It is going to be one of three things.
1. The charging system in the Windstar is not up to the task.
2. Heat build up in one of the components.
3. Inadequate wiring(Goes with #1 &#2)

Are you trying to run it with the Windstar engine running or were you expecting to be able to cool a camper with the AC unit.

You're asking the Windstar's probably 200 Amp hour battery to deliver 300 amps continually. It cannot deliver that much current for very long. It's only intended to run the starter motor for a few seconds.

Even with the engine running I doubt your alternator can keep up with such a HUGE electrical load. There is no such thing as a free lunch. A 12 volt DC electrical system has to deliver 12 times the current to produce the same wattage as a 110 volt system. If you put about 8 of those batteries under your seat, got a 48 volt inverter and wired up your batteries in pairs then parallel for 48 volts you could probably run that AC unit for 20 to 30 minutes before it too would die from low voltage. That assumes your alternator still works after charging all those batteries. It's a simple matter of power. The battery in your van does not hold enough power to run an AC unit. Just start the engine or plug the thing into an extension cord, it's cheaper and it will run longer and it won't burn out your alternator and cost you $300 in repairs to learn the formulas for power/voltage/current.

840 watts drawn off 80-90% efficient inverter draws approximately KW,80 amp alternator at 14 volts is kw,you cant draw 100 amps continuous from an automotive flooded lead acid battery,there are so many components of this equation that scream insanity,i run 3kw inverter in my 5th wheel rv but i run six group 27 deep cycles.i use 80 amp charger/converter for deep cycle bank daily at 1kw charge rate.this is so basic i cant believe the question or the responses,of course the voltage is going to drop,dude do the math.its going to be one of one things,1kw charge rate,no reserve capacity,3kw inverter=no worky likey thisey any wayey

I have a large inverter in a ford transit connect... but similarly to unk1's setup... I had extra batteries... I used 3 blue top marine spiral cell wired with 2 gauge and an 80A fuse to the stock battery and kept the engine running as much as possible... I am amazed you didn't trip the inverter with the in-rush current... I wonder if the AC unit spins the motor (without the compressor) for the first couple of minutes then when the compressor kicks on the extra load causes a higher current draw (thus dropping the voltage)... one thing about inverters... they normally show wattage capacity on a resistance load only (not motor) and a/c motor wattage rating is almost always lower.... esp in-rush current... also normally they only produce square wave (or triangle wave AC which can run an AC motor hard, hot, and cause jittery starts... real sine-wave inverters are normally very expensive...

id say up the capacity with at least 2 additional deep discharge cells in parallel... then if in-rush is an issue try something like:

http://www.parts-express.com/pe/show...umber=267-7056

to prop the sags...


in an ideal world dual alternators.. or an upgraded alternator 130A+ would help (some new cars do put out 100+A but they run a lot of stuff... )

kid twist brought up a good point,dirty AC makes motors run much less efficient,subsequently with the same inertial load they will consume amperage at a greater rate for a longer period of time,therefore further increasing surge requirements,compounding the problem.