Dump load questions

[440rt]

Hi, I just had a question on dump loads. I probably won't need to hook up a dump load, but I am very curious how I would hook one up. I have a Morningstar Tristar MPPT 45 controller. It looks like I need to get the optional relay driver and hook it up off of that. I downloaded the manual for it at:

http://www.morningstarcorp.com/en/support/item.cfm?ItemId=28

but I quickly got lost in what they were saying.

I was just thinking of what to do, if I had extra power coming in from the panels and the batteries are full. I know most people would grid tie but I am not on the grid. Also most of the time I will be at work during the day so manualy turning on hi power stuff also won't work.

So I guess my question is if I could hook up a load dump to say a 12 volt water heater or an extra inverter to run some kind of 120 volt device. How would I hook it up to the relay driver. Thanks, -Bryan

10 evergreen 110
tristar mppt 45
xantrex 1800 pure sine
xantrex 40 amp charger
xantrex link pro monitor
8 egc2 6 volt = 880 amp hr total I use about 200-250 or about 20%-30%

[kmmtnman]

Thinking about the same thing

I was wondering about using the surplus power to heat the water line from the tank. I have a 2" line and it gets damn cold in the winter. Also because its in granite, I could only get it 2' deep with a pick, shovel and electric jackhammer in some places. Too steep for equipment. Fortunately its on a south slope and get's a lot of sun and hasn't frozen yet. The cabin is a weekend place so there would be 500+w available to heat water on weekdays.
I wonder if the hot water would thermosiphon in the line and rise to the tank or over heat in the line?

[Sunking]

Well if all you are doing is panels and batteries you have no use for a dump load. Dump loads are primarily used for wind generators, because when the batteries are full you have a place to DUMP the power from the windmill. Otherwise it would likely over speed and destroy itself.

[Mike90250]

The hard part will be finding 12V heat tape.

Solar does not really need a "dump load" but you could use it top power gear when the batteries finish charging. I don't know if you will find much in the way of 12V heating gear, maybe stuff for RV use.

[Naptown]

In order to dump load you would need another charge controller if I read their site correctly. These are only capable of one function at a time. And yes finding 12V heaters could be a challenge. You could use 120V but the wattage would be 1/10th of rated

[mitre]

I'm new to solar and I was thinking about doing the same thing with a 1Kw emergency backup system for a refrigerator/freezer. I see that a xantrex c-40 will dump load. I was looking at a 40amp DC water heating element and a used 10 gal electric hot water heater as a preheater for the gas hot water heater. Then again, I could just use up some of the battery for lighting, laptop, ect.

Is a 1Kw system too small to even have a dump load?

[Sunking]

Is a 1Kw system too small to even have a dump load?

So how are you going to control the water temperature to keep it from getting to hot?

[mitre]

Good point. Unless max amp output per day was only enough to just keep the water warm. More research is needed on that one.

[tandrews]

You can find a cheap 12v dump load in the form of a 12v car heater.
The type that has a peltier element and small fan.
They come as small as 150w and as cheap as 20$.
Where you put it depends on your seasonal needs and surplus.

http://www.princessauto.com/pal/product/8237190/Interior-Heaters-%26-Fans/12V-150W-2-in-1-Ceramic-Heater/Fan

for example.

[Mike90250]

The problem with using water heater elements for dump loads, is you need a very large tank to store the water in, one so large you cannot overheat/boil it. The water heater thermostat cannot be used to control the water temp, if it shut off, the turbine would become unloaded, also not rated for DC.

[TheAwkwardGuy]

How would you make a "dump circuit"?

I don't use much power from my system either and most of the time my cc indicates over current. I'm looking for a way to automatically divert power from the panel to a separate circuit to run a ventilation fan to cool down the house.
My idea is for the fan to run only when my batteries are full. I would use the fan to circulate cool air from the basement without draining the batteries. If it's cloudy out the fan would not run but the house would not heat up as much either so it wouldn't be needed.

[Mike90250]

How would you make a "dump circuit"?

You need a charge controller, that has relay or control outputs, so that when a preset voltage & time is reached, the relay activates and turns on your load.

You can get voltage sensitive relays or switches, but you loose the absorb time sense.

[billvon]

How would you make a "dump circuit"?

You need two things. A controller that lets you do this (an old Trace C40 works well) and a dump load. Biggest warning: the dump load MUST be larger than your power source. If you have a 1500W wind turbine you need a >1500W dump load.

I don't use much power from my system either and most of the time my cc indicates over current.

Something doesn't make sense here. Overcurrent is not a problem you can solve with a dump load. Indeed, it makes it worse. Dump loads reduce the voltage in battery banks by INCREASING the current drawn from them. They only make sense for wind, hydro and other sources that cannot easily be disconnected without an overspeed.

[TheAwkwardGuy]

Something doesn't make sense here. Overcurrent is not a problem....

You're right. I meant to say the cc indicates Over Voltage meaning that the battery bank has reached full charge.
I don't have Wind or Hydro, just Solar.
If it's a sunny day, I reach full charge around mid-day. The cc allows ~1Amp to go to the batteries and should I get ~4Amp max output at that time of day.
That means I have ~3Amps of energy available but not being used? I'm just looking for a way to use this extra energy.

It looks like a voltage sensitive relay or battery isolator might be what I'm looking for.

[billvon]

It looks like a voltage sensitive relay or battery isolator might be what I'm looking for.

Sort of. Here's the problem:

Batteries go through three stages of charging - bulk, absorption and float. Absorption is the highest voltage but the charge isn't "done" at that point, so if you used just voltage on the battery you won't get a full charge before you draw the battery back down.

However if you use a PWM charge controller and you put the voltage sensitive relay on the panel side, then set the relay to higher than your max battery voltage, then you will get power only when the charge controller doesn't "need" the power. Note that your load will have to deal with full panel voltage (17-22V for a 12V system) for this to work.