>"To jumpstart a car battery you must not connect the negative terminal directly, the spark could ignite the hydrogen generated by overcharging the battery and cause an explosion."

imho this is pure bullshit. hydrogen can hardly be generated when the battery is not being charged yet (duh), and the amount of potential hydrogen generated AND escaping is absolutely negligible. even so, if there was a significant amount of it (that I highly doubt, since the hood is open and it escapes upwards rapidly), do you remember what happens when you ignite H2 in a glass tube? it just "barks" away, there is no chernobyl-tier explosion, it is not a TNT