Sorry .. no pics.. Please bear with my descriptions.

The new Interstate MTP-91 battery I picked up for my 3.0i Coupe doesn't quite fit. The foot print is identical to the BMW battery. The overall height of the Interstate battery is 3/8" shorter. More importantly, the positive terminal post on the Interstate is 1/2" lower than the BMW Positive post.

This is a problem, because the positive lead coming from the car is not able to "seat" square on to the battery post. BMWs have the extra pyrotechnic de-coupler device built in to the cable / lead. It's bulky, and completely rigid. This assembly runs along side the edge of the battery well (frame of the car). It then has a "dog leg drop" shaped metal terminal lead sticking out at a right angle from the red cable. Like I said, the whole cable / lead assembly is very rigid and there is very little play in the cable.

The cable lead can "drop" about 1/4", but then it encounters / comes to rest on the battery well edge / car body-frame. It can't make it that last 1/4", so it won't seat firmly on the battery post. Sits at about a 10 degree angle. This is not good. I've always been taught, that with battery terminals you want a 100% solid tight connection. In fact, the terminal posts are just barely conical shaped, so that you can twist it on there a bit and get a super good purchase on the post. I'm no where near that. Visible .5 mm gaps on both sides.

There's a plastic shield under the positive cable lead assembly. Stoooopidly, I was thinking, "Hey, I'll just cut that outta there, for slack". Bzzzzz... That plastic is what insulates the positive cable from contacting the frame!

Next option is to raise the battery some. There are three fitment / tie down points. The side strap is no problem and a non-issue. The bumper side clamp foot has lots of threads and some play it... also not a problem.

The front most anchor point is the problem. A 6" long, 1/2" wide piece of 1/16" sheet steel formed into the universal 30 degree angle that would mate up with any batteries plastic molded bottom edge / foot. It's spot welded to the frame of the car. Fits the Interstate perfectly. I thought about taking a pair of channel lock pliers and straighten / pry up the bracket enough to get me 3/16" play, but still have just a little bit of an angle to it so it held the battery. Then I'd buy a sheet of 3/16" thick rubber (from Goodyear industrial rubber supply here in town), and place it under the battery.

The problem is, the sheet metal bracket has the typical "blips" formed into it (the trick for making sheet metal stronger). Three of them. I haven't tried to bend the bracket yet. Wanted to talk to you guys first. From just looking at it... I have a feeling I'd have to cut out the reinforcement blips to slightly weaken the bracket first, and then It would bend.

I double checked with Interstate. They say the MTP-91 is the right battery. I asked, "For the Z3 Coupe.. not Roadster.. not M Coupe". He said, yes. Definitely Z3 Coupe, all years. They have no post adapters to make the post taller.

He also says there is NO Optima battery that will fit my car (They are an Optima dealer as well). This is odd, as I've read on this board that Optima was the other choice to go with. He says the foot print is completely different. Do you guys know of an adapter or something?

I don't particularly want to go with a BMW battery. only a 2 year warranty, and clearly... they're not nearly as tuff. They only last 3 to 4 years. Plus like Ron said, they weigh 9 pounds more than the Interstate. The Interstate has an 8 year warranty and they're just as available across the US as BMW.

Modifying the bracket feels wrong. But... what choice do I have.

Also, thinking of the community here.. most Z3 owners aren't going to want to, or necessarily have the skills to.. modify their car's bracket. Need an easier solution. Tell me I've overlooked something here.

Any ideas?

P.S. Just saw pictures of the M Coupe's battery well. Yeah.. the Z3 Coupe's battery well is completely different. It's over on the far passenger side, and the cables come out from the car at a different angle.