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Before we get started, there are plenty of stock-sized batteries the will fit nicely in the E46 with no modification to the hold-down bracket. This won't be helpful in that case. I just so happened to have a reasonably new Optima Red-top 34R (Group 34 with terminals reversed) from my Audi, and wanted to replace my 10-year-old OE battery. I wouldn't go out of your way to do this swap, but rather I present it as a workable option.

I'm sure it's understood, but I accept no responsibility if you destroy your car or any part of it doing this. If you can't install a battery without breaking things, you might want to look into a new hobby, but that's neither here nor there.

On to the Optima 34R battery. It provides 1000 CA and 800 CCA. It weighs approximately 38 pounds, compared to OE which is somewhere around 50. The weight advantage provides little to no real-world benefit, other than the satisfaction you get from knowing you've shaved a few pounds and have added a few CCA. As a comparison, this is similar weight and nearly twice the CCA of the "BMW Lightweight Battery" sold by some vendors. It's also fully sealed, meaning you can tuck aside the vent tube.

There are lots of battery install DIYs, so I won't go into that detail. I'm standing on the shoulders of giants.

Once you've got your battery in the well and terminals hooked up, you'll want to reinstall the hold-down bracket. Due to the small size of the 34R, the height-adjustable plate on the bracket provides almost no clearance between the terminals. I've attached a picture to illustrate. I didn't want to take the risk of the battery moving in the well under hard driving and contacting the plate, so I modified the bracket to a safer design to allow for clearance between the metal bracket and the terminals.

Items required:
- Jewelers or other small-blade flathead screwdriver
- Large Phillips driver
- Ice hockey puck from your local Sporting Goods retailer (Made in Czech Republic for maximum street cred)

This is simple. With the battery bracket off the car, flip it over. Looking at the bottom of the plate, you'll see a round retaining clip. Pry it off with your flathead. It may take some persuasion.

With the clip off, the plate is released from the bracket. Put it aside. Note that the height-adjustment screw has a "nose" on it, to which the retaining clip attached. This will come in handy.

Lay your puck onto the battery. Place your plateless bracket into it's mounting position, line up the puck centered under the screw, and bolt the bracket into place.

Now, drive the screw into the puck. The "nose" acts a pilot, allowing the screw to be driven into the puck with hand-torque only. Secured, the puck provides a high-friction, non conductive anchor for the battery. On a battery with a stock-like wider terminal spread, you could easily leave the plate and place a puck underneath, as many have done, but with the smaller battery this is a relatively simple fix.

Godspeed, battery installers.
 

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Good post ... I think I used a portion of a cutting board from the local dollar store to bridge the gap when I installed my last battery.
 

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Nice.

I was going to try to Plasti-dip my original bracket, but just happen to have a hockey puck left over from the 6 pack I just bought to make jack stand pads!

Think I'll try this before i crack open the yellow dip (even though it would look cool with my Optima Yellow top). :)
 

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i used a wood block 2" thick and put it under the battery and removed the roundish metal holder and the block lifted the battery high enough for the bracket to hold it into place. actually fit pretty snug.
 
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