Snapped bolt on HV battery after too much torque

Bolt extractor bit? It's got aggressive, reverse threads. They usually come in a set, an array of diameters for different bolt diameter. They likely spec the bolt diameter at the threads, not the socket size. It's more'n likely either 6 or 8 mm.

You need to carefully drill a pilot hole (diameter usually spec'd with the set). I've found it helps to drill that diameter, then maybe enlarge it with a slightly larger bit. Also good to centre-punch before drilling the pilot hole. Then using the the appropriate extractor bit, and the drill set to counterclockwise, push firmly, press the button, and pray.

That said, I doubt one bolt missing will cause any problems; you can leave this till convenient.

Using an electric impact to install bolts?

 

I doubt that one bolt would be important, either structurally or electrically. And the car's running. Yeah if you can grab the bolt from below that's much easier.

 

This is good practice... It's rare to find a snapped bolt this accessible...

First step it to use a center punch into the middle of the bolt. Automatic center punches are ideal. Then you need to drill a narrow pilot hole into the bolt, then make it large enough to twist in a bolt extractor.

Or alternatively open the case and grab the back end of that bolt with vice grips and unscrew it.

 

You should be able to grab that by the threads and remove it next time you have the cover off. No big deal.

Sometimes it helps to start a bolt with fingers only, and whether you use a tool or not it always helps to rotate the fastener backwards until you feel it drop into the thread before changing to clockwise for tightening.

 

I drive with no cover ... keeps battery cool...er?