12V Pure Sine Wave Power Inverter 300W To AC 110V High Efficient DC Inverters For Smartphones, Tablet, Laptop (300W)

I ordered this to use with a deep-cycle battery as a battery backup for my furnace (boiler plus circulator pumps). After wiring it up I tested it. For the first few minutes it handled the load (~240W) fine, but then it started to make maybe a buzzing sound, voltage dropped, and when it got down to ~85V (from 110V) it turned off. I tried again, and the same thing happened. In the attached pictures the kill-a-watt is measuring power out of the inverter, and the voltage/current meter is measuring power into the inverter.

This is an excellent 300watt inverter. The charge the phone quickly on the charge the computer to other charge the iPhone I like it is still fast for charging 👍👆🏽It is good quality