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.
Do not connect the earth cable first. Fitting the earth cable first means when you go to fit the live side you are actually working on a live terminal. It is all too easy to short the live side of the battery to some part of the vehicle with subsequent molten bits of melted spanner flying about etc. This is a very real risk in the confined area between the bonnet and battery of early jags including Mk7, 8,9, Mk 1, Mk2 and S types. It is also easy to short the live terminal to earth in an XJ6 courtesy of the metal battery box surround. In addition if the battery has just been charged and is still venting hydrogen there is an extreme risk of an explosion from the sparks generated from the shorting to earth and you [and you vehicle] can suffer horrible injuries from this explosion as well as being sprayed with acid. I kid thee not.
Whilst I was an apprentice a trade-training instructor using an old battery and a remote switch purposely demonstrated this. Even though we were over 20 metres away, the explosion and battery destruction left a lasting impression on the apprentice group that included myself.
The correct method is to fit the live side first. Even if you do touch the earth with a spanner while doing up the battery cable clamp nothing will happen, as there is no circuit for the battery to discharge, as the earth side is not connected. The earth side is then connected to the battery. If you touch a vehicle earth while doing up the battery earth side nothing will happen.