorting their wedding ring to ground and the positive battery terminal. If you think about it, that ring, being made of gold in most cases, is an excellent conductor of electricity and can get 'white hot" in seconds. It's definitely not a good way to start a camping trip.

[B&T]

Yep, knew a mechanic from years ago that was wearing a metal watch band -- burned an imprint of the band on his wrist -- but amazingly, he actually managed to do it while disconnecting the negative terminal -- go figure.

But we are talking about installing a disconnect switch to disconnect the battery and will not be using tools to disconnect the battery once the switch is installed.

Technically, i see no harm from placing it either on the positive or negative side.

If it counts -- Pretty sure keystone puts the battery fuse on the positive side (too cold and rainy to go look)

[BullDog33]

The question is what is safer - The answer is as I stated, to put the disconnect switch at the positive terminal. If you put it on the negative terminal you increase the risk just because the entire trailer is effectively charged and looking for a path to ground. It can find a path many ways to ground including through you, your truck, your kids, generator cord or any number of ways.

We had a Sprint cell site on a trailer that had the battery disconnect on the return side instead of the load side. A tech stopped, dropped the trailer from the hitch and then touched both the trailer and his truck at the same time. Similar to what others have seen happen, the current went through him and he had his watch heat up and give him a second degree burn. This was roughly a 3000 amp battery backup system but it outlines my point better than I had made it earlier. Had it taken another route through his body it could have killed him and the root cause was ultimately an improper install.

The safest choice is to limit the power to the smallest footprint possible. This same philosophy keeps the vampires at bay as well.

[raytronx]

I've spent 28 years as a electronic tech and your DC current and battery logic baffles me. Guess I better brush up on 12 volt systems again.

[B&T]

My thoughts, exactly the same!

[gepaine]

I just have to say "adios" to this thread.

There is only so many ways to try to explain basic dc circuit theory.

If someone refuses to accept that a path back to the negative terminal is required for current to flow, then so be it.