I should say it's the reversal.
Connect the positive first.
Well done Bob.
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I made the mistake of replacing the screw caps on a battery that I was charging years ago.
I went in the house for a cuppa.
A little while later, I was connecting the battery back up and the spanner slipped, and caused a short accross the terminals. There was a massive bang, and I got a face full of plastic shards and sulphuric acid. I was very lucky not to lose my sight.
So, be carefull
Jim