power inverter and back up power

That's correct. Many things will not work with an inverter like a UPS. Nothing wrong with your setup or equipment. Your inverter is a modified sine wave. A real inverter is a pure sine wave but much much more expensive and your UPS would work with that one.

Just as an experiment. If you have something else plugged into the inverter like a TV or Microwave, turn them on and see if it kick starts your inverter to charge the UPS.

 

how about fixing the problem and see what happens first before trying another solution.

properly functioning batteries shouldn't be draining down to 11.7 volts over a 10 hour break. my truck has a fridge, and 4 brand new batteries. voltage drains to 12.3 after 10 hours. in a normal operating environment. perfect sleeping temps so truck not needed for a/c comfort. naturally, you wouldn't leave the fridge on. if parking the truck in the hot summer heat while your on hometime. the fridge running inside a hot truck will kill the batteries in 10 hours. not an issue in the winter time.

if you don't have a fridge, you should be able to run them batteries on a full 34 hours. without firing up the truck.

my invertor didn't work when the voltage drops to 11.7 either. on them older batteries.

 

This a big problem with opti-idle or whatever they call it. A truck engine starting is going to cause a huge momentary drop in voltage and few inverters are going to "like it". That said, if this is a mfg'r or dealer installed inverter, they (warranty or authorized service) may be able to correct this with another inverter or some other means

 

all i have is a cobra 400 watt plugged into the 12v cig socket with a 5 in 1 plug. it runs my 9 inch fan when needed, (consumes the most power) laptop, printer, and a small heated glade scenter. not all at the same time though. last year i also had a tv and roku.

have no idea on the fridge power supply. but when the old batteries went down, the truck just shut off. no electrical power till the truck was fired up.

 

I've got an 1800 watt Xantrex in mine, it will run the fridge, tv, and a fan if I need it all night, volts drop to 12.4 to 12.5. This is will be the 3rd year on these batteries but this inverter has the shore power charge feature so when I'm home or able, I usually plug in to keep them topped off. So far, so good.

 

What make and model is that UPS and inverter. Let me look at the specs. The inverter may not be big enough to charge the UPS.