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Looking for ideas on a DC disconnect. 1

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itsmoked

itsmoked

Electrical
Feb 18, 2005
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I'm looking for some ideas from the Brain Trust.

I'm building a high energy off-grid power source. It has lithium batteries. Of course the finicky lithiums cannot be over charged or over-discharged or charged when too cold (on pain of death to lots of things including one's wallet) The system's Battery Management System (BMS) needs the ability to cut the power to and from the batteries. Commonly used for this are the typical power solenoid relays:

Battery_Solenoid_wwa5b4.jpg


But those are abhorrent because they consume a ton of energy just being kept closed. They typically are too hot to touch making them scandalously power wasting. Not doing that.

Much much better is a motorized disconnect that ceases to draw any power after it's open or closed.
I use these cool valves frequently. Look at the price for a stainless steel ball valve version!!

Motorized_ball_valve_cuet38.jpg


Amazing! So now turning to switching electricity instead of water, a much more trivial effort made of some stamped metal contacts and a little bending of it and you get this:

DC_motorized_Disconnect_hte6wh.jpg


The problem is that ridiculous price! I'm not doing that. I need about 10 of these. But damned if I'm paying that exorbitant price for it.

Here's the non-motorized price.
manual_switch_yhr83b.jpg


While OFF and ON would be nice I can live with OFF only. This means a solenoid type action where a PLC generates a brief ON could maybe pull something. Perhaps something like one of these kill switches.

Kill_SW_qf5eko.jpg


Or perhaps twist something like one of these:
Chrome_Disc_SW_mcyixj.jpg


Or pulls open one of these:

KnifeSW_cgvcho.jpg


I could run a motor too.
I have a couple of 3D printers and I'm not afraid to use them :) I can print most any cam, lever, shaft, housing, frame, linkage, or mechanism.

Suggestions?



Keith Cress
kcress -
 
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Hey MikeL. 24V though a buddy would sure like some for his 48V service.

Spar!! That is... um.. well! Damn. Pragmatism - defined.
Amazing they found a linear actuator that big that short!

NC would require power to keep it open... <shudder>


Keith Cress
kcress -
 
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How about a 12 or 24 volt starter solenoid? They take a surge of current to pull in and then very little to hold in.
If that NVX XSB 1500 Smart Relay will serve electrically, you can fit an economizer resistor to drop the hold-in power.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Keith,
How much current? The top one is 500A, the others are not, so it’s hard to tell from those what you need. Switching DC is tougher than AC because of the arc drawn. Most low cost “battery isolators” are not rated to be switched under load, which when automating something like this, is a probability. The contact separation must de higher for DC, or you can put contacts in series to get the same effect.

I know of some industrial DC switching contactors, used in DC drive applications) but not likely rated for 500A. They are usually rated for higher DC voltages, which is not a bad thing. I’ll dig them up in the dusty archives. If you get ones with DC coils and an “economizer circuit” the coil consumption is low and it doesn’t get excessively hot.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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How fast do the contacts need to move, Keith? The servo motor drives for ball valves can be had for cheap if you look, but a lot of the cheaper ones that we use have 30-90 second 90 deg. swing times.
 
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MikeL; Current rating needs to be around 150A continuous.

While I had no idea there were so many bi-stable contactors out there it's not helping. I didn't see a single one that was below $300.

Bill; Virtually all of them that are available are 12Vdc coils which does me no good. They also use about 24W that's 600Whrs a day times 10 is 6kWhr/day. Not good.

Jeff; 150A. Typically LiFePO battery stacks are happy delivering 100~150A. I have 10 of these I need to individually be able to isolate.


Keith Cress
kcress -
 
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Did you look at this one?
So at Arrow they are close to $100 each if you buy 10,
For Bill's suggestion, there are plenty of 24VDC mag solenoids, problem is for what you want they have a fairly large hold in current, and it is harder to find a "continuous duty" rated solenoid, but here is a link to the current Cole Hersee product line, But to get the right one, they ain't cheap.

MikeL.
 
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I was going to try an RC servo, using it to pull a lever and things started getting complicated since the servos need that problematic pulse train to be happy. I went with the GIGAVAC since they have one model that has a built-in hold circuit (Bill) that drops the power to 1W. Found three newish ones on fleabay for $32. I'll worry about the others later.

I couldn't see the coil current for the Hersees.

Thanks all.


BTW MikeL there's a thread crying for your knowledge:
thread816-470873

Keith Cress
kcress -
 
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That is a design choice that is seldom applied to small systems.
While pulse to open is common in substation switching, the monitoring circuits and backups including a trained staff, are not present on small solar systems.
Latching contactors or pulse to open are used on non-critical applications such as large lighting circuits.
Kieth wants a system that will fail open.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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stevenal said:
Why not a latching relay? Pulse to open, pulse to close.
Click to expand...

Thanks Bill.

And!

Violation 1: Control voltage not 24Vdc
Violation 2: $300/piece X10 = $3,000!
Violation 3: On a massively paralleled system it could be hard to tell which pulse you were on and whether you actually had turned a battery bank on or off.

Thanks though.



Keith Cress
kcress -
 
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I’m Steve, not Bill.

A latching relay has two coils. The close pulse goes to the close coil and will never open it unless you wire it wrong.
Try the link. Didn’t take me long to find 24V device.
 
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