How diversion charge controllers and dump loads work

Here at Leading Edge Turbines one of the most frequent technical questions that we get asked is about how diversion controllers and dump loads work. After all it seems a little counter intuitive to have to 'dump' energy in a renewable energy system!

Firstly dump loads (or diversion loads) are only used to protect the deep cycle batteries from becoming damaged through over-charging. In some grid-tie systems the dump load is used to protect the grid-tie inverter from damage due to excess voltage. The dump loads aren't usually needed to control or protect the turbine itself.

Imagine a typical off-grid system with a small wind turbine charging a bank of batteries. The small wind generator will be continuously charging the batteries according to the fluctuations in the wind. The system will probably also have a number of 'loads' - appliances that consume electricity from the batteries at the same time as they are being charged. The dump load operates with a diversion charge controller and is much like the overflow on a bath tub. The bath tub is the battery bank and the wind turbine is a tap. The tap (wind turbine) continuously fills the bath with water (charge) until the level of the water reaches the level of the over-flow hole (dump load). When this happens excess water flows into the overflow and flows away. This stops the bath from over-flowing or in reality your batteries from becoming overcharged and damaged. The dump load only comes into action when the batteries are full so if you have other electrical loads draining charge from the batteries then the dump load may come on quite infrequently. Alternatively if you have very little in the way of electrical loads on the system and the batteries will be full more often then the dump load will be working quite frequently.

One final point to note is that all of our diversion charge controllers use a proportional control system. This means that the excess power that is sent to the dump load may be 10w or 25w or 150w or 300w or 1000w anything in between depending on the size of the dump load The system quickly and automatically decides how much power needs to be dumped at any one instant. So the dump load may not be dumping 500watt continuously.

So why not just disconnect the turbine when the batteries are full?

Disconnecting the turbine from the batteries and leaving it disconnected from anything is a very bad state of affairs for a small wind turbine. This will mean that the turbine is open circuit and off-load. The turbine will free wheel to incredible RPM's - This will be noisey and will mostly likely lead to the destruction of the turbine!

So why not just switch the turbine off when the batteries are full?

Although some other micro turbine manufacturers use a charge control system that switches the turbine off when the batteries are full there are a number of disadvantages to this. Firstly it is not especially good for the turbine. Small wind turbines are usually 'stopped' electronically by disconnecting the turbine from the batteries and then shorting out the turbine. This is often called dynamic braking. Under high wind conditions when the batteries are more likely to be fully charged by the small wind generator the dynamic braking can be overcome by the force of the wind. This leaves the turbine sending the generated current through the short circuit and back into itself. This is bad for the turbine and will often result in a burnt out stator.

Furthermore this frequent stopping and starting of the turbine can be bad for the batteries themselves as it will cycle the state of charge more quickly. This can reduce the length and quality of service that the deep cycle batteries can offer.

Please see our Diversion Charge Controller equipment in the 'products' section of our website for more information.

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