June 2002
911 Battery relocation
By Steve Grosekemper
Over the years, 911 battery location has migrated all over the front of the car. In the beginning, the battery was placed in the extreme front left corner of the trunk. That seemed to work just fine for a several years. Then someone figured out that it wasn't’t such a great idea to have the battery weight on the same side of the car as the driver.
So in 1969, two batteries replaced the single left side battery, each placed in boxes under the headlights. This was a step in the right direction to get the weight distributed more evenly over the front of the car, but there was a problem. The engines started getting bigger and oil coolers became mandatory. The perfect place for an oil cooler is, of course, exactly where that right battery box lives. So in 1974, Porsche went back to the left front corner of the trunk to house the battery. As if this wasn’t bad enough, the single battery’s weight had also increased to nearly 50 pounds! see figure 1.
Figure 1.
Standard Battery Location
The battery remained there for quite some time, until it got moved up and
back to the rear of the trunk in the 996 cars. Batteries are an obvious necessity,
but with their excessive weight they must be place in the right spot. Weight
placed beyond the wheelbase of the vehicle is much harder to control than
weight within the wheelbase. It’s like trying to spin left and right
while holding a 5-pound sack of flower. If you hold it close to your body,
you can spin back and forth fairly easily. Now hold it 2 feet out in front
of you and do your spins again… Not so easy to change direction now is
it?
There is another reason to move the battery from the stock position. As we all know, batteries tend to leak sulphuric acid, and the front suspension is right below the battery just waiting to get eaten up.
So what’s a poor 911 owner to do about this? The answer
is simple, if you have a pre-1989 911 without air conditioning. A new battery
can be placed in the cubbyhole, which usually holds the A/C evaporator housing.
This box is very small, and the stock battery will not fit. The stock battery
can be replaced by a new smaller battery, which will fit in this location,
and is also going to be lighter than that old stock battery of yours. The
battery of choice is an Interstate DCS-33 battery. This battery is intended
for golf cart duty but works very well in this situation. On top of the excellent
power/weight ratio, the battery is also sealed, so there is no danger of acid
leakage and the associated metal corrosion. This gives us 3 benefits to this
upgrade: weight savings, better location for vehicle dynamics, and increased
safety.
The battery relocation procedure is as follows:
1. Remove the stock battery
2. Unbolt the battery ground strap from the chassis and reattach
the remaining chassis ground wires
3. Separate the battery positive cable from the three red accessory
wires (we will come back to these wires a little later)
4. Cut the battery terminal off of the end of the cable
5. Free the cable of all tie-downs between the original battery
position and the firewall leading into the passenger compartment
(Proceed to driver’s foot well for next procedure)
6. Remove the carpet and foot well cover to access the driver’s side
pedal assembly
7. Locate the battery cable where it goes through the firewall
(below clutch pedal). Release the cable tie-downs and pull the cable back
into passenger compartment
8. Drill a hole from the passenger compartment to the new battery box and install a cable grommet. Insert the battery cable through body grommet and feed the cable into the battery box. Use a body plug to seal off the old cable hole and install a chaffing cover over the battery cable where it encounters the metal brace under the brake pedal, see figure 2.
Figure 2.
New Cable Routing Behind Pedal Assembly
Note: The arrow on the left shows where the cable came from and the arrow
on the right shows the cable inserted through its new hole. Be sure to use
a wire grommet through this hole as not to cause the battery cable to short
out.
9. After the cable is run, reinstall the pedal assembly floorboard and carpet
Where the original battery was located, there is a wiring harness that cannot
be moved, so we need to get direct battery power to these wires from our newly
relocated battery. To do this, we install a 928 power junction assembly where
the battery B+ post used to be. The junction assembly consists of 3 pieces:
-928.612.867.00 - $5.87 - Connector
-928.612.868.01 - $7.13 - Cover
-28.612.073.00 - $5.23 - Screw
From there, we run a 6-gage wire (with a 6mm eyelet connector)
behind the fuel tank and into the battery cubby. Use the same care when drilling
and installing the cable through the sheet metal as we did in the drivers
foot well.
(Do I need to mention how careful you are going to need to be while drilling
next to the fuel tank? - I hope not!)
With this new battery access post, we will be able to test and/or charge the
battery without opening the cubby cover. The 928 power junction neatly does
this by joining the 6-gage wire from the battery to the factory wiring harness,
see figure 3.
Figure 3.
The 928 Power Junction
Now that we have all of the wiring run we can get ready to install
our battery. Before we do, we must address the new wiring connectors for the
battery since the new battery attaches with 3/8” bolts (8mm) instead
of standard battery terminals.
10. Solder an eyelet connector to the large battery cable; solder another
eyelet connector to the 6-gage wire coming from the 928 B+ junction. Both
of these eyelets should have 3/8” (8mm) holes
11. Install a 12-inch long 2-gage battery cable to the chassis of the car
in the battery box. This cable should have standard 3/8” eyelets on each
end
12. Install the new DCS-33 battery in the battery box. Using
the stock battery clamp, drill and install the clamp to hold the battery down
from the top edge. The battery will be very secure here. You can also cut
and install some small Styrofoam blocks on each side of the battery to keep
the battery from shifting from side to side if you wish, see Figure 4 for
battery installation.
Figure 4.
Battery Installation
When the battery installed, we can see how well it fits with
the cable routing and battery tie-down clamp in place.
With the battery relocation complete, it’s easy to see the advantages
of this conversion for the car that is driven in competition, where overall
weight and weight location is paramount. The conversion takes a 48 pound battery
from the farthest forward corner of the car, and replaces it with a 24 pound
battery behind the front axle and to the right of center. We lose 24 pounds
of weight and transfer weight to a much better location. The car is now more
nimble, and has better turn-in characteristics.
You may be thinking
that this is a "For Racers Only" trick, but you would be wrong.
While this is a great set-up for a racer, it transfers to several street cars
as well. Let’s look at a few advantages for different street cars.
65-68 Cars:
These cars are magnets for any type of rust or corrosion. Getting the battery
off the left front suspension pick-up area has obvious advantages. With the
battery in the cubby hole you also free up valuable storage room in the trunk.
69-73 Cars:
These are the twin battery cars. Twin batteries have a host of problems that
we could eliminate. When two batteries are used, there are twice as many connections
to get dirty and corrode. The voltage regulator will also only see the voltage
of the best battery, so uneven battery condition is also an issue. Weight…
remember there are two batteries up there, at 38 pounds each that is a total
battery weight savings of over 50 pounds. Price… these batteries are
not only hard to find, but they are also expensive when you do find them.
The DCS-33 battery costs less than one of the twin batteries. Space…
with the new battery hidden away you have two new storage compartments where
the old batteries used to live. The trunk is now a clean and environmentally
friendly area. There are no more worries about battery acid on your carpet,
car covers, or gym clothes. If you are adding a Carrera oil cooler to the
right front fender, you can now remove the battery box to make room for the
whole cooler/fan package.
74-89 Cars:
The advantages for these cars is similar to the earlier models, clean extra
space in the trunk and no corrosion issues for the front suspension pick-up
area.
While originally intended for racing applications it is easy to see how this
battery conversion could fit into the modification list of any pre-1989 911.
Good Luck
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