When the battery is charging when it is 80 percent recharged the per cell voltage is 2.36 volts the battery will gas it is breaking down the h20 it will vent the Hydrogen and the oxygen, Hydrogen at 4 percent per cubic volume you have a potential bomb all it takes is a spark.

 

Ever heard of "bilge gas" exploding a boat? Hydrogen in close quarters (like in the spare tire well of a HHR) is "bad". Not so much worried about the battery itself exploding.

GM may be a bit over protective, but it could happen.

 

And thats the reason there is a vent tube that exits the bottom of the car, so there is no build up,, therefore lessens the probability quite a bit...otherwise we;d have cars blowing up all over the place, yes? If it was that bad GM would have never put them there for liability purposes. And lets not forget the thousands and thousands of hot rods with the batt in the trunk...two of them sometimes....

 

...... and old VW bugs -