DUMP LOAD

[lglagasse]

Hi everyone:

Just completed this dump load. Would anyone please take a look and post a comment.

48. volt system
    
1920. watt, each resister is 240 watt .5 ohm resistance, there are 8 resistors.
      
      

at 48 volt should be at 40 amp.

The snap switch is from a water heater. The switch is adjustable and will shut down if it gets too hot, I think?? The ajustment is from 110 to 170 this is degrees.

I plan to use a tri star ts 60, the dump load capacity is 45 amp, this should be safe.

Thanks in advance

lglagasse

[lglagasse]

should be 1920 watt not 920

Thanks

lglagasse

[ghurd]

Be aware the resistors will get very, VERY hot.

Might be a good idea to get them away from the backing to allow more circulation.  At least a couple inches.

What is the backing material?

Wouldn't hurt to add a couple fans too.

Not sure the snap switch will snap.  The distance between them will greatly lower when it switches.

If the snap switch opens, the input current will be high.  Meaning when the circuit is too hot is when it is needed most.

I expect it would be fine if the input is always less than 1KW, but the resistors being so close to the backing concerns me.

I'm working on one (slowly) with a 300W resistor that will only take 130W, 2.5" from the backing material, a heat sheild between them, and it will have a fan.

.

Does anyone care to venture a guess about the pipe changing the inductance?

G-

[lglagasse]

The backing is made of alumalite,{Bilboard sign material} thin aluminum each side with a sandwich of plastic in the middle.

I do plan to put a shield around the front side and spacers in behind to keep away from the wall.

I will have another dump load in my hot water system but that is not made yet. They could be made to switch back and forth.

Will the 12 volt glow plugs take a 48 volt system?? Might burn out??

Thanks Ghurd, I'm learning

lglagasse

[TomW]

Might be worthwhile to use copper tube for the holders thru the center of the resistors to heat water. Would serve double duty then shed the heat and heat your shower. Or just let it thermosiphon thru a tank to pull heat off the resistors. Just off the top of my head.

Tom

[ghurd]

Might add spacers and a sheet of thin aluminum between the resistors and alumalite.  Keep most of the heat from the alumalite.

You might be underestimating how hot they can get.  300'F wouldn't impress me anymore (scare me yes, but not impress me).  The extra flow would help keep those expensive resistors alive.

Glow plugs?  I wouldn't bother if they were free.  More so for 12.6V glow plugs working on 57V.

Mine were used, average, maybe 3 times a day?  1,100 times a year? Average 15 seconds each? 4.6 hours a year?

I probably averaged changing 2 a year?  Life span of about 9.2 hours?  NO WAY I'd spend the money on wire and connectors, plus labor, to connect them.  Have a battery overcharge when one failed.  Then reconnect the new ones.  

Watt-Hours.  Not sure if this holds up, but...

If mine lasted 9.2 hours on 12.6V, and 12.6V is 22% of 57V, then a brand new one could be expected to last 2 hours?

However, heat kills.  More volts = exponentially more heat.  I doubt the would last a couple minutes.

Start rant.

I'm too lazy to change light bulbs that often.  Far to lazy to get them from the junk yard (UK: breaker yard).  They are too expensive for how long they last.

Nichrome is the next best bet, IMHO, but it is not easy either.

Resistors are worth the cost.

End rant.

You are doing fine.

G-

[lglagasse]

Hi Tom:

I never thought of that idea of heating water in that fasion but it's a good one.

I heat my water with wood [Winter], oil in Summer. I thought of making a manifold to insert some 48 volt heating elements to use as a second source of dumping-heating for Summer use.

I can easily move the pipes away from the backing and add a fan. I have no way of testing at this time since I only have 12 volts to test with. I could run copper pipes through the resistors and pre heat my water as it is drawn to the other heating sources.

Are you saying this design is unsafe? Looking for feedback.

Thanks for all the help.

lglagasse

[TomW]

I wouldn't say it is "unsafe" but I would say that 2KW of heat is a lot if you cannot dissipate it. Wouldn't surprise me if those resistors get hot enough to ignite paper touching them in short order.

I would err on the side of too much spacing rather than not enough. Certainly if there is plastic involved.

The resistors will last longer if they are well cooled, also.

I would also place the resistors so air naturally flows across them at 90 degrees to their axis rather than down their length. They will cool much better then. Personally, I would not even consider them mounted vertical like that. Top part will be "cooled" by the hot air from the lower hot parts.

Just thinking out loud.

Tom

[lglagasse]

Hi Ghurd:

I just did not have a clue how hot these things get. My only testing was on a 12 volt battery and things got hot.

I will re-think this issue. I measured some copper pipe but its not sized right to fit the resistor core properly.

Was not aware that things might get 300F degrees.

Glow Plugs, I do not need the aggrivation either if they need to be changed out that often.

I can easily re-configure the present one to work safely for now and work on water heating as a back up.

Thanks Ghurd, you have been very helpfull

lglagasse

[lglagasse]

Hi Tom:

Thanks for your coments. I will re configure, I can always use more hot water. I agree after your coments and Ghurds that this is not safe or bordering on being safe. I was not in tune of the heat produced here. I would rather be safe than sorry.

lglagasse

[ghurd]

Yes!    Rethink the distances.

I am using (similar but smaller) 25W resistors at 19W.  They get HOT.

Testing with a 12.8V battery, meaning only 16.4W in a 25W resistor...

From dead cold room temp (70F?), they gain 1F per second and very linear, until they reach about 200~220F.

Then they get hotter much faster.  And take a lot longer to cool down between cycles.

"Fahrenheit 451" could be a few short minutes.  Much over 451F can take a long time to cool down below 451F.

Not sure what the plastic in the middle would do with that.  AL is great, but it's still a lot of heat to move.

It is a Lot of heat.

A space heater on high only uses a bit under 1500W.

G-

[getterdone]

have you thought of a bigger batt. bank.

if you have enough batt.s' you wont need a wasted load.

you do need some way to dump every now and then., hot water element would be my choice. and will be the way i'm setting mine up.

don't waste your power.

just my 2cents.

[richhagen]

I was thinking the same thing on that backing material.  1920 Watts is a lot of heat that will be rising into the switch and meter as well, I am not sure I would locate them too close above.  My little 600 Watt 48V dump load (old toaster elements mounted on sheet metal) is such that you would not want to keep your hand in a similar place to where the meter and switch are located once it has been in operation for a bit.  I cannot see the long term effects on the plastics or most non metallic materials being that good.      

Being a ferrous pipe, I would venture that it would change the inductance, and that might have some effect upon the switch as the inductor releases the energy stored in the magnetic field by increasing the voltage when the circuit is broken.  I'm not sure if that would effect the function or life expectancy of the water heater switch, but I would guess that it would have a negative effect on both.  Rich

[SamoaPower]

lglagasse,

I think you have more serious problems than the heat issues raised by the posters so far. The following comments are based on what my tired old eyes tell me from your photos - could be wrong.

It appears that you have all eight 0.5 Ohm resistors connected in parallel, which would result in a load resistance of 0.0625 Ohms. A 48V battery system will have about 57.6V at the absorption point while charging, which is when the controller would begin to dump excess available current. This would result in a peak current of 922A. Obviously, the controller wouldn't be very happy.

This is a common problem for people not very familiar with Ohms Law, where they are thinking in terms of resistor wattage instead of the electrical circuit.

It's obvious that you need a much greater load resistance than 0.0625 Ohms to limit the current for the controller. A single 0.5 Ohm, 240W resistor has a maximum current rating of 21.9A (I=Sqr Root(P/R). Since you need a much greater resistance, you will need resistors in series to obtain it. The single resistor current rating will then determine the maximum load current (21.9A). To limit the current to this value would require a minimum resistance of 2.63 Ohms at 57.6V (R=V/I).

You can get close to this by using seven of your resistors. Connecting five of them in series plus two in parallel (the combination also in series) would give you 2.75 Ohms total. This will give you a load current of 20.9A at 57.6V and a dissipation of 1204W. The maximum dissipation of a single resistor would be 218W.

Whether this solution would be suitable would depend on your maximum available source current. If it's greater than 21A, then you would need different value resistors.

[TomW]

Samoa;

Good catch. I didn't look that close at the wiring scheme. It is kind of busy and not enough contrast for me to make out easily what is going on. I am still not absolutely sure.

It is easy to forget that not everyone understands Ohms law and the relationship between volts amps watts and Ohms. I don't even think about it myself usually it is so automatic.

Tom

[lglagasse]

Hi all:

Thanks, I will start over. There is good information here so that I can still use the resistors that I have.

Might even help others that are not so savy with this stuff like me.

lglagasse

[finnsawyer]

You need two more resistors.  Each resistor can dissipate 240 watts safely with adequate ventilation.  With .5 ohms resistance that means no more than 21.9 amps (I^2xR).  The resistor would then have 11 volts drop across it. To reach a total voltage drop of 48 volts with a safe value of current you would need to put five of the resistors in series.  That would give you 48/2.5 = 19.2 amps , slightly less than ideal.  So, you need two of the five resistor series packs in parallel or ten resistors.  Each resistor would be dissipating 184.3 watts for a total dissipation of 1,843 watts.  Pretty close to what you want.  Get two more resistors.

Can you cheat with the eight with two series packs of four in parallel?  Well, at 48 volts that would get you 2,304 watts or 288 watts per resistor.  Maybe, if you mount the resistors in air, not against a board or vertically as you show, and if you blow air past them with a fan.  Maybe do some tests.  Are you also sure that 48 volts will be the dump voltage and not say 56.8?  I don't think those resistors are intended to be mounted the way you have them, in any case.  They need air flow past them at the rated power.  

[lglagasse]

Hi and thanks finnsawyer:

I have 2 more resistors. I will mount horizontally and hook 5 in series and 5 in parallel. I do not yet have the controller but as Samoa Power says it will most likely be 57.6 volts from the controller. I will use a fan and mount a heat shield behind the resistors and also put more clearance betweem the heat shield and the resistors.

Your ideas along with Samoa Power will help me configure this so it will be safe.

I have no real way of testing with 48 volt yet but will have soon.

Thanks again for taking the time to set me straight.

lglagasse

[coldspot]

 lglagasse-

Nice work, but as you know by now,it needs a bit more.

Tom-

"use copper tube for the holders thru the center of the resistors to heat water."

 Now I'm glad I'm slow, and used mostly nichrome wire coils.( Did use one power resistor that I'd like to use for that very good idea).

 Thats the best idea for using those!!!!

Today I checked mine with a half inch copper tube and all but the two 8 ohm ones would work. But, I spotted an old copper tube salvaged that looked smaller than 1/2" and grabbed the resistor out of my truck that I was hauling into town as a size tester and the old tube would fit and it is straight and long enough to work with.  

 So, thank you Tom!

(thats another project on the list)

[TomW]

Coldspot

My pleasure. Glad it clicked with someone.

Just be sure the water can move freely through the tubes. Steam is nasty when it gets loose.

Tom

[finnsawyer]

"I have 2 more resistors. I will mount horizontally and hook 5 in series and 5 in parallel."

I hope you just mis-spoke here.  You need two packs of five in series, the two packs connected in parallel.  At 57.6 volts the dissipation per resistor will then be 265 watts.  With forced air cooling that might be acceptable.  Or maybe you could lower the dump voltage to 55 volts.

[lglagasse]

Hi Finsawyer:

You need two packs of five in series, What does this mean packs? I have 10 resistors, Sorry I do not follow this. It is my ignorance in understanding not the way you are explaining it.

Thanks for taking the time.

lglagasse

[TomW]

Lg;

Well, he means you put 5 resistors in series [a "pack"] and then put those packs in parallel. At least thats my take on it.

Might [will] be a good thing for you to hunt down some information on basic electricity. Watts Ohms Amps and Volts all work together and it is a very simple set of formulas that help you figure this stuff out.

I highly suggest studying it some if you intend to play in this hobby.

Tom.

[lglagasse]

Finally, I understand this set up.

Thanks

lglagasse

[jzeveney]

You device looks cool however here's something I was planning on attempting.

This has not been built yet so feedback please.

Why not just use your existing hot water heater?

The elements in them should respond to DC the same as AC.(I think)  

Example: The element in my existing hotwater heater is 37.5 Ohms +/- a few.  This results in 4500 watt at 120volts.  At your 48 volts this would be 1800 watt.  Use a simple powered relay to transfer from GRID to supplus power.  This will allow you to use the existing heaters thermostat for safety and you done.

Z

[lglagasse]

It has been said here [in previous postings]that the 240 volt heating elements will not hold up. There are 12 to 60 volt elements available. Here is the link

http://hydrogenappliances.com/hotwater.html

I have two water storage tanks with heating tubes going through my wood furnace. This heats my water in the Winetr Months. All the fitting holes are used up on my two tanks.It is a very coolway to heat water.

lglagasse

[ghurd]

I believe lglagasse misunderstood the earlier posts.

The 4500W element should hold up fine, but it would be nearly pointless.

Watt's Law  4500W/120V= 37.5 AMPS.

Best bet is the voltage is 240V instead of 120V.

So 4500W/240V= 18.75A.

And Ohm's Law 240V/18.75A= 12.8 ohms.

A 12V system dumping excess power at 14V, into a 12.8 ohm resistance is less than 1.1A, or 15W.  15W is about half of a car tail light bulb.

A 48V system dumping excess power at 56V, into a 12.8 ohm resistance is less than 4.4A, or 245W.

A 4500W heating element using 245W (or 5% of rated watts) is likely to last a LONG time.

What TomW said...

"Watts Ohms Amps and Volts all work together and it is a very simple set of formulas that help you figure this stuff out"

G-