There's been loads of threads on this. The issue is with a larger battery the fuse on the positive battery terminal fouls the bulkhead making it difficult/impossible to fit.

Very sensible quote from Craig: -

Cam-Tech-Craig wrote:

What is wrong with a correctly sized JLR battery sir?

However, we're mostly not that sensible. Also, we all like to spend money on a pointless upgrade.

Modifications to 'fix' this range from standing it on a piece of wood.

To cutting part of the battery to make it fit. Not something most of us want to do with a brand new battery with a long warrantee.

There are all sorts of posts about what this little black box does. This from Miggit.

miggit wrote:

Hold on there.... the Range Rover +ve terminal is an explosive coupling and part of the air bag (safety, not EAS) system. I would be very careful about removing it... for a kickoff it will be monitored and as such removal may throw up a fault with the airbag system...

There's even a diagram.

Click image to enlarge

However, the diagram doesn't look like the part attached to my battery.

I decided to have a look. It's easily opened by removing the sticker and prising the connectors with small screwdriver. All it is, is a 400 amp megafuse.

It's an inline fuse so there's no way it could know anything about the air bag or any other system. Maybe the airbag fires an electrical spike up the cable to break it, but that's the only way There's no way the rest of the electrics could know anything about it unless it broke. I ran my car briefly without it before I did the mod I will describe. I didn't get any error messages. I wouldn't like to run without it for long though as it's there for a reason.

The fuse is made by Ford (unsurprisingly). So I bought a similar fuse box from a Jaguar (JAGUAR XF X250 3.0 D Fuse Box F6HT-14A094-CA 3.0 Diesel 202kw 2010). It came with a 150 amp fuse, so I swapped them over.

Click image to enlarge

I bought a short extra lead to fit between the battery and this fuse, and screwed them together.

Fitted the lid.

You can use the bolt from the old fuse box to connect the lead to the battery. It's a T40 hex I think.

Fit the red thing over the top.

I then pushed it to the same side as the old fuse box. I will tidy this up later.

Cheap, easy mod.

Please let me know if I've done anything stupid. I don't think I have apart from replace one fuse box with another.

That's one of those things that seems so obvious once someone else does it

I remember trimming a lip off a brand new battery to make it fit - and even then it would only go in with some leverage from a bar.

Yes - two threads in the wiki.