The Battery Box

One of the biggest problem areas in our cars is the battery box – specifically rust.  When we look at neglected cars to possibly rescue, one of the primary decision points goes directly to one of the only places in the galvanized bodywork where rust occurs.  If the battery box is rusted, perforated or completely rusted through, it’s a parts car.  If it has been poorly repaired with fiberglass, seam sealer and chewing gum, it can also be a goner in our opinion.  But why?

The battery box is not a structural part of the body.  It is a sheet metal area that is located under the hood at the bottom of the windshield, but it is also open to water, snow, ice, salt, leaves and other things that touch the outside of the body.  There is a drain that allows the water to run through the engine compartment near the exhaust onto the ground under the car, but if that drain is clogged with leaves and debris, the water can sit under and around the battery for a long time.  With all those invisible electrons moving around the battery, the paint in this area loses its magical protective powers and the water and chemicals can then cause rust under and around the battery.

The battery box is also located directly above the air conditioning box, specifically, the air conditioning fan and motor.  When water and chemicals leak through the metal “ceiling” from the battery box “floor,” water gets into the fan motor and does what water does to electrical parts, bearings and such.  Over time the entire AC system is ruined, and the water also gets in and under the carpet, promoting rust and mold.  That water actually makes its way back to the passenger rear floor, pooling there.

So when we find one with a rusty battery box, not only do we need to fix that, but we also need to probably replace the AC system completely, strip out the moldy carpets and other interior pieces, but also refinish the floor on the passenger side.  With the amount of work involved in a normal rescue, this additional work is normally too much to consider.

But bad things can happen to otherwise good cars, so people want to know if it can be fixed…but first, let’s talk prevention.


To preserve and protect the battery box area, here is what we do.  (Some may disagree with this, but this is what we do here, and after five years we think that it is effective.)

  1. Remove the battery, tie the cables out of the way.
  2. Thoroughly clean the battery box area with soap and water.  Use degreaser where needed.  Make sure it is clean, let it dry overnight.
  3. Get some Flex-Seal tape.  We have found that the 12″ wide stuff works best.  Also get a small can of Flex-Seal and a couple of small paint brushes.  The tape comes in Black or White, and the liquid also comes in Clear.
  4. Cut a piece of the tape to cover the area where the battery sits.  Do not cover the drain hole!  Put it in smooth and straight, and roll it with a small roller to ensure that it bonds to the metal.  Follow the manufacturer’s instructions for a perfect seal.
  5. Open the can of liquid and brush it around the edges and across the tape.  Make a nice thick layer of liquid Flex-Seal that goes up the sides and around the drain hole.  Let this dry for a few hours, then apply a second coat.  This stuff takes a little while to dry and cure, so reserve enough time to let it do its thing.  Again, read and pay attention to the instructions on the can.
  6. Replace the battery when the Flex-Seal is dry.


  1. Remove the battery and clear away the cables.
  2. Remove any loose rust, debris, etc. from the battery box and surrounding area.  Use elbow grease and a shop vac.
  3. Evaluate the damage.  Is it rusted through to the interior?  Is the area just cracked and weakened?  Is it just surface rust and pitting?
  4. Do the “screwdriver test.”  Take a screwdriver and probe the surface to see if the screwdriver will go though the metal…if it does, you have very weak, very thin metal and repairs are indicated.
  5. If the rust is primarily on the surface and there are no weak spots, then go to “PREVENTION” above after you thoroughly clean rust, debris, etc. from the area.
  6. If the damage is severe, patching and welding is required.  Please do not try to fix it with fiberglass cloth.  The battery is too heavy and will eventually separate the fiberglass fix from the metal, causing it to leak again.  And you will find yourself doing the repair again, but you will have to contend with removing your fiberglass repair too.
  7. From here the fix is just like trying to fix a hole in a quarter panel!  Find the edges of the rusty metal and cut away the rusty metal using the appropriate tools.  You want good solid steel for welding.  Protect the interior from pieces of rust falling into the AC box or carpet.  This will probably mean removing the glove box and other items on the passenger side of the dash.
  8. Create a patch panel with appropriate steel.  If possible, duplicate the stud that mounts the hold-down using an M8x1.25 bolt.  Do this before you weld the patch in place.
    1. We like to use heavier gauge steel for the patch since we do not have the equipment to put beads or groves in the sheet metal to give it extra strength.  If you have a metal fab shop nearby, you can take the finished patch panel to them and have them put a couple of strengthening bead lines in it.
  9. Before welding, protect the air conditioner box and other interior pieces under the area to be welded with welding blankets and/or scrap sheet metal.
  10. Weld the patch panel in place, including weld across the entire edge with no gaps.  When complete, grind the welds and check for any imperfections in welding.
  11. Sand, prime and paint the repaired area.
  12. Once the patch has been welded in place and prepped, go to PREVENTION and complete the job.

There are, of course, variations and differences in this important repair that are dictated by conditions, equipment and skill level.  But here are some “fixes” that we have encountered that definitely do not work.  (Sorry, we don’t have photos, but…)

  • Duct tape over the holes, then spray painted with Rustoleum.
  • Small pieces of sheet metal riveted over the holes, then duct taped and spray painted with Rustoleum.
  • Spray foam through the holes to fill up the area under the holes, then spray painted with Rustoleum.  (This particular repair was found to have ruined an otherwise okay AC unit.)
  • Fiberglass repair “Tiger Hair” and duct tape.  The “technician” in this case stuffed rags up under the dash, then filled the holes with Tiger Hair, sanded it (lightly), covered it with duct tape and spray painted it with Rustoleum.  The rags were still stuffed up inside the car!  (See a pattern here with duct tape and Rustoleum?)
  • AND THE WINNER IS…a properly-sized tupperwear plastic pan secured to the car by screwing through the bottom of the pan into the rusted-through battery box, with the battery then sitting in the pan.  Unfortunately, this raised the battery so that it hit the hood.  To fix that, the battery was then insulated by covering it in…you guessed it…Duct Tape!  And of course it still leaked!

This repair is almost always worth doing correctly as the consequences of a rusty, leaky battery box are quite severe.  Take the time to do it thoroughly and correctly – and permanently.

Author: Kevin Duffy, LLC, DeLand, FL

After retiring from a career in Law Enforcement, Kevin Duffy turned his attention to one of his passions, Porsche 944's and 924S's. He owns LLC in DeLand, FL, rescuing and restoring forgotten Porsches, bringing them back to a useful life. He is especially interested in the rare-but-beautiful 924S Special Edition. He can be found at Porsche Club events, including track days, tours and shows, as well as other car-focused events around the southeastern United States.

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