(ET) ET and a Redi-line inverter

[tb <tbamc gbta net>]

I’ve got a decades old Redi-line rotary inverter, 120V, 1000W, that I’ve 
used a few times with a car battery, for tools and lights, and it’s worked 
well.  Uprooted tree from a storm the other night pulled the electric 
service off the house wall, and wife wanted coffee in the morning so I ran 
an extension cord from the Redi-line—attached to the front 2 batteries of 
the ET—in to the kitchen and the coffee maker.  Power disconnect on the ET 
was off.  She turned on the coffee maker and I hear shrieking from the 
kitchen.  Strong smell of smoked electronics and the maker’s switch was 
fused open.  

Actually measured the voltage where I hooked up the Redi-line and it 
measured 12V just like it should so I don’t think the ET was the problem.  
I’ve been told that the Redi-line ac wave may have been too square, and 
that fried the switch.  Does that make sense?  

Thanks.

Thon