We know that Dewalt uses 2 FlexVolt batteries to power their radial arm saw, so the concept does work for big power demands at 120v. Why doesn’t DeWalt make a power station with only flexvolt batteries. $500 is way too much for four 20v batteries that are compromised by the lowest common denominator battery. I tried buying the module from the replacement parts site, but they do not sell the module.
I would use it for a CPAP machine, for which there are millions in use.
I suspect the flexvolt miter saw runs on DC voltage internally and is not their 780 saw with a battery kit. In that the motor is a brush less DC 54 volt nominal drive machine. TO get the efficiency out of it. So that AC module they make for it - is just an AC to DC Transformer Rectifer Unit. Or realy just a capacitor bank and a smoothing circult to make a DC solid voltage at 108 Volts and I assume the saw runs at 108 volts due to dual battery.
Which makes alot of sense.
However That doesn’t say they couldn’t make a widget like you are describing. 2 flexvolt batteries gets you to 108 volts though and you would want to get up to 115 so there would be some extra draw to make that difference - it’s not much. Then you would have an AC inverter circuit which would take more draw to even out. SO your 4 AH flexvolt batteries might not last that long.