This is a photo of a battery that I blew up. I was hooking up the last few wires before hitting the starter for the first time on my new engines. I was placing wires loosely on the terminal studs as I figured what went where. All went well until I connected the engine ground cable to the engine. BAM, my ears were ringing & everything including me was covered in acid. I was sitting in the bilge between the engines two feet away from it when it blew. Lucky for me my face was turned away. I grabbed the hose off the dock & blasted myself & my brand new engines. I thought, I must have connected something wrong. That was not the case. Apparently one of the first connections was the battery charger. The charger was just doing its job charging the batteries & creating hydrogen gas. When I connected the ground, a loose wire on the battery terminal arced. The rest as they say is history. You can do this stuff thousands of times & get away with it. But every once in awhile things can get real interesting real fast.