One of my coworkers came in to work yesterday with a four-inch gash in his forhead.
He had charged up his lawn mower's battery, and when he turned the key, it exploded, sending shrapnel and acid all over his garage. He thinks he was hit by a filler cap as it flew by. Acid was sprayed all over the garage ceiling. He got lucky.
ARRL Field Day is coming up on the 26th and 27th of June. Our club uses batteries for some things even though we have a generator for most of the operating positions.
When a battery is charging, it will release hydrogen gas, and a spark from an ignition system, or even a bad internal connection is all that it takes to set it off.
My high-school chemistry teacher said that of all of his senses, the last one he would want to lose is his sight. I have decided that I will never again work on lead-acid batteries without safety glasses. I will be buying a set for each vehicle, to keep with the jumper cables.
