I just about have a heart attack when I see people incorrectly connecting or disconnecting batteries in vehicles [or aeroplanes]. Do yourself a favour and read this carefully. I will refer to the batteries “live” side being the terminal above earth and the “earth” side as being the terminal, which connects to earth. This is because the original polarity of a lot of early Jags was positive earth and later models were negative earth. In addition a lot of early models have been converted to negative earth. The use of the terms positive and negative is inappropriate in these circumstances.