Diode for Battery Bank Merge Solenoid

[David Anderson]

I have a 400 amp continuous duty solenoid to merge the house bank with the alternator.   This is the second one I have installed.  It seems to be failing  as the first one did.  The big ring that snaps to the two contacts in the solenoid gets dirty and of course begins to heat up and pits the ring causing more and more resistance.  Eventually it won't carry any voltage through the switch at all.  I can recycle the coil a few times with the dash switch and see the volt meter jump up when it makes a good contact.  The coil switch is #12 wire through a 30 amp relay, so the coil should be getting ample power to engage the solenoid. 

I talked to Dick Wright about this two years ago and he quit selling them because of the above described failure.  He only recommends a battery isolator now.  I know several of you guys have these solenoids and wonder about your success.   On my last trip I couldn't get more than 12.7 volts on the Trace meter while the bus alternator was showing 14.2, and I wasn't running any appliances off the inverter.   I have 0000 wire from the alternator to the Trace in the circuit.   When it was new I was getting 13.9 volts at the Trace meter which is more than enough to run one rooftop air. 

I do suspect that having the solenoid in the engine room subjects the coil to a lot of heat, maybe weakening the strength of the switch closure.   I located it there out of convenience but it could be moved into the bay near the inverter.   Where did you guys mount yours? 

Any ideas as to what I'm doing wrong here?  It's nice to tie those batteries together when you drive and don't need the genny to charge things up.

David

[RTS/Daytona]

see--> http://www.hydrogenappliances.com/solenoidrelay.html

400 amps & silver alloy contacts

[Jerry Liebler]

David,
     I've had similar experience using 200 amp rated contactors, with silver alloy contacts, to connect my buss battery & alternator to the house bank and inverter. Mine look just like the ones Pete gave a link to that proport to be 400 amps, I noticed that they had a 200 amp fuse in their test circuit so I question the rating.  Mine works fine for awhile then contact resistance seems to build up and eventually there is 1/2 volt drop (while under full inverter load) across the contactor. So I replace the contactor.  But I take apart the bad one, I file the contacts, put it back together and, when I feel like it swap the repaired unit back.  It seems like I get about a year of use before the problem resurfaces.  I carry a near new one and the tools to make the swap.  I think it's simply the nature of the beast.  My contactor is mounted in the former HVAC bay so heating isn't the issue.  I think it's simply airborne dirt eventually leads to burned contacts.  The good news is after disassembly and filing them, it seems to last just as long as a new contactor.
Regards
Jerry 4107 1120 

[Tim Strommen]

It sounds like you're using contactors rated for a continuous duty without due regard to the connect and disconnect surge and arc currents.

The fuses that are rated 200Amp for typical inverter applications are usually designed to blow after a high average current (the rating of the fuse), while sustaining brief surges (around 1000amps).

If the soleniod is connecting a high load (like a discharged battery bank to a charging supply) you can easily exceed the rating and pit the contacts with the arc.  Diode battery isolators have no contacts to arc and ar always "on" whenever there is enough voltage to bias the diode, it will begin to conduct.  This makes the flow of current/voltage more gradual and "less damaging".  The drawback is that the diode requires that voltage rise to get across the diode gap - which in some diodes can be as high as 1.5volts.  This voltage is lost as heat within the diode package.  This also requires that the voltage regulator be turned up to overcome the loss from the diode (or the batteries won't see the correct charge voltage and will fail sooner than they should).

Diodes can fail if there is a short burst of high current too, so it's important to be careful when hooking up a jumper cable (go directly to the batteries the input to the isolator).

Cheers!

-Tim

P.S.  if you want to tie the banks together for a prime-mover start in the cold, try an IGBT or MOSFET rated at a couple hundred volts and 1000Amps.  Putting a resistor or halogen light bulb in parallel with the IGBT/MOSFET will reduce the surge a bit (if you wait until the light bulb is dimmed, it means the differential between the packs is almost less severe, and the IGBT/MOSFET can be turned on without "punch-throught"). -T

[TomC]

I used two 150 amp continuous duty solenoid relays that are in parallel with a heavy strap between them.  I know it is capable of handling starting current since I've started the bus when my starting batteries were dead.  I do not though run the current for the inverter through this solenoid-it is only for tying the starting batteries and deep cycles together.  Good Luck, TomC

[JohnEd]

Jerry,

I missed the part where you described H O W  you took that thing apart and i really want to know.

Thank you,

John

[Jerry Liebler]

Johned,
   My solenoid switches had a cap over the cylindrical section that was swagged over a slight flare on the cylindrical portion.  I pried the swagged material outward, very small sections at a time and working around the circumference, with a pair of  end cutting pliers.  Once the cap is off the rest is obvious.  When I reinstalled the cap I only swagged a few points, no sense making future disassembly more tedious.
Regards
Jerry 4107 1120

[kd5kfl]

I think it's simply airborne dirt eventually leads to burned contacts.

Wrongo. It's a laws of physics thing.

You send electrons through a wire, it becomes an electromagnet. You move a magnetic field past a wire, it becomes a generator.

When you remove the current flow from a wire, the magnetic field around that wire collapses. This collapsing magnetic field is in motion; it's moving inwards. This collapsing magnetic field generates voltage. This voltage is the opposite polarity of the voltage which caused the magnetic field to expand. A big honking negative voltage spike. About 300 volts.

This spike creates arcs and sparks. These arcs and sparks burn and sandblast the contacts. This explains the fat blue spark when you unplug things.

There is a solution, for DC circuits. Commutating diodes. Commutating describes the job the diode is doing, not the diode itself. Don't ask the guy at Radio Shack for a commutating diode; you will just get a blanker blank stare than the usual blank stare.

http://www.allaboutcircuits.com/vol_3/chpt_3/9.html

Bottom line: Big diode. Cathode ( the end that normally connects to the negative ) on the load side of a switch. Anode ( the side that normally connects to positive, has a band of paint on this end ) to ground. Shunts the spike to ground, not through the contacts.

No, I can't give you a part number and source. I would just go to my friendly neighborhood surplus store and get the biggest diode there.

[Kristinsgrandpa]

kd5kfl hit the nail on the head.  His explanation was great.

Anytime you break a DC circuit it produces a high induced current, which causes arcing/pitting of the contacts. This can be reduced by silver alloy plating but not completely eliminated.

Reducing the load before switching helps.

That is also why you should use only DC rated switches on DC circuits. AC only rated switches aren't plated to withstand the arcing.


Ed

[Jerry Liebler]

Kd5kfl,
    I mostly agree with your explanation, the disagreement is with '300 volts'.  In fact there is no limit to the voltage, it increases very rapidly until something breaks down.  In the case of these solenoids what breaks down is the air in the gap between the contacts, hence the arc, which may well exceed 300 volts.  Diodes  are a solution ONLY IF the direction of current flow prior to the contact's opening is known.  In the case of a merge solenoid what's needed is something that can breakdown, nondestructively, before the airgap, regardless of the current direction.  I will try a pair of 'back to back' Zener diodes.  In fact I have a collection of devices made just for this purpose They are called, among other things, 'double anode transient suppression zener diodes'.    I'm embarrassed to say I hadn't thought of what's really going on.  Thank you for pointing it out.
Regards
Jerry 4107 1120   

[DavidInWilmNC]

I'm curious as to what solenoids you guys are using that are failing.  I've seen generic ones that look like a Ford starter solenoid called 'continuous duty', but I didn't think they looked up to the task.  I used a White-Rodgers from www.grainger.com/Grainger/items/6C025.  I'm wondering if this is any better (or worse) than some of the solenoids others have used.

David

[TomC]

Mine are the type that looks like the old Ford starter solenoid-the key is that it is rated at continuous duty.  I've left them on for days at a time with no problem.  Good Luck, TomC

[Barn Owl]

We use the White-Rogers solenoids that David mentioned where I work in 36v mules (forward/reverse and interlock switching). They are incredibly robust and have an almost not existent failure rate in our application.

http://www.taylor-dunn.com/vehicle_details.aspx?mode=base&id=22

[David Anderson]

I'm kind of leaning to a manual switch and merge the banks only before I start the engine and disconnect them only after stopping to avoid the voltage differential problem.   How would that idea work? 

David

[chris4905]

I am using this, for the past 7 years, no problems works great.....

Cole Hersee
Continuous Duty Solenoid
24V
Part Number:  CH 24144
200 Amp SPST w/ Silver Contacts
Normally Open

I use it to connect my house 24V bank with my starting 24V bank.  Charge with the bus alternator going down the road.

I have a switch on the drivers panel which I turn on to activate the solenoid and connect the batts systems together.  The switch is connect to the start system, so if I turn the bus off, it also disconnects power from the solenoid and separates the batt systems when stopped.

No problems with arcing or anything else.  I believe I also have a wiring diagram of how I installed it, including a diode to resist back flow/arcing.  Just have to locate it.   Email me if interested and I'll try to find it and email it back to ya.

Has worked flawlessly for the past 7 years, in 2001 it cost me $33.34 for the unit at a truck supply store.  I believe these can even now be ordered right off the Internet at CH's website.  If you go into their website you can of course search for this solenoid, it will give you a picture, measurements, specs, etc., etc.

Chris