some reading
More reading
(the product name in the link is Guardian, but it's generically called a "battery isolator")
You can't just plug in a second battery to positive and negative in your vehicle. All sort of funky things can happen from eletronics issues to dead batteries. You need to wire it properly so your charging system will charge both batteries while you're driving.
You also need to mount it properly. If you get inot an accident, that suker will take your head off! It doesn't take much of a hit to get it airborne either.......
As to killing you, the poster about is right. Batteries give off hydrogen gas when charging anddon't when discharging. Even so, hydrogen given off by the battery isnt going to kill you, but it can explode! I've seen a battery go up in the shop once, had to change my shorts after that. A guy was charging a battery, sparked the charger cables and the hood damn near came off the car.
Best advice I can give is get a battery box (boat stores carry them cheap) and vent it properly if you're going to keep it inside the car.
There's a reason they're outside on cars and campers.............
Cheers