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Old 04-09-2020, 11:13 AM   #21
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Here's the switch I used... I think the brand is "Battery Doctor" or something like that... 2 years old now and no issues so far. This one seems to be OK quality, but as others have said, the BlueSea models are the best.

I had enough slack in my cables that I didn't need to add in any cable, I just cut my negative cable at the appropriate location, and then crimped some cable lug rings on the ends.


For the life of me, I don't know why the manufacturers don't make these standard equipment
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Old 04-09-2020, 11:15 AM   #22
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Is there another one of these blue seas that can be remotely switched or are they all like the one in the first link where you have to run large gauge cables and the entire battery load directly through the actual switch? The only 1 I see is $189 but it has a 24V not a 12V or 6V coil on it.
https://www.bluesea.com/products/cat...s/ML-Solenoids
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Old 04-09-2020, 11:17 AM   #23
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Originally Posted by Jmac1 View Post
Thanks for the quick responses! Is everyone that's installed these switches really wiring them to a relay? Every DIY video I've watched has them hotwired but I do want to do it the correct way.
Most folks that install a battery disconnect switch are installing a manual switch and not that described by Ampdraw.

I'm not saying his post isn't relevant... just saying... of the hundreds of prior posts on this subject I've seen on this forum over the years, I believe his is only the 2nd that suggested it.

If your statement about 'hotwired' has to do with which wire to use... that debate (positive or negative side) has been hashed to death and the bottom line is... either is correct. You'll get LOTS of arguments over which is MORE correct.

I always like to use the negative side as there are typically less wires on the negative side. (usually one) Use the same gauge cable as the original cable that went to the battery.

Remember... if you completely isolate your battery with a disconnect switch... be sure to reconnect the battery BEFORE towing as the on-board battery provides power to the trailer brakes in an emergency breakaway situation. (breakaway switch plunger pulled/7 pin plug disconnected)
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Old 04-09-2020, 11:20 AM   #24
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I was thinking of mounting a switch on top of the battery box. The reason is to avoid having to undo the strap and pull the lid every time I want to use the switch. I'll be inches from the battery. I was, however looking at the 24inch cables so I could just set the box lid aside when access to the battery would be needed. Not sure how that would work with using a 12" cable.
With a travel trailer like yours, its kinda pointless using a relay. Use a switch.

As far as positive or negative, electrically it doesn't matter. It's just convention to put it on the positive side.
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Old 04-09-2020, 11:28 AM   #25
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Since you want the switch right by the battery any suggestion anyone gave you here is the “right way” and a relay would seem like over kill. I wouldn’t worry about just running 24” of extra battery cable if I’m understanding you correctly, just use the same or thicker gauge wire that’s already on it.
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Old 04-09-2020, 11:40 AM   #26
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Most folks that install a battery disconnect switch are installing a manual switch and not that described by Ampdraw.

I'm not saying his post isn't relevant... just saying... of the hundreds of prior posts on this subject I've seen on this forum over the years, I believe his is only the 2nd that suggested it.
There’s only 1 other actual Engineer here?

You know what they say, I’m an Engineer so lets just save time and accept I’m right.

I wouldn’t take accounting advice from an Engineer, just like I wouldn’t take Engineering advice from an accountant. It sounds like this forum is full of accountants? Lol
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Old 04-09-2020, 11:40 AM   #27
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For those wanting to use a relay, some important notes.

First, the relay MUST be rated for continuous duty.

It must also be rated for the maximum current flow expected, especially if you are (or ever plan on) using an Inverter.

A typical relay is just a solenoid operated switch and does draw power all the time it's on. Power that does nothing but keep the switch closed.

Rather than a typical solenoid type relay, consider a Solid State Relay which draws (and wastes) far less power. They're not mechanical nor do they have contacts that can arc, corrode, or just plain stick.

FWIW a regular battery switch can be mounted much farther than 1' from the battery(s). Just make the connection with larger wire. I used to have a 35' boat that had a battery bank located probably 10 feet from the battery switch. Just ran 2/0 wire from batteries to switch and back to the power distribution panel.

Every installation will be different and one needs to take into consideration the current, total length of wire run, and then consult any one of the numerous online wire size calculators to determine wire size.

Back to relays, If you want total, 100% reliability, use a quality manual switch. Using a relay requires a second set of wires and another switch. More chance for a problem to arise. If insisting on using a relay, use a solid state relay and don't waste power. Remember, the large the relay, the more the coil has to draw to activate it and hold it closed. Continuous duty, high current relays, draw far more power than the small relays used for other functions in automotive applications. Remember that the "holding current" is continuous and can amount to several amp hours of wasted energy every day.

Solid state battery disconnects are like LED lights. They draw 1/10th the power that old school solenoid type relay/switches do. A typical solid state 200 amp battery switch will draw .125 amp and the conventional type can easily draw a full amp or more, depending on it's rating.
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Old 04-09-2020, 11:47 AM   #28
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Yes I like that, 500A rating too, that’s a sexy switch. Replacing the one that was on my RV when I got it with that one might get added to my project list unless it requires a power draw to hold itself open or closed.
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Old 04-09-2020, 12:02 PM   #29
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For those wanting to use a relay, some important notes.

First, the relay MUST be rated for continuous duty.

It must also be rated for the maximum current flow expected, especially if you are (or ever plan on) using an Inverter.

A typical relay is just a solenoid operated switch and does draw power all the time it's on. Power that does nothing but keep the switch closed.

Rather than a typical solenoid type relay, consider a Solid State Relay which draws (and wastes) far less power. They're not mechanical nor do they have contacts that can arc, corrode, or just plain stick.

FWIW a regular battery switch can be mounted much farther than 1' from the battery(s). Just make the connection with larger wire. I used to have a 35' boat that had a battery bank located probably 10 feet from the battery switch. Just ran 2/0 wire from batteries to switch and back to the power distribution panel.

Every installation will be different and one needs to take into consideration the current, total length of wire run, and then consult any one of the numerous online wire size calculators to determine wire size.

Back to relays, If you want total, 100% reliability, use a quality manual switch. Using a relay requires a second set of wires and another switch. More chance for a problem to arise. If insisting on using a relay, use a solid state relay and don't waste power. Remember, the large the relay, the more the coil has to draw to activate it and hold it closed. Continuous duty, high current relays, draw far more power than the small relays used for other functions in automotive applications. Remember that the "holding current" is continuous and can amount to several amp hours of wasted energy every day.

Solid state battery disconnects are like LED lights. They draw 1/10th the power that old school solenoid type relay/switches do. A typical solid state 200 amp battery switch will draw .125 amp and the conventional type can easily draw a full amp or more, depending on it's rating.
The latching relay I posted the link to at the top of the page defies your logic and thinking of how a standard relay works, here’s the description (and its only $33 with free shipping)...

Quote:
Description
The Intellitec Battery Disconnects provide a simple, safe and reliable means of remotely disconnecting 12 Volt or 24 Volt battery systems. These unique latching disconnects designed and manufactured by Intellitec are capable of carrying heavy currents. They require no power to stay open or closed, the Intellitec battery disconnects only draw power during activation.The disconnects are sealed against the environments and are designed to withstand the shock and vibration experienced within commercial vehicles, trailers, coaches bus or boat applications.
A solid state relay as you’re describing also typically needs to be mounted on a heat sink with paste or it’ll fry itself under normal use. Not very efficient if its converting (wasting) your power into heat, you’re only considering the activation power draw, not the loss taking place within the SSR from your load passing through it which is what creates all that heat.

Your “quality manual switch” has the same design of contacts inside of it that as you put it “arc, corrode or just plain stick”.

Do you seriously want to have this argument with an actual Engineer? Just be warned in advance, you’re bringing a knife to a gun fight and I love a good debate, especially when I’m right lol.
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Old 04-09-2020, 12:06 PM   #30
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Thanks guys and thanks Ampdraw for your willingness to walk me through this. I'm sure I'll be back with more questions because yes, I am electrically illiterate. I'm just a humble beer delivery guy who likes to camp with the wife and kids.
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Old 04-09-2020, 12:16 PM   #31
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Yes I like that, 500A rating too, that’s a sexy switch. Replacing the one that was on my RV when I got it with that one might get added to my project list unless it requires a power draw to hold itself open or closed.
It's pretty easy to read the spec...it's latching.
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Old 04-09-2020, 12:23 PM   #32
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The relay I posted the link to at the top of the page defies your logic and thinking of how a standard relay works, here’s the description...



A solid state relay as you’re describing also typically needs to be mounted on a heat sink with paste or it’ll fry itself under normal use. Not very efficient if its converting (wasting) your power into heat.
The switch your referenced is NOT your typical relay that's sold in most locations but rather a specialized design. If using this switch then yes, no current is drawn. Conventional continuous duty relays like the Cole-Hersee type repays sold in most auto parts stores work exactly as I stated.

As for power dissipation on a solid state relay? Just how much heat is generated by 125 milliamps? At 12.5 volts it's a whole 1.6 watts. That translates to about 5.5 Btu/hr. As for the continuous current being switched, most RV's heavy power use is intermittent so any heat generated by high current use will be limited and relatively insignificant, unlike high power switching applications in industrial use where a heat sink is required.

I know that even when watching television at night my load is between 5 and 7 amps depending on the number of lights turned on.

Biggest load is when charging batteries and then the load goes up to 60 amps (LiFePo4 batteries). Then the heat generated goes up to 204 Btu/hr.
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Old 04-09-2020, 12:35 PM   #33
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A solid state relay has an on resistance. If it was 0.1Ω, that would mean a 100A load would dissipate 10 watts in the switch itself. Not that much but would require a heat sink. Solid state relays are often used for battery combiners. Not sure I would use one for this application ever. Of course I would not be use a latching relay for the OP's application either.
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Old 04-09-2020, 12:52 PM   #34
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A solid state relay has an on resistance. If it was 0.1Ω, that would mean a 100A load would dissipate 10 watts in the switch itself. Not that much but would require a heat sink. Solid state relays are often used for battery combiners. Not sure I would use one for this application ever. Of course I would not be use a latching relay for the OP's application either.
The average person is going to have no clue how to diagnose a problem in an SSR, or worse take it off to “look at it to see if they can find the problem” and not clean the heat sink or reapply fresh paste. They also don’t get my vote for reliability mainly just because they are so sensitive to burning up. I’d prefer having as few things as possible on my RV that are prone to burning up.

I haven’t had time yet to look at the full specs on that other switch. I have a “project list” on a white board in the garage of my toy hauler and looking into did get put on my list. I’m curious to see how they justified the $200 cost over the $33 price of the other one I just found after a quick search.
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Old 04-09-2020, 12:53 PM   #35
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Did this on mine....simple, quick install!Attachment 226530
Ditto, works great
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Old 04-09-2020, 01:03 PM   #36
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The switch your referenced is NOT your typical relay that's sold in most locations but rather a specialized design. If using this switch then yes, no current is drawn.
That was the only relay referenced in this entire post, so why bring a multi paragraph discussion/confusion about relays that no one was even talking about into the discussion?

Quote:
As for power dissipation on a solid state relay? Just how much heat is generated by 125 milliamps?
Apparently enough heat to require the thing to be mounted on an aluminum heat sink with heat sink paste.
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Old 04-09-2020, 01:04 PM   #37
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The average person is going to have no clue how to diagnose a problem in an SSR, or worse take it off to “look at it to see if they can find the problem” and not clean the heat sink or reapply fresh paste. They also don’t get my vote for reliability mainly just because they are so sensitive to burning up. I’d prefer having as few things as possible on my RV that are prone to burning up.
We actually used these all the time on some of our large chassis which cost over $400K, so not sure where you get that they aren't reliable. Saying that, I would never ever use them on an RV for the main battery disconnect.

Can we keep to the topic at hand where the OP needs a simple battery switch?
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Old 04-09-2020, 01:21 PM   #38
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. I’d prefer having as few things as possible on my RV that are prone to burning up.
That alone would be a huge argument for using a simple, time tested, manual battery switch.
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Old 04-09-2020, 01:39 PM   #39
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That alone would be a huge argument for using a simple, time tested, manual battery switch.
X2
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Old 04-09-2020, 02:55 PM   #40
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That alone would be a huge argument for using a simple, time tested, manual battery switch.
I’ve never seen a mercury switch fail outside of burnt connection terminals from someone not properly torquing the lugs on it, which is what that relay I posted looks to me to be.

Are you claiming a simple throw switch never fails? I’ve seen plenty, the spring steel on the inside weakens over time, contact points rust and corrode, plastic cracks. The contact points on a mercury switch do not have any of those problems.
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