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Old 04-01-2016, 05:22 PM   #1
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Battery Disconnect Switch

I want to add a battery discount switch. Is it best to put the switch in the positive line or the negative line? What is the reason one is better than the other?

Also, if you charge your battery with an external battery charge, (in other words a battery charger other than your on board converter), do you have to completely disconnect the battery during the charging process?
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Old 04-01-2016, 05:30 PM   #2
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Doesn't matter which lead you insert the switch in, pos or neg. I think it is generally accepted that the pos lead is the norm. I would think there is no need to disconnect the battery with an external charger WHEN no devices are on in the rv, but if you have the disconnect I would switch it off anyway.
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Old 04-01-2016, 05:54 PM   #3
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I would cut off the positive side with the switch, but technically either side will work.

It is my practice to only have one charging source at a time. The chargers (if relatively smart) will see each other then "think" the voltage is the battery(ies) without "knowing' the higher voltage is being produced by another charger. You may or may not get a good charge by using more than one at a time and if you do it could take a long time.

Make sure the converter/ charger you have charges correctly, most do not; causing premature battery failure or starting the evening with 80% or less of its (their) potential.

Also, don't expect the +B from the tow vehicle to charge the battery while traveling.
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Old 04-01-2016, 06:13 PM   #4
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X2 with WolfWhistle on the positive side for the switch.
Also, on the trailer plug connection, just because you have an alternator that is rated at 150 amps [hypothetical], doesn't mean you will be charging the batteries at anywhere near that amperage. Not even close. I would venture a guess at 10-15 amps tops. I may be wrong and I've never checked it, but the wiring would be much larger if it was more than that.
If you run two chargers [converter and external charger] at the same time, they are "reading each other". One at a time as WolfWhistle said. You can just hit your breaker for the converter when using an external charger or your disconnect switch.
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Old 04-01-2016, 06:38 PM   #5
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The reason I am charging with an external is to see if I can get the battery fully charged. As I understand fully charged, I should be 13 volts plus. I am only reading 12.6 after being plugged in all weekend, (and I have checked it a few times). So currently I am not on shore power at all. I have the battery totally disconnected and have a battery charger on it set to charge at 2 amps. I want to take a reading in the morning and see if I have more than 12.6 amps showing on the battery.

Also, I will add the discount switch to the positive side. Thank you for the input. Anyone with any input on the battery voltage, I would be happy to read.
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Old 04-01-2016, 06:46 PM   #6
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The standard car battery voltage in today's vehicles is 12 volts. Each battery has six cells with 2.1V. ... A fully charged car battery has 12.6 volts.
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Old 04-01-2016, 06:59 PM   #7
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Here is a handy chart.
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Old 04-01-2016, 07:31 PM   #8
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Well I guess my battery was fully charged. For some reason I had in my mind that normally a battery will read over 13 volts. Learn something new every day.
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Old 04-01-2016, 07:53 PM   #9
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Quote:
Originally Posted by Asrabbit View Post
Well I guess my battery was fully charged. For some reason I had in my mind that normally a battery will read over 13 volts. Learn something new every day.
That is the typical charging voltage. Easy to misinterpret.

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Old 04-01-2016, 08:25 PM   #10
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In many forums. one has to get to the fact vs opinion. A review of the previous post have some validity, but ... I'll try to help.


The switch MUST be on the Positive lead and as close as possible to the battery. If not, you still have a HOT battery that can cause a short, .... Granted the circuit would be open, but if you touch ANY positive wire inside to ground, you WILL have a complete circuit and KABOOM!


I would disconnect the battery whenever doing external charging so that the charger can get proper load measurements it needs from just the battery alone.


Battery Charging is a long story - You can read the following FAQ from a website that explains it in detail, but it has a lot of extras too. ZRD FAQ - Charging, Remote Charging, Battery Requirements Personally, I ensure charge is initiated when the battery bank gets near 12.5volts and certainly by the 12.3-12.0 volt range to ensure long life.


Most standard cells are 2.2Volts when they are fully charged (times 6 = 13.2Vdc). Charging Voltage has to be 1.0 Volt above battery voltage to work. This is why the typical single stage automotive type alternator is set to output at 14.2-14.4 Vdc. This will enable the battery to reach the 13.2Vdc to complete a charge. Read the FAQ above for additional info on 3stage charging. For a single automotive type battery, it is not necessary, but if you have a large expensive battery bank it is imperative.


Multiple charging sources are great, but only use 1 at a time to charge a battery/ bank. Ensure your solar is turned off/disconnected or it will confuse everything if you are not using the solar exclusively.


BTW, one of the biggest killers of batteries is leaving them almost permanently on a charger. Consider it eating one teaspoon at a time, but never stopping.

I hope this helps/clears some thing up. Ask away if you need additional info.
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Old 04-01-2016, 09:37 PM   #11
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Battery Disconnect Switch

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Originally Posted by 25FKS View Post
In many forums. one has to get to the fact vs opinion. A review of the previous post have some validity, but ... I'll try to help.


The switch MUST be on the Positive lead and as close as possible to the battery. If not, you still have a HOT battery that can cause a short, .... Granted the circuit would be open, but if you touch ANY positive wire inside to ground, you WILL have a complete circuit and .


So this video is wrong?

http://youtu.be/NPI3f3edL9U

http://www.hotrod.com/how-to/interio...e-or-negative/
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Old 04-01-2016, 10:05 PM   #12
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Another video recommending negative side

http://youtu.be/qM7mpOnSjDM


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Old 04-01-2016, 11:25 PM   #13
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25FKS, have a question for you.

If the battery negative has been disconnected, how can you possibly have a complete circuit by touching a positive wire inside to ground. Unless those negative electrons are floating through the air to complete the circuit.
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Old 04-02-2016, 06:56 AM   #14
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Generally most of us are attracted to the positive side of a battery and totally forget about the return path through the negative. Most mechanics will disconnect the negative when working on a vehicle. On an RV, disconnecting the negative will insure that all loads are disconnected, including those that go directly to the batteries without going through the battery control center, disconnect switch, etc.

I go with disconnecting the negative since there are lots of way that you can inadvertently short a wrench from the battery to any point on the chassis.
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Old 04-02-2016, 07:29 AM   #15
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After watching the videos it looks like the negative side is the way to go. I do wonder however if the frame is used for a ground anywhere? I guess, there really would still not be a path back to the battery. A question I really thought would be easy, just does not seem all that easy. I guess I can go one way and always change it later on if I need to.
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Old 04-02-2016, 07:55 AM   #16
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As there is no way to guarantee what someone does / mods installed, ... or something happens over the years, .... The safe way is to remove power! If you have worn, bad, replaced wiring, ... list goes on, there seems to always be a way for power to somehow find a way to ground eventually.


The only safe way is to remove power completely with the shortest wire path. BTW That is what circuit breakers & panels do. I won't go in circles/debates here - Do whatever you please. I just offered my assistance and years of experience, I do not need ____. I'll leave that to Politicians. Last post on this subject.
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Old 04-02-2016, 08:43 AM   #17
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I'm so glad others addressed the, I can't say because this is a family place, given here.

The disconnect is usually on the positive side for convenience, the ground usually comes out of the battery box, runs about a foot and is solidly connected to the frame. The positive side is routed out of the box, along the frame, to the breaker where it's then distributed throughout the unit. It's easier to add the disconnect to the system right next to the breaker. Here it doesn't matter which side is disconnected, but to disconnect the ground it would require extra work that isn't necessary.

The statement about mechanics pulling the ground first and connecting it last is true for a few reasons. If we drop a wrench on the battery, then yes it can hit the fender, ground out and be a problem. The battery is surrounded by grounded metal, so by pulling the ground, you make that metal inert.

When working with the ground first/last it's safer. There is most always a spark when completing the circuit, when connecting the ground the spark flows from the battery to the cable and is dispersed through the framework, but when connecting at the positive it flows from the cable to the battery which at that proximity could cause a problem if the battery has been gassing. The spark created by connecting the positive last is much larger compared to connecting the ground last. Within the confines of a disconnect switch this is a moot point.

Finally, auto electronics are designed to wake up by seeing the ground last. Believe it or not, they are sensitive to it and while the exact how's and whys are best explained by an engineer in Detriot I know you can put a car in logic lock by improperly installing a battery. I've fixed several simply by pulling the ground cable, discharging the entire system, and reconnecting the ground. This isn't an every car every time problem, but does exist.

In the RV world, so far as I'm aware with towables, we don't have those issues, which is why it's still okay to switch the positive. It's absolutely best to put the switch on the ground side, but there's not enough difference to justify adding that much cable to put the switch in a convenient spot.
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Old 04-02-2016, 09:37 AM   #18
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My camper and boats have the perko cutoff on the positive side and close to batteries.

On my boat i use my perko switch to disconect, use one battery, OR the other, or both combined. Can't really do that if your hooked to ground wires. Besides that, its all about eliminating sparks in boats.....or things can go boom!

Vehicles and equipment.....disconnect the ground from battery.
Campers and boats........disconnects on positive.
Thats what I've seen and recommend!
( like most have already stated)
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Old 04-02-2016, 09:45 AM   #19
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I would have said Negative but could see where Positive may be chosen especially if cutoff is for a portion of the circuit.


The battery mount type I must agree need to be Negative mounted for safety reasons. The Positive should always be protected if exposed. Marine and most cars today have red covers over the positive terminal to prevent accidental contact. The negative off first and on last is why I would side with the Negative side of the discussion.

On the Positive side I see my Coachman Class "C" wire diagram has Cutoff on the Positive with a few circuits still powered when cutoff is OFF.


I believe there may Not be a all inclusive correct answer. Correct may be when done safely for the needs of the unit. Trailer would have some different considerations than say a Motorhome.
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Old 04-02-2016, 09:55 AM   #20
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The battery disconnect on a marine and camper application on the positive side, may also be because both are often times charged with built in chargers.

Vehicles usually are only charging while they are running.
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