I received your email and replied, but at the time I was thinking you had a Xantrex inverter, I reposted the main contents of my e-mail response here and then edited them a bit. I don't have an Outback inverter, but I hear they are reliable, but the basics are below. I don't claim to be much of an expert, just trying to help, so don't send me your bill if things go wrong, in other words verify for yourself before application.
It would be a good idea to look at the battery fact sheet that our hosts here put together as it is a good source of basic information. It is located off of the front page and I have copied a link here:
http://www.otherpower.com/otherpower_battery.html
The Outback probably has a three stage charger, which charges the batteries in three stages, bulk stage - where a basic constant current is applied to charge the battery to past 80% or so, absorbtion stage where an elevated voltage which is applied to the battery while the charging current diminishes until the battery is essentially completely charged, and float stage at a lower voltage where the battery can be maintained from that point on in a stand by mode. The exact charging alogrithms for these stages vary with manufacturer. The charger likely also has an equalization mode, which applies a higher voltage to the batteries for a period of time, allowing all of the cells in the battery to slightly over charge, to equalize the voltage and state of charge of all of the cells. The purpose of equalization is to prevent the state of charge from different cells of the batteries from varying too greatly, causing some cells to be chronically under charged and others over charged. In extreme cases of imbalance and then discharge, an individual cell can become completely discharged and reverse, often ruining it. This is not likely to occur in a situation where you are overcharging the batteries a bit with the wind turbine every once in a while before the dump load kicks on.
In your case, you will probably want to disable the equalization mode, which is most likely possible with your charger, check your manual. The reason for disabling equalization is that the higher voltages during equalization will likely turn on the dump load as the C40 will sense the higher voltages and think there is power to be dumped. You can manually equalize if needed, (disconnect the C40 and dump load) but if the dump voltage is high enough this will likely not be necessary.
The next thing to do is to look at the voltage settings on the C40. If you have flooded type of batteries, set the dumping voltage a little bit higher, a slight over charge will not hurt them, and the voltage should rise higher once they are charged if significant input current is applied. The voltage at which it cuts off the dump load should also be adjusted to turn it off before voltage drops low enough on the batteries to kick the charger back into its bulk charging mode.
If adjusting the C40 alone does not solve the problem, then you will also want adjust the voltage on the charging stages of the inverter to prevent the voltage during charging from exceeding the voltage at which the C40 load controller begins to dump power to your load. Hopefully on your Outback inverter, if it has a three stage charger, the voltage of each stage can be independently set, but refer to the manual for the one you have for the details of whether this can be done and how to do this. You will also want to have a meter so that you can verify the voltages and make adjusting easier. If the dump load kicks on during any of the charging stages (other than float stage if it is maintained indefinitely), then you will have to make adjustments to the inverter and load controller to prevent this.
Additionally, you will want to check your dump load. It should be able to handle the maximum voltage your battery bank is likely to see (not just the nominal voltage of the battery bank) and dump as much power as you are likely to generate. It should be robust, so that it will not burn off, start a fire, melt plastic, or have other difficulties during periods of sustained high winds, or other generation. Some inverters can control loads for dumping as well, that might give you additional options, or the capability of having a redundant dump load, so check the manual for your inverter.
Good luck with your system, Rich Hagen
