Welding Using Car Battery. You can damage electrical components you can start fires. This is going to be pretty much a link-only answer and behind a paywall at that but Digital Machinist magazine had a 2-part article last year on building a spot welder using a car battery as the source.
Before we begin you should understand that welding with car batteries can be dangerous and should only be attempted in an emergency situation and even then the batteries should be covered to reduce injuries you could incur from hydrogen gases building up in the battery and exploding. What you need is a total of 30-36 volts. 1 never attempt this on a battery that is near or above a 70 charge 1232 volts on a digital multimeter.
I have assumed the car battery voltage is 12V and the welder voltage is 100V.
2 never leave it unattended. I would also say it might not have been a sparked hydrogen explosion that Loosie found. But I have heard of it being done in a pinch. Before we begin you should understand that welding with car batteries can be dangerous and should only be attempted in an emergency situation and even then the batteries should be covered to reduce injuries you could incur from hydrogen gases building up in the battery and exploding.