Years ago, my father-in-law had a battery hooked to a charger, the battery was finished charging, but still hooked up trickling. I turned the battery charger off, unplugged the charger from the wall and waited for the chargers' capacitor to discharge.
When I went to disconnect the clamps from the battery, it blew, in my face and in my eyes.
I made it to a faucet a few steps away and layed under the faucet with my eyes open for about 15 to 20 minutes. I sure am glad father-in-law was only a few steps away!
That day and the next I couldn't see anything, along about the third day, things began to visualize, but as in a deep fog. It took the better part of a week before I could drive and 2 weeks before I could return to work.
I really now not only respect a battery. but I also keep my face out of the danger zone when connecting or disconnecting a battery.