Use external charge controller rather than internal inverter charge controller?

[Dan Lenox]

All,

I have a Xantrex XW-6048 inverter that I recently purchased, it will be used in a 48v environment, grid-tied, along with a battery bank. It will not be put into service until spring time.

The inverter user manual states that the battery bank should be connected for proper operation. From what I understand, in a wind turbine environment that the battery bank caps the turbine output voltage to their float voltage.

Xantrex does have an MPPT Solar Charge Controller that they recommend for use with solar applications on the XW systems, their installation manual states that it is not necessary with a wind turbine system.

I understand that the inverter contains battery charging capabilities, but I am thinking of adding an external charge controller and a load controller.

Here is quick system diagram of what I am talking about. Grounds and neutrals on the AC and DC distribution are not shown, as well as auxillary power panel:
374

Since a battery bank tied to the turbine output caps the voltage at the battery banks float voltage, it would seem ideal if a fully charged battery bank can be disconnected from the turbine output so that the voltage is not capped, and more power/efficiency could be extracted from the turbine output.

One could use 2 devices: both a charge controller and a load controller to selectively switch the battery bank in and out. The charge controller is used when the battery bank needs to be charged, and the load controller is used to sense when the inverter wants to draw from the battery bank.

There is Always a dump controller connected to the turbine output, which can keep the turbine loaded in case the grid utility was down.

Wouldn't this scenario provide the maximum extraction of power from the wind turbine?
Another question is - is this a good idea?

I have contacted Xantrex and asked their opinion as well as if I void any warranty by doing this. I have only sent the support request today, Xantrex typically takes a week or two to respond.

thanks,
Dan Lenox

[Rob Beckers]

Hi Dan,

Your Xantrex inverter has a charge controller that can keep the batteries topped-off from the grid. It will not do any controlling when it comes to charging from other sources (solar, wind or any other source). Unless you add charge control to those other sources they will over-charge and ultimately kill your batteries.

There are two general types of charge controllers, in series, and (you guessed it) in parallel. For solar panels the series type is generally used. It simply disconnects the source (the PV panels) from the batteries when they are full. For wind and hydro the parallel type is generally used, it's now called a diversion controller, because it diverts energy from the batteries to something else, such as a a big resistor, or maybe a heating element for hot water. The reason that a diversion controller is used for wind and hydro is because they run away if left unloaded. They overspeed and destruct themselves, so a series controller that disconnects the batteries can't used. As you mention, you could switch from batteries to a load instead, and there are charge controllers that do just that. They aren't used much, at least not for wind, for a couple of reasons: One is that switching a big alternator as used in wind turbines will create big transient voltages. That's a good way to kill electronics involved in the switching, or even contactors. If the switch does not work properly you'll again get a run-away wind turbine. Also, a battery bank provides a nice low-impedance load to a wind turbine, keeping it under control and loaded at all times. A diversion controller is an elegant way to do the job. It never switches the wind turbine directly, and keeps it connected to the batteries at all times. Instead it switches from the batteries to a diversion load, that is a much nicer type of load than switching a nasty inductive alternator.

If I recall correctly, your wind turbine was meant to be hooked up directly to the batteries, its alternator is such that it matches reasonably well with 48V batteries. So in your case you need a diversion controller that can divert as much energy as the turbine can produce, to avoid overcharging the batteries. You don't need any other (dump) controller for the turbine. As long as the connection to the batteries doesn't come loose the turbine will never run unloaded.

In theory you can do what you say; add a switch to divert the wind energy output to another load when the batteries are full. If that produces more energy or not is a big question mark, that will only happen if you can load up the turbine better. Generally only a wind MPPT charge controller will be able to do that. The downside is that you don't load up the turbine enough and it self-destructs.

Does this answer your question?

-RoB-

[Rob Beckers]

I should add: Both the wind turbine (after the rectifier) and the diversion controller hook up directly to the batteries. They don't connect to the inverter as your diagram seems to imply.

-RoB-

[Dan Lenox]

Rob,

my diagram shows the diversion controller connected to the DC distribution box and not the inverter.

The distribution box contains breakers so that any sub-systems can be taken off-line for maintenance, the assumption is that if the turbine is taken off-line that it is shut down.

Dan

[Dan Lenox]

Rob,

As you stated, and I reconfirmed my inverter will only accept 44 to 64 volts, I had looked at so many inverters I should have double checked this. So for my system this question ended up being a moot point.

But as a general statement, yes I agree that using an MPPT charge controller 'could' be used if one existed were versatile enough for wind turbine (rather than solar). Assuming that enough of a load were put on the wind turbine to keep it in check.

Another assumption is that there is *always* a diversion load connected to the turbine output so that at 'some' point it kicks in...

Dan