systems these days have computers
that stay connected all the time. There is always a small current
drain because of the computer draw, about one ampere. One ampere is sufficient to cause a spark. A spark could easily ignite the hydrogen gas that may be given off by a battery
; however, a battery
usually only gives off this gas when being charged. It dissipates very rapidly and does not "hang around" the battery caps very long. Still, hydrogen gas is very explosive and it will blow your battery to pieces and splatter you with hot acid if you ignite it. Dangerous. Accordingly, you should ventilate the area around the battery before removing any connections on it - just waving a rag over the battery ten times will get rid of any gas outside the battery. Now, you cannot get rid of the gas inside the battery so you still must be careful. Even a small explosion is not a good thing. When disconnecting terminals or checking a battery with a tester it is best to cover the battery with a small sheet of rubber so you do not accidentally cause a short circuit. Always disconnect the negative connection first. The reason for this is that those computers
run on 5 volts, not 12 volts. If you touch the frame of the car with the positive lead or a wrench connected to it you will not only get a nice shower
of sparks that might cause a gas explosion, the frame forms the negative side of the feed to the computers. It is possible to fry the computers by placing even a transient 12 volt power surge to them as might occur with the negative lead still connected and the positive lead arcing to the frame. Reversing the leads when jump starting the car is a common way that computers get ruined but arcs from the positive terminal can do it too. In summary, be very careful around batteries
because they hold a great deal of energy, emit explosive hydrogen gas, and the entire system is designed for the battery to be connected and disconnected in a certain order - take off the negative first.