Simple for many but not everyone even knows where it is.
Mine is left hand drive but all have the battery on the left of the cab regardless.

If you have a cab radio that has a secure code have it to hand before you start
If your alarm has special instruction for resetting? either have the instructions or contact the supplier/installer

Be absolutely sure you have the correct replacement battery! physical size capacity and terminal configuration. (Consider a bigger battery whilst doing this) I paid £95 for a 110ah Varta Silver 5yrs warranty delivered. Standard size for 3.0 litre is 100ah usually

Remove the cab carpet and there is the floor panel with coin turn fixings a 50p piece is good, just half a turn on each

Now you have the battery box cover, just remove the two nearest 13mm bolts.
The lid hinges up and then pull it away from the hinges towards you.

This is what you will find

Now remove the battery clamp 13mm nut

Now disconnect the Negative clamp, no spanner needed here but best to remove the cable out of the way altogether by removing the 13mm bolt to the chassis also.

Now remove the terminal cover at the +ve end

Remove these two screws and the red plastic expansion plugs.

Then slacken the +ve clamp nut 10mm ring spanner and lift the lot clear


Pull out the old battery

Present the new one

Be sure to remove both transit plugs. The red bung pictured, one each end.

Reverse the whole procedure