Battery acid clean up

You've spilled sulfuric acid in the back of your car. This is a very efficient oxidizer and will happily eat it's way through many things (think Alien blood from the movies). It won't eat through as fast as in the movies as it's been diluted to roughly 37% acidity and if you're lucky, the battery was extremely discharged rendering the acid somewhat less effective.

@CharlieRB's advice is sound, use baking soda and water to both neutralize (baking soda), and dilute (water) to further reduce it's ability to eat into things. If you've left it for long and it's in contact with your floor boards in the car, it's likely already eaten through the paint and is etching the metal of the floorboards. The sooner you clean it up, the better.

Wear clothes you don't like, as they're likely to get ruined by the acid. Wear eye protection & gloves, wash up carefully afterward and change your clothes. Open all the doors to the car while you're working on the spill to provide as much fresh air for you to breath as possible. Put the clothes into the wash as soon as possible after you've finished, or at least soak them in your tub or shower to dilute any acid they may have gotten on them. If you don't, you're liable to find that you have some interesting bleached areas, if not totally burnt through.

A bit of an anecdote, I was filling the batteries on a Mike Boat. They required a very large volume of acid for the very large batteries to start the two Allison V-12 engines down in the engine room. The guy I was working with had decided to drink his breakfast (whiskey), and managed to dump one of the acid canisters on my legs. Fortunately I was wearing a pair of USMC BDU trousers which are very-very tough, and there was a running hose only a couple of feet away. I was able to quickly rinse off the acid, then strip off the pants so my legs wouldn't get burned by the acid.

I used the advice on here for a similar issue. The (new) battery acid was sitting on the floor of the backseat for 2 weeks before I saw it. After doing research I thought it was going to eat through my carpet and metal of the floorboards. I quickly drenched it all in water and then covered it in baking soda 3 times (until it stopped reacting) and then cleaned it all about a week later with a carpet cleaner. The carpet looks good now and there isn’t any visible damage.