I want to remove the back-up time (12 minutes) of my Liebert PSP500-115 300w. Because I want to use it like juste a converter DC-AC with battery hight capacity. But the probleme is after 12 minute UPS cut. How can I do this modification please!

it converts Tesla to Edison.
AC to DC,

Yes, you go through a converter.Check the attached links,
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DC AC Power Inverters 12 Volt DC to AC Inverter

12 Volt DC To 110 Volt AC Power Inverter for Auto RV and Boat compatible...

The Parallax 7155 converter should not require battery support for any 12 volt load amperage that is less than the maximum amperage capacity of the converter. A standard RV incandescent 12 volt lamp draws about 1.5 amperes for each bulb. With the battery disconnected and other 12 volt loads off, turn on lamps (not fixtures) and calculate the number of bulbs which can be turned on. If you run out of lamps, turn on vent fans etc. The idea here is to try to get to 55 amperes of DC load if possible without going over 55 amperes. At a sustained DC load of about 25-30 amperes or higher, the fans should cycle on continuously after about 5 -10 minutes. Lamp brightness should be normal and the measured voltage from the converter should be above 13 volts if under 55 amps of DC load. If the voltage drops with only a few lamps on, a converter issue would be likely.

Diagnostic tests an RV technician should make would be to:
1. Disconnect the battery system.
2. With the converter system powered on, measure the DC amperage with a DC ammeter capable of reading at least 60 amperes DC from the blue lead at the DC fuse panel terminal VCC.
3. Measure the DC voltage at terminal VCC and DC ground.
4. Record both readings.
At any DC load amperage less than 55 amps, the voltage should be above 13.0-13.2 volts DC. This method should take out all guesswork and confirm a DC converter section issue or not.

first is no i would not recommend feeding the 9 volt dc through the actual 9 volt ac it may damage the bridge rectifier that converts the ac to dc

The only way to do that is to buy a DC to AC converter...that is if you want to run your tv off of 12Volts DC. The converter converts the DC to AC so you can just plug up your set to the converter. The higher the ampage the better the results. Match your ampage of your tv to the converter ampage output. Just do a google search for "DC to AC converter". There are many availiable and many different prices. It is not practical to change the TV to receive DC voltage.

Yes you can - purchase a 12 Volt DC to 110 Volt AC converter (you can get at Staples or Home Depot). This will allow you to plug your adapter into the converter while it's plugged into the cigarette lighter jack in the car.

you need a voltage doubler circuit which feeds from your 6V DC system and converts it to AC and steps it up and converts it to 12V DC. This device might be difficult to find. OR use a 6Vdc power inverter and just use a 120volt AC fan if much easier to source out in your area.
You might want to try a 12VDC high speed fan and see if the 50% reduced speed is already sufficient for you.

Vacuum works with the battery but does not work with 120 volt. Every time I plug it in it blows the internal fuse. I have replaced the fuse inside the vaccum several times and every time i plug it in it blows. What do you think it could be?

The best you can hope for is 100% efficiency. Sadly, most chargers are only .707 percent efficient when they convert the AC to DC. So charging the battery is going to lose 30% of the electricity your paying for, even if the inverter is 100% efficient, which they aren't, they are actually probably close to 90%, you lose more money converting the DC power back to AC. Not to mention that the average battery loses 1% of its charge over a month. Your better off just following published conservation topics to save on your electric bill.

I'm looking for that TV For my van..we had same tv and now it doesn't work. How much would you sell it for?
Thank you