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Author Topic: pressure switch  (Read 2006 times)
Skykingrob
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« on: December 13, 2013, 08:27:36 PM »

I am wanting to charge the house batteries from the DN 50 alternator when the engine is operating. I have the line run but not yet connected to the house battery bank. I need some kind of pressure switch that will close to allow the current to go from the alternator to the batteries. I do not know which is best, air or oil, but think it would be easiest with an oil switch. I do not what to look for. Please give me a link or specific name/number of an oil pressure switch that would work on a 8v92 whose standard oil pressure is 15-25 psi and an alternator whose output could be as much as 250 amps. Please also give me the name of a electrical switch that can tolerate the amount of current from the alternator and can be triggered to close by a signal from the oil pressure switch.
I did search the archives but did not find anything, most likely because I didn't know what to search for.
Thanks

Rob
91 XL 40
Missouri
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luvrbus
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« Reply #1 on: December 14, 2013, 02:44:06 AM »

A regular oil pressure sender will work but I only seen the hookup used with a relay www.texasindustrialelectric.com  they can help and may have what you are looking for

good luck
« Last Edit: December 14, 2013, 02:49:34 AM by luvrbus » Logged

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eagle19952
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« Reply #2 on: December 14, 2013, 08:33:48 AM »

You will (should) need a relay/contactor....
I know of no sender unit that will carry charge amps by itself.
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Donald PH
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« Reply #3 on: December 14, 2013, 09:31:15 AM »

Why do you want to switch the alternator output?
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Cary and Don
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« Reply #4 on: December 14, 2013, 10:10:55 AM »

Why not use a battery isolator switch?  Bus generator to the input,  bus batteries to one output, house batteries to the other output. Works great on our system.

Don and Cary
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bevans6
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« Reply #5 on: December 14, 2013, 10:15:52 AM »

Switching the alternator connection to the house batteries is a simple and easy way to automatically charge the house batteries from the alternator yet ensure that the start batteries are not drawn down by extended house usage.  A 5 psi pressure switch connected as the signal for a high current relay would work, you may need to have the pressure switch connected by a small relay before the big relay, you need to research the pull-in current of the high-amp relay coil.  My MCI has such an oil pressure switch as stock on the pressure manifold, it usually lights up a tell-tale light on the dash.

There is a side-effect to doing this with an oil pressure switch.  It needs to be a low enough pressure switch so the relay isn't cutting out whenever the engine idles.  Cranking the engine over will normally produce enough oil pressure in two or three seconds to operate the switch.  At that point you would see the house batteries switched into parallel with the start batteries, which you may not want.  I personally wouldn't use an oil pressure switch, I would use an air pressure switch as stock on the MCI.  It is used in my bus to turn the alternator on when air pressure is up to 70 psi or so, it could easily also switch the house batteries into the circuit.  Another caution, though.  It's very possible for the house batteries to be quite discharged.  The start batteries will also be a little discharged, having just started the bus.  If both sets of batteries are hooked to the alternator simultaneously then the current draw can spike to very high, with the resulting load on the alternator drive system peaking.  If your drive gears are weak at all, this will put the highest load on them and possibly instigate a failure.  I know that on my belt drive alternator, with 4 well tensioned belts, the current load when that happens can cause the belts to slip and squeal until I switch the house batteries out of the circuit.  For this reason I have implemented a manual switch system so that if I know the house batteries are drawn down a lot I can let the start batteries charge for 5 minutes before I switch the house batteries onto the alternator.

Brian
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« Reply #6 on: December 14, 2013, 02:05:00 PM »

Your best bet is a simple relay and toggle switch, then you are independent of any pressure system and can switch when you want - like adding on the house batts for an emergency start with weak start batts.

Keep it simple!

Even simpler is a battery disconnect switch on each batt, but that means you have to go outside to make the switch. There should be a disconnect on each batt anyway for emergencies - been there -  stuck start solenoid!!
« Last Edit: December 14, 2013, 02:08:28 PM by gus » Logged

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Skykingrob
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« Reply #7 on: December 14, 2013, 03:21:33 PM »

Thanks for the ideas guys, keep them coming. I wondered about the demand on the alternator when the oil pressure reaches the cut in pressure and as Brian says, that may be a failure point. That is why I was using some kind of pressure switch, to keep the alternator from going from doing nothing to full work load in short order charging both the start and house battery systems. Likewise, I thought the air pressure switch could cause an undesired effect in that while there is air pressure, the systems, house and start batteries that is, would stay connected even though the coach is not running which could lead to the start batteries draining.
I like the idea of a high current relay like those Clifford mentions from TI and the toggle switch Gus mentions is certainly simple but the same problem exists in that if I forget to shut off the toggle switch, the two systems are connected resulting in the start batteries possibly draining down. However, that is only true if there is not a diode in my battery isolator system which like Don says, is how his is set up. I will check to see if there is a diode in my battery isolator but in the meantime, Don, how is yours set up and what brand battery isolator do you have?

Rob
91 XL 40
Missouri
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Cary and Don
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« Reply #8 on: December 14, 2013, 05:20:40 PM »

We have a toggle switch that operates a relay that will let our house batteries charge while running.  It is energized by a circuit that is hot only when the run switch is on.  Turn off the engine and the circuit is off.  This is in addition to a jump start circuit that runs on a momentary switch.

Don and Cary

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« Reply #9 on: December 14, 2013, 05:40:25 PM »

Just to chime in.  I agree with Gus, Don and Cary and others that use/suggest a dash toggle energized through the master switch for this job.  Sometimes automatic is better but since one can conceive of situations when you would want control here, it is better to have a manual switch.
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« Reply #10 on: December 14, 2013, 08:40:31 PM »

I charge my house batteries from the alternator using a circuit that was originally for the A/C unit.  This circuit only has power if the engine is on and the alternator is working.  I added a solenoid with a switch on the front dash so I can turn off the power if I need to reduce load on the engine.
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« Reply #11 on: December 15, 2013, 05:50:40 AM »

Manual switch works for me and also helps if you need a boost on start. Jerry
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Jon
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« Reply #12 on: December 15, 2013, 06:08:25 AM »

Rob,

I think the 50DN has a relay output post. If it does then a simple way to send juice to the house batteries is to use a solenoid such as the firewall mounted ones found on old cars. They are available from NAPA, are cheap and when the alternator is not putting out power the solenoid opens so the two battery banks are not tied together.
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Jon

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« Reply #13 on: December 15, 2013, 06:20:07 AM »

Just buy the right stuff I have a friend that stops here every year and we change out his relay some here call a solenoid you cannot convince this guy a 50amp continuous duty relay (solenoid) he pays less than 20 bucks for will last with a alternator putting out 250 amps charging the house batteries 

The 6 years I know of him replacing the relay he could have bought one that would carry the load
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gus
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« Reply #14 on: December 15, 2013, 09:27:07 AM »

A relay to a toggle switch carries a very small load, insignificant actually, so that is not a problem.

My method is to drive a while to let the starts get a little charge before connecting the house batts. However, with a 300 amp alt that is probably not necessary!
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« Reply #15 on: December 15, 2013, 09:34:38 AM »

You are 100% about the load required to open and close a relay but that nothing to do with the amps a relay or solenoid will carry
« Last Edit: December 15, 2013, 10:31:45 AM by luvrbus » Logged

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« Reply #16 on: December 15, 2013, 11:20:49 AM »

I have the dash switch method with a 400 amp solenoid. I have three dash switches I turn on in the process of starting the bus.

1 to charge house batts
2 to power the 12 converter
3 to provide 12 volts for the 12 volt guages I have

Of course there is a special order to turning on the switches --- Just like the order of checking and starting the bus up.

HTH

Melbo
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« Reply #17 on: December 15, 2013, 11:37:48 AM »

I think you should also include a small LED warning light on the dash, or an illuminated switch that indicates the circuit is "on."

If you could figure out a way, have it flash when you turn off the ignition switch as a reminder the batteries are still interconnected.

FWIW & HTH. . .

  Wink
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Skykingrob
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« Reply #18 on: December 15, 2013, 12:15:41 PM »

Well, I think I have found the relay to use to control the charge current to the house batteries. Here is the link, see if you agree (may have to cut and paste into a browser window if the link doesn't start on it's own):
 
http://www.texasindustrialelectric.com/Relays_24824_01.asp

What I have not settled on or found yet is a switch to control the relay. It could be air, oil or just a toggle switch. Advantages and disadvantages to each of them. I will need to study the manual on the DN50 Jon, to see if it has a relay on it that will work.

If I have the relay right, let me know.

Rob
91 XL 40
Missouri
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bevans6
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« Reply #19 on: December 15, 2013, 01:06:24 PM »

All 50DN alternators have the R terminal, the bus wiring would have a relay that might be connected to it.  It's a tap onto the center of one of the three diode pairs, so it puts out a ripply half voltage signal.  If yours is a 24v alt, it will put out a 12v signal.  All it does is tell something, usually a relay (because a half voltage signal full of ripple isn't good for much other than turning on a relay) that the alternator is operating.   So if you plan to use this, it's not only exactly what the R terminal is supposed to be used for but you need to look for a relay of appropriate current capacity but it has to have the correct voltage for the control coil.  As with all things, there are caveats and things to know...  On many buses the R-terminal is already connected to a relay and that relay works (among other things) the Alternator tell-tale light on the dash.  With many buses (my MCI for example) the 50DN will start to flicker the No Gen light if there is little load on and the batteries are fully charged.  If that should happen with your bus and you are using the R-terminal to control the relay, the relay will be toggling open and closed with every flicker.  That is something to be aware of.  There is a reason an awful lot of people decide to use a manually controlled switch - or just combine the start and house banks permanently, the idea being that the generator (with a separate start battery) can be the back up if things get drawn down too far to start the bus.

Edit - I think the relay is fine.  For me, the number to look at is the surge rating, which in this case is 600 amps, for boost starting the engine.  If you do the R-terminal deal discussed above, then you would use that relay, driven from the output of the R-terminal relay.  The 50DN would not switch that big relay directly from the R-terminal.

Brian
« Last Edit: December 15, 2013, 01:17:24 PM by bevans6 » Logged

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« Reply #20 on: December 16, 2013, 10:03:31 AM »

I agree with Brian, that relay should work fine. Or you probably can find one at your local auto supply store, depends on the prices. Just make sure it is a full time. It will look like a generic start solenoid/relay on older cars except it is rated full-time (Constant current).

The only time it will be carrying many amps is if you ever have to use it as a start booster, a rare thing normally. I've maybe done it three times in seven years.

It still won't take anything more than a simple toggle switch, holding current is small even for a high capacity relay. Simple is better!!
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« Reply #21 on: December 16, 2013, 11:31:30 AM »

On my MCI there is a relay in the rear junction box that is activated only when the air pressure is up, the engine is running and the alternator is operating. It is on the OEM A/C wiring circuit. It originally prevented the A/C blower to come on before the engine was running and the alternator was operating.  If memory serves me correctly, that relay can provide a signal to your battery solenoid. You can route that signal through a toggle switch and also use an empty telltale socket for an 'on' light. I also added a small timer circuit in between the relay signal and the toggle so that there is a 30 second delay after I flip the switch between the alternator coming on (Gen Light goes out) and the solenoid is fed the signal. If you use a toggle switch that has an On Off MOM positions, you can manually bridge the house batts to the starter batts for emergency starting situations. Use the On for your full time connection and the MOM for the emergency bridge. That MOM has really come in handy a few times when the start batts got run down. If you have a sufficiently large inverter, you can run some heavy loads from it while running down the road with your house and start batts bridged to the 50DN (280 Amps!)   
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« Reply #22 on: December 16, 2013, 01:36:17 PM »

Rob, That switch looks like it will do the job. As an alternative you can use an isolator such as have been used on our store bought conversions. They are a little pricey and they take a little more room.

I have had both types that I mentioned on my coaches and in 15 years I replaced two solenoids, so far no isolators have failed in about 10 years of experience with them. http://www.allbatterysalesandservice.com/browse.cfm/4,6120.html
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Jon

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« Reply #23 on: December 16, 2013, 01:49:24 PM »

Isolators take more cable than a relay to hook up but they work good,the battery separator gizmo is getting popular but I have no idea how they work   
« Last Edit: December 16, 2013, 01:52:56 PM by luvrbus » Logged

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Skykingrob
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« Reply #24 on: December 16, 2013, 06:54:21 PM »

Thanks to everyone for input. Still studying the pros and cons of the switch. The reason I was looking at the air or oil switch was to make it as automatic as possible so if I forgot, then drained batteries was not the consequence but as you all have pointed out, there are other consequences to air and oil switches that I had not anticipated. So, still thinking that decision through.
Thanks again. If more ideas, let me know.

Rob
91 XL 40
Missouri
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« Reply #25 on: December 17, 2013, 10:52:57 AM »

Just my $.02 - battery isolators produce heat and as such are less efficient than a solenoid. If you want one, contact me off the board as I have a nearly new one available.
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