Dump Loads for Solar, Wind and Microhydro

May 2, 2013 · 16 comments

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Dump loadDump loads a.k.a. diversion loads are are an important part of the off grid electric system.

Almost all wind and water turbine installations will require them and most solar installations can benefit from a dump load.

As we discussed in the dump load intro, a dump load is simply an electrical device (load) to send electricity to when the batteries are full or the extra power is not required.

Solar panels are unique in that they can be short circuited (positive and negative connected) or disconnected (positive and negative open circuited) without any issue. If your batteries are full and the solar modules are still making power, you can simply short circuit (not so common) or disconnect the solar modules from your batteries with a charge controller.

Wind turbines and water turbines generate electricity by rotating and need to have a load on them at all time. Without a load (such as a battery or a dump load) they will over-speed and possibly be damaged. Water turbines will generally turn 3 to 4 times as fast without a load while it can vary greatly with wind turbines as the wind speed changes.

The best dump loads are made from resistive loads such as air heaters or water heater elements. Resistive loads like previously mentioned are:

  1. Durable and long lasting
  2. Can accept AC or DC current
  3. Not polarity specific (+ or – is irrelevant)
  4. Inexpensive
  5. Useful for heating water or space heating

Incandescent light bulbs are not suitable for dump/diversion loads. Although you may have seen them used in the past think about this. Your system is working smoothly and you have 10 x 100 watt light bulbs as a dump load. On a bright sunny day your system is dumping 1000 watts into your light bulbs and one of them burns out. Oh no, now your dump load is only 900 watts and the voltage starts to climb.

incandescent bulbs make poor dump loadsAs the voltage starts to climb, the bulbs will actually consume the 1000 watts or more. When the bulb went out, the resistance dropped. Now the bulbs are operating above their rating and the second one burns out. Within minutes they will all burn out leaving your battery bank at the mercy of your system.

A second and often overlooked problem with the standard tungsten incandescent bulb is the fact that the resistance of the bulb’s element changes rapidly as the bulb heats up. In fact the resistance will increase by 15-20 times from cold to hot.

A cold 100 watt 120 volt tungsten light bulb has a resistance of about 9.5 ohms while the same bulb has 144 ohms of resistance when hot or in about 1/10th of a second.


You can use an AC air heater or water heating element for your DC dump load but the element will not dump as much power as you might think. Click here to learn about using off the shelf AC elements as a dump load.

In the past the normal way of installing a dump load was to use a voltage controlled switch. This switch would turn the dump load on at a specific voltage (maybe 14.6 in a 12 volt system) and off at a specific voltage (maybe 14.0 in a 12 volt systems). The larger the dump load, the larger the difference between the on and off set points. This larger voltage difference was to prevent cycling of the dump circuit.

Dump load using voltage controlled switch

This on/off dump load system worked but it was not the best. Batteries work best (and last the longest) if the bulk/absorption voltage is steady instead of cycling up and down. When you purchased your batteries you were told to set the bulk/absorption voltage to 14.6 volts (just an example voltage), not somewhere between 14.0 and 14.6 volts.

DC current is very difficult to regulate. You can change the power out easily in AC current by using something as simple as a dimmer switch. DC is not that simple. It can only be on full or off full without expensive equipment.

Then how can we regulate (and hold the voltage steady) DC current for our dump load?

The answer is pulse width modulation or PWM. Don’t worry PWM is not really that complicated.

Let’s pretend we have a 200 watt air heater but we only want to dump watt 100 watts. All we need to do is make sure the 200 watt dump load is only connected 50% of the time. If we connected the load for one second and disconnected it for one second it would only use half of the 200 watts (100 watts). This is all PWM is. Pulse width modulation turns the load on and off many times per second. The more power that needs to be dumped, the more on cycles there are. The less power that needs to be dumped the less “on” cycles there. Of course these cycles are so fast you will not be able to keep track or count.

Below you will see the most common way to install a PWM dump load.

Using a PWM Dump Controller

The above system uses a PWM Morningstar Tristar TS60 charge controller/diversion controller. The TS60 monitors the battery voltage and only dumps what is necessary to hold the voltage steady at the bulk/absorption voltage. The TS60 either works as a 60 amp solar charge controller or a 60 amp diversion controller. It does not do both at the same time.

We do not show the solar panels, micro hydro turbine or wind turbine on purpose. You need to treat the dump load circuit as a completely independent system. Every item you add to your system should be treated independently.

It can be very confusing if you start thinking about installing a wind turbine, a solar array and a water turbine on the same battery bank. But, if you treat each as an independent system you can just think about one step at a time.

The solar/wind/microhydro connect directly to the batteries (with over-current protection) and stay connected to the batteries. Every watt hour they produce goes to the batteries. Every extra watt hour produced goes to the batteries first and then out to the dump load.

If you are using a dump load you do not need a charge controller on the solar array. The only exception would be using an MPPT solar charge controller to get the most out of your panels. In that case you would set the MPPT controller to disconnect the solar array at a volt or two above what the dump load is set to turn on. You do not want the solar charge controller disconnecting the array as you want to use the excess power to operate your dump load. However if the dump load were to fail, the MPPT controller will disconnect at a volt or two higher than usual to protect your batteries from over voltage.

Another option for a dump load circuit is to use one of the programmable auxiliary outputs from either your inverter, charge controller or battery monitoring system. Here is an example below:

Diversion Load using AUX OUTPUT

Most of the high end inverters and charge controllers made by Outback, Xantrex, Midnite Solar, Apollo Solar and Magnum Energy include an AUX OUTPUT relay driver. This relay driver is a programmable 12 volt circuit. You can tell it what voltage to dump and it sends a 12 volt output to operate a solid state relay which turns the dump load on and off.

You MUST use a solid state relay if you want to use the PWM dump mode. A standard relay will not handle the many on and off cycles per second. Make sure you take advantage of the PWM setting on your AUX OUTPUT relay driver. Some are labeled “dump load using SSR” and some are labeled “Pulse Width Modulated” in the menu of the charge controller.

There are many other ways to make your diversion load circuit. The one’s listed above are the common methods. If there is a method we have missed and you think we should add it, let us know in the comment section below.

You can make your dump load to divert AC current instead of DC and use a standard AC heating element. The only problem with dumping AC current is if there was an inverter failure. If the inverter were to shut down for any reason, your dump load would not work. It can done, but it is more risky than using DC.

Another problem with using an AC dump load in conjunction with an inverter is that you cannot use the PWM method. It must be installed with a standard relay (or SSR) that closes the circuit (powers the dump load) at a certain voltage and opens the circuit (disconnects the dump load) at a certain voltage.


diversion load controller 45 ampsOne last IMPORTANT thing to remember.

Your dump load (diversion load) controller or solid state relay must be sized large enough to handle the full wattage of your dump load.

THIS CANNOT BE OVERSTATED!

YOUR DUMP CONTROLLER OR SSR OR STANDARD RELAY MUST BE SIZED ACCORDING TO THE SIZE OF THE DUMP LOAD AND NOT THE MAXIMUM AMOUNT OF POWER THAT WILL NEED TO BE DUMPED!

When the dump load is turned on (even if it is only a fraction of a second) it is consuming the full rating of the dump load. It has nothing to do with the charging capacity of the solar modules, wind turbine or water turbine. Please keep this in mind when sizing the dump controller or relay, wiring and over-current protection such as fuses or circuit breakers.

FOR EXAMPLE:

You have a 500 watt water turbine and a 1000 watt dump load and you are operating a 24 volt system. The likely dump voltage is going to be about 30 volts. If we divide 1000 watts (dump load) by 30 volts we get a result of 33 amps. When the dump load is turned on it will consume 33 amps.

Even though our turbine can only produce 500 watts or 17 amps and the most we will ever need to dump is 17 amps we MUST size or dump controller or relays at the amperage of the dump load, 33 amps in our example.

When the batteries are full, the dump controller will use PWM to dump the 17 amps but it can only do this by turning the dump load full on and full off. For this example it would be on roughly 50% of the time and off 50% of the time. Something like the Tristar 45 would be perfect for this example.


Things to Remember:

  • AC heating elements make good DC diversion loads.
  • Polarity does not matter with any resistive load such as a heating element.
  • Heating elements work the same with AC and DC current.
  • Pulse width modulated dump loads always work better than simple on/off.

Leave a Comment

{ 16 comments… read them below or add one }

Trevor July 6, 2016 at 6:36 am

Brilliant website – I installed a dual solar/wind turbine system to my camper van. The solar panel 100w is permanently fixed. The wind turbine is 300w removable. After connecting it didn’t take long before the air heat dump load device I had fitted (600w), started dumping. It obviously produces heat as the coils accept the dump load. Something I hadn’t thought would be an issue, however in the summer time!
After reading your website I have come to realise that the panel can be disconnected safely without damaging it! Great – so now I can introduce a manual switch into the charging circuit to physically disconnect the panel rather than covering the panel – haha!
My biggest worry was the continuous switching noise of the control relay and the heat being produced from the dump. The switch will obviously and simply fix this problem with minimum effort which in turn will prolong the life of the relay and give me control over charging and any heat produced. It may mean however that I will have to move the relay and dump to an external position under the camper to illuminate the noise and heat issue when on and no one in!

Reply

Jody Graham July 6, 2016 at 9:59 am

Thank you Trevor for your kind words. We try to provide new information based on experience rather than copy all the same info every other solar website has. Your system sounds great. You are correct, you can disconnect any solar module without any problems. Just make sure you use a switch rated for DC and not just a normal 120/240 volt 60hz or 230 volt 50hz AC switch and you will be fine.

If you are using a mechanical relay (one that uses a magnet to pull contacts together) they can be quite loud. One option to make your system quieter is to change the relay to a solid state relay like this 60 amp DC solid state relay. It is made to switch up to 60 amps DC at up to 110 volts DC. It is triggered (or turned off and on) by any voltage from 3-32 volts DC making it perfect for a 12 or 24 volt system. This is what it looks like.

It does the exact same thing as your current relay but has no moving parts and will not wear out or make any noise. Your dump load controller will still make some humming noises but at least you won’t hear the clicking of the relay. The only difference is you will need to mount the solid state relay on a heat sink like those used for computer processors or a piece of aluminum. SSRs can get hot when there is a lot of current going thru them. Here is an example of a heat sink. They are less than $10 usually.

Again thank you so much for your comments. I appreciate it! Keep up the great work…Jody

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Christian T B June 28, 2016 at 8:50 pm

I am in the process of installing 2x205W mono crystalline solar panels and a 400W Air-X wind generator to get the best out of the shifting weather conditions for charging my 1040Ah 12V AGM battery bank in my off-grid cabin in Sweden. I plan to use a heating element in the buffer tank in my central heating system as a dump load.
In the article you state: “If you are using a dump load you do not need a charge controller on the solar array”.
Would that also be true for mono crystalline solar panels with an Uoc of up to 46V? Or would it be necessary / safer / more efficient with a MPPT regulator for the solar panels anyway?
The Air-X has its own internal regulator and doesn’t need a dump load, but this regulator can be bypassed by turning the adjustment screw to its stop.

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Jody Graham June 29, 2016 at 12:32 pm

Hi Christian,

Thank you for your question. If you are using modules with a Voc of 46 volts they are likely a 24 volt nominal solar panel. If you hook those modules directly to a 12 volt battery bank you will get less than half of their rated output. I would definitely use an MPPT controller to connect to your solar panels at the MPPV or MPV (maximum power point voltage) and then the MPPT controller will reduce the voltage to that of your battery bank’s.

I would then set the MPPT controller’s bulk voltage one volt higher than your dump controller’s bulk voltage. That way if the dump/diversion controller or dump load ever fails, you will have the added protection of the MPPT controller to stop the voltage from going too high and damaging your batteries.

AGM batteries are not forgiving if you over charge them. Overcharging an AGM will boil the electrolyte which will escape as a gas. Since it is sealed you can not return any distilled water to the battery as in a flooded lead acid battery.

Then I would turn the adjustment screw on the AIR X to defeat it’s charge controller. Let the dump load take care of that. If the dump load fails the AIR X will not make enough amperage to overcharge your battery bank unless you live in a very windy (40-50km per hour constant) place.

As a general rule, if your power source (in amps) is less than .5% of the battery bank’s C20 rate, you do not need a charge controller or diversion controller. It is simply too small to harm the battery bank.

Your AIR X will likely average less than 75 watts (from years of installing them) unless you live in a super windy place. 75 watts is about 5.2 amps at 14.4 volts (your battery bank’s bulk voltage or charging voltage). Your battery bank is 1040 amp hours. 5.2A/1040AH=0.005 or 0.5%. It is on the limit but like I said before you will likely get less than 75 watts on average. If the dump load ever failed, the AIR X won’t put out enough current to harm your battery bank.

If you are in a windy spot or are concerned about overcharging your battery bank, then set the turbines short circuit voltage 2 or 3 volts above the bulk voltage of 14.4v. I say 2 or 3 volts as your turbine is likely a great distance from the battery bank and you are likely using the smallest wire you can get away with. The voltage at the turbine will be a few volts higher than the voltage at the battery bank due to the resistance (voltage drop) in the wire. If you have sized the wire to handle the whole 400 watts without voltage drop, then adjust the turbine to short circuit at 1 or 1.5 volts above the bulk (charging) voltage. Then if the dump load fails, your batteries will be ok.

Your system is well designed and should give you years of performance with little to no maintenance (since you have chosen AGM batteries). Good work…Jody

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Dan T September 15, 2015 at 9:50 am

I’ll have to say your site is very informative and understandable. I’ve been a micro-hydro owner for 20 plus years and finally…someone who understands and can communicate it.
Here is my dilemma. I am trying to control my diversion load. I currently have a 2 part diversion load, hot water and an resistant heater. Switching between the two has been my problem. . ( The original hot water thermostats can’t handle DC.) I have DC relays that switch between diversion receptacles but recently discovered the 48 volt hot water elements not taking enough power and before the relays would switch some sort of back feed boiled my batteries to death.
What I am looking for is some kind of load controller that will feed power to the hot water heaters but be able to send any/all power to resistant heater without relying on hot water thermostats if there isn’t enough capacity or demand (not an electrician so hope my terminology is right). Is it all/none or can something regulate the power to both?
Also wondering about solid state relays vs. mine.

Reply

Jody Graham September 15, 2015 at 4:57 pm

Hi Dan,
Thank you for your kind words. Good for you living off of micro hydro for 20 years. Are you using a Stream Engine or Harris Hydro? I only ask as I used to manufacture the Stream Engine years ago. I am writing an article about exactly what you want to do right now. I will try to have it finished by the end of the day. I will update this reply with a link to it….Jody

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Nisatirut July 10, 2015 at 7:02 am

If the charge controller does not have a dump load, is it possible to make one up?

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Jody Graham July 15, 2015 at 5:32 pm

Thank you so much for your comment and question. You would either need to purchase a charge controller that can operate a dump load like a Tristar TS45 or TS60 or a Morningstar relay driver (with a relay or solid state relay) or a high end controller with a 12 volt relay driver built in (with a relay or solid state relay).

The TS45 would be the least expensive option. There are folks that can build dump load controllers on places like youtube but you will likely being doing the best thing buy getting a Tristar TS45 or TS60 and using it as a dump controller.

Jody

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alan ali May 20, 2015 at 10:07 pm

Hello. Your dump load methods are quite informative. Is it possible to hook up a 220v inverter to a diversion load controller and send this power to a water heater element. I’m reluctant to buy heavy 100ft of heavy gauge wire to run from the diversion load controller to the heater element. Will the inverter run or will it shut down and indicate a fault. The inverter is 12v dc to 220v ac.

Reply

Jody Graham September 10, 2015 at 2:15 pm

Hi Alan,
It is possible to dump AC electricity from your inverter although there are a few rules. First the dump load must be smaller in wattage than your inverter. Second…you must use an on/off relay (SSR or standard) that is turned on and off slowly instead of a solid state relay that is controlled by pulse width modulation (PWM). You cannot use PWM with the inverter’s AC as it is very hard on the inverter to make electricity that is turned on and off many times per second. You would need the on voltage to be a volt or two from the off voltage so it doesn’t cycle too fast. Another consideration is that if the inverter were to fail your batteries could be severely overcharged. For those three reasons most folks decide to setup a DC diversion load setup.

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Terry Kailaoha Kumu Pa'a Manaiakalani Ridge July 13, 2014 at 2:47 pm

Aloha ‘oe,

You would think there would be a plethora of information about Solar and Wind generated power systems all over the internet and there probably is, BUT, in my EXTENSIVE search/research efforts all I find is bits and piecese hither and yon about these systems. I think Solar Homestead may be a one-stop-shop for this info and I GREATLY appreciate it ~ ~ ~

Mahalo and MUCH Aloha

Kai

Reply

Jody Graham July 15, 2015 at 9:26 pm

Thank you so much for the kind words. It is a work in progress and will likely take many years to complete if ever completed. Thanks Again Jody

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Maxime October 1, 2013 at 1:31 pm

Hi Jody,

Thanks a lot for the great website.

I have a friend who is an electrical engineer and he was telling me that it’s physically impossible to charge and discharge a battery at the same time since the electrons turn in one direction when they are getting charged and the other when they are being discharged. I was also able to confirm this from documentation I found on the Internet.

Now, every solar diagram I see show the solar panels plugged in to the charge controller and the charge controller connected to the battery and then the load/inverter/equipment connected directly to the battery. How does that work and not explode?

Maxime.

Reply

Jody Graham October 1, 2013 at 7:00 pm

Awesome Question Maxime,

I think I will make a webpage that explains this but I will try to address it here as well. Your friend is correct. A battery will either be charging or discharging at any given time. But that does not mean you can’t send power to a battery and remove power from a battery at the same time.

If you have a charging source that is 30 amps and a load that is 20 amps, you will have a net charge of 10 amps. Current is only flowing one way…into the batteries.

If you have a charging source of 20 amps and a load of 80 amps, you will have a net discharge of 60 amps. Current is only flowing one way…out of the batteries.

If you have a charging source of 50 amps and a load of 50 amps, you will have a balance. Current coming from the charging source will go directly to the load without affecting the battery.

Think of a battery like a pail of water. You have one hose adding 10 gallons per minutes and another hose adding 5 gallons per minute and another adding 20 gallons per minute. Every minute you have a gain of 35 gallons into the pail. Now you have a hose that is removing 8 gallons per minute and another removing 2 gallons per minute. Every minute 10 gallons is being drained. But cumulatively you have 25 gallons per minute going into the pail (35 into the pail minus 10 out of the pail).

You can have as many charging sources connected to your batteries as you wish and as many loads connected to your batteries as you wish. There will always be either a net gain, a net loss or it will be balanced.

The second thing you need to remember is electricity moves from a higher voltage to a lower voltage. If you have a 12 volt battery that is 12.5 volts and you apply 14 volts to it, the current will flow into the battery. If you connect a 6 volt battery to the 12 volt battery the current will rush from the 12 volt battery into the 6 volt battery. If this reaction was uncontrolled it would likely result in damage to the 6 volt battery.

With a solar system, you have higher voltage coming from the solar controller and that is why the current flows into the battery. If the solar module’s output voltage was only 12.4 volts (still talking about the 12.5 volt battery above) no current would flow. If the solar module’s voltage was 13 volts, the current would flow into the battery. In a solar electric system, the charging sources will either have a higher voltage than the battery and charge or they won’t and nothing will happen.

At any given time the battery will either be 1. Charging 2. Discharging or 3. Balanced and doing nothing.

Within the next few days I will create a whole web page to explain this better with drawings. Hope it will help. Have a great day….Jody

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joannf May 22, 2013 at 2:16 pm

I really love your website. It is soooo hard to find the truth about solar energy and living off the grid. There are so many sites that just have the same generic info all the other sites have. I can tell you have experience and you are not just copying and pasting from other websites. Keep up the great work. J

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Jody Graham May 24, 2013 at 11:05 am

Thank you so much for the positive feedback. Living off the grid has its good times and its bad times. But I wouldn’t trade it for the world. Cheers…Jody

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