Dump Loads for Solar, Wind and Microhydro

May 2, 2013 · 4 comments

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

Most wind and water turbine installations will require them and most solar installations will 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 and disconnected 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.

As 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.

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 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.

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 and stay connected to the batteries. Every watt hour they produced 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 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.

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

{ 4 comments… read them below or add one }

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

Reply

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

Reply

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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