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How to Relocate the Battery to the Trunk with e30 Parts

Written by Charles Elliot Goldstein Saturday, 01 October 2005 How to Relocate the Battery to the Trunk with e30 Parts By Charles Elliot Goldstein

Parts Needed:

  • e30 battery cable and plastic protective sleeve (or length of old garden hose)
  • e30 engine-compartment distribution block
  • e30 starter cable
  • e30 battery cable grommet or regular large grommet
  • NAPA or similar small marine battery box
  • 4' - 1/8"x3/4" aluminum angle
  • 6 - 3/4" long stainless steel machine bolts, washers, and nylocks.
  • approx 30" 2 gauge side-post terminal battery cable
  • approx 15" 2 gauge top post battery ground cable
  • Dremel with cut-off wheels and grinding stone.
  • Drill with bits the size of the SS bolt you choose.
  • Auger bit


Step one: Pick a spot to drill a hole for the battery cable to enter the engine compartment. I chose that weird angled surface right above and behind the clutch pedal, where it touches the firewall when depressed. Now, this is where you can decide whether you want to cut an oval hole like I did to fit the oval e30 grommet, or make it easy on yourself and use a regular round grommet. I needed to borrow a sheet metal auger bit from a machinist friend; I guess they cost alot of money so you might want to consider that. Now, if you're taking the e30 grommet road you'll want to hold up the e30 grommet from both sides of the firewall and make sure there is room to cut. If you have a late car there isn't much choice; it's pretty crowded. Hold up the grommet and trace around the shape and try and find the center points of the radius' as best you can. Now buy, borrow, or steal a sheet metal auger bit that's slightly smaller than the waist of the grommet. Drill pilot holes for the auger bit. Cut the radiuses with the auger bit and use the dremel to open up the two holes where they cross each other, so you have an oval the same shape as the e30 grommet. Good job, this was the hardest part.


Cable enters engine compartment just under main harness.


Other side of the firewall.

Step two: Whip out the aluminum angle, and cut four pieces to surround the base of the small marine battery box. Drill holes in each piece for the SS machine bolts, one at each end. Now, measure the width of the strap that is included with the battery box. Use the dremel cut-off wheels to cut a slit in the center of the two longest pieces of aluminum angle for the strap to lace through and get a strong hold on the battery box once the aluminum is bolted to the trunk floor. Now say to yourself, "I like long sentences."


Pretty, secure, AND solidly mounted...

Step three: Center the battery box in your trunk with the ports for the battery cable facing towards the rear of the car. Surround the base of the box with the four pieces of aluminum angle so it is tight, this will prevent the box from sliding when your driving around the city like a mad-man. Drill holes in the trunk floor to match the aluminum bolt holes, being aware of the diff hanger reinforcement underneath the trunk floor. Bolt down the aluminum angle with the battery box strap laced through the slits. Rob sez: If your battery does not SNUGLY fit the battery box, you should probably fit a seperate battery tie-down inside the box for the battery itself!

Step four: Run the e30 battery cable through the rectangular hole above the drivers-side rear wheel well and slide the plastic protector sleeve over that portion of the cable. If you didn't snag this sleeve from the boneyard, you can use a piece of old garden hose, which is generally extremely tough stuff. Some (RobS.) would recommend running the entire length of cable inside some nice tough garden hose just for safety's sake. Now take the cable through the back seat bulkhead down by the seat belt reel/mounting point. Then go through that little tunnel in the seat rail and up and over to the grommet in the firewall.


Using all (more or less) stock BMW parts, the job is complete...

Step five: Mount the e30 distribution block. I chose to bolt it onto the washer bottle holder right above the brake booster (my car is a '76). The holes line up perfectly with the distribution block mounting bolts. If you are using your washer system or you have an early car, you might want to mount it on the drain gutter sheet metal, you know what I'm talking about. Bolt the tapered lug end of the e30 battery cable to the distribution block and then remove the stock starter cable and engine block to battery ground. Take the e30 starter cable and run it from the tapered lug end of the battery cable at the distribution block to the starter (relay). File the non-recessed side of the side-post terminal end of the cable you bought so it's nice, flat, and shiny. This is going to rest against the block and provide a block to chassis ground. You might want to scrape the rust off the block where the stock ground cable was bolted on. When I did this, the bolt that was in the block fit prefectly through the side-post terminal end, and the washer fit perfectly inside the recess in the terminal end, so just bolt that end of the cable to the block. Finally, find a nice spot for the other end of the new block ground. I chose to bolt to the common chassis ground point right next to the relays. If your car doesn't have this, then take off an upper strut bearing nut and sand off some paint around the bolt, then bolt it there with the strut bearing nut.

Congratulations! You should feel a subtle but noticeable change in your handling balance as well as have more room up front for sidedrafts, fuel injection, or just general wrenching! If you have any questions, feel free to post them to the Message Board!

Also check out Zenon's Hawker battery installation for under the back seat. FYI you can also put an Optima red-top batt under the rear seat, laying on its side.

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