Relocate battery
 

I just purchased a Odyssey PC925 to reduce weight. When I´m about to put the new battery in I cant help but thinking of how high the battery box is located. The PC925 is considerably lighter than the original, but is still a respectable weight. Wouldn´t it be great to place it at the bottom of the frunk to lower COG... Hmmm cant hurt.

So I start looking at how to fasten it and how to relocate the wires.. Strange thing, I found absolutely no example of others on the forums performing this operation.. Is it just me?

So what would you give up. Trunk space? The spare tire? A simple connection? Dangling wires?

IT is just you!!!! ;-)

Make so little difference in CG and lots of headaches.

Extending the battery wires is always a bad idea.
You will interduce unnecessary resistance to the power distribution.

Had the Porsche engineers thought that the battery should reside lower in the body they would have done it. A lot of work for likely no improvement.

It's just you. :)

The battery location was carefully chosen on the centerline of the car to protect it in an impact so the car doesn't catch fire with you in it. The rest of the car will be sacrificed yet the battery, fuel tank, and passenger compartment are well protected.

Spec boxster's have to keep the battery in the stock location but almost every other type of boxster or 996 race car I've seen moves it to a lower position in the car. The optimal place on a boxster is in the passenger foot well from what I've heard. A lot of them move it down in the frunk. That's where I moved mine and I put it right in front of the spare tire. You're only extending the cables 2 feet at best. That won't cause electrical issues. Heck, people have been relocating batteries to the trunk for decades. Many many vehicles have the battery located in a very vulnerable spot with only a little bit of sheet metal protecting it. I've bought quite a few cars where the battery was crushed in the accident. No fire. If you don't believe me, go on copart.com and you'll see thousands of cars/trucks with crushed batteries and no fire

Wow, great response!

Thanks all.. I´m not to concerned regarding the safety aspect, and if I´ll do it it will need to be a very neat installation, no cheesy Cables hanging around.. I guess it all comes down to wether I should prioritize this before all other stuff I plan to do on the car this Winter. Still if someone has done it, it would be very nice to se some pics or get some hints.

If I go ahead I´ll show & tell :-)

Edit: To answer a previous question: I have already given up the spare and replaced it with a small compressor/repair kit.
A lot of effort for very Little improvement is my trademark ;-)

If you go on here http://986forum.com/forums/show-tell-gallery/59449-woodys-build-thread.html with google chrome you can see some pictures of my install.

Maybe you install that lighter battery you got and see if it makes appreciable difference to you. If not, probably you won't notice much difference moving it down to the frunk floor and you can prioritize other things.

I like the way it is installed, protected from impat and weather. With 914 they installed it in the engine bay half exposed to the environment so it spits acid right over the longitudinal frame to corrode it.

My solution. Of course, I've also dumped all of the trunk interior, etc. Part off the goal was to try to balance out the brake booster. The car is naturally driver side heavy.

http://986forum.com/forums/uploads02/2951513370119.jpg

Nice!
Think I will do this.

If you look at the bottom of the frunk in J.Fro's picture. Those two rubber plugs come out easily and make a great vent for the battery tube

I have a aluminum sheet mount for a Odessey 925 for sale if interested in buying

You went to the trouble of relocating the battery and you still have a spare tire in the trunk ?

Thanks! However I live in Sweden so I guess its a bit far :-)

I found a "Heavy" metal plate under the battery, thought that I could remove it to save a few grams.. Then realized that it is probably vital to keep the gasoline pump assembly safe in case of an accident, so this plate will stay in Place.

https://i.ebayimg.com/images/g/Ph0AA...G2J/s-l300.jpg

Maybe a step away from the main topic, but.. When relocating weight, I Believe we all can agree on that low is good. However do we have consenus regarding:

1) Center of mass (I Think it´s called).
I find the box to be very agile in turns, this is nice as long as you stay in Control, but when you go over the edge the Quick behaivour related to high amount of mass in the center also makes is easy to spin around..

2) Balance front/rear.
Some say... (And this time not related to The Stig) That the 911 is quicker also due to the weight distribution:
a) Better balance front/rear when breaking.
b) better grip on acceleration.

So.. How does this affect the aim of relocating weight in a Boxster? Shold we strive to relocate as much weight as possible to the back trunk?

I believe so, to reduce under steer & improve traction for rear tires.

911's advantage is coming out of turns quicker due to rear engine weight & superior rear suspension

It only is about thirty minutes of work to relocate the battery. It's still a street car and the spare comes out at the track

No!

The 911 is quicker because it has more power. It doesn't handle as well as a Boxster, because the 911 has all the weight in the rear. Good for acceleration, but bad for everything else.

Moving weight to the rear of the Boxster would make it handle like a 911, which is bad. The Boxster handles better because of the weight distribution that the mid-engine layout gives you.

To the OP, relocating your battery would be one of the last things you should worry about. Yes, the battery is located high in the car, but it is still a lot lower than in most cars, because the Boxster is a low car. For street driving, you'd probably never notice the change if you moved the battery to the floor of the trunk.

Polar moment of inertia is the point where the vehicle seems to rotate on an axis.

Not quite right, IMO the lower the polar moment the better, not only will it change directions more quickly, but it will also recover more quickly. A car with a high PMoI may "stay in control" longer, but once you loose it, it will be much more difficult to recover. Not to mention the entry understeer and exit oversteer that comes with it. If you're not trying to change F/R weight dist. , put it low and in the center of the car.

As far as weight dist. , around 55% rear (like the Boxster) is generally considered ideal, 60% rear like the 911 is too much, and can lead to quirky handling issues. This is assuming there's no odd rules in place regarding staggered tire sizes.

Keep the battery where it is and put in an electric power steering pump in the lower trunk. Cuts even more weight and eliminates the problematic PS pump on the engine. Win-Win

The polar moment of inertia is a way to describe how mass affects rotation. The lower the PMOI, the easier and faster an object will rotate. The classic demonstration is how a figure skater increases the rotational speed of a pirouette as the arms and legs are brought closer to the body (center of mass).

Stephan Wilson's explanation is spot on - a car with a lower PMOI will rotate easily and quickly which is why it takes quick hands on the wheel to avoid a spin in a Boxster. Driving a 911 takes a completely different driving technique to manage the higher PMOI (slower but much larger wheel corrections to avoid a spin).

Here is a video showing how quickly I had to move my hands to avoid a spin at Laguna Seca...

<iframe width="560" height="315" src="https://www.youtube.com/embed/BtJiBHyA7bY" frameborder="0" gesture="media" allow="encrypted-media" allowfullscreen></iframe>


Since grip is determined by the stickiness of the tires combined with the weight on the tires, more weight means more grip. But remember that grip is often a trade off; increased front grip can mean reduced rear grip - thus the concept of balance becomes important.

The secret to fast driving is managing the balance of the car to move the weight of the car to the location (front or rear) where it will provide the most benefit at any given moment without creating a condition where the reduction in grip in another location induces handling issues.

Boxster's are generally evenly weighted (front to rear) so its fairly straight forward to manage the balance of the car for maximum grip. For example, braking during corner entry puts weight on the front tires which increases the front grip for good corner turn in and reducing understeer. Then transitioning from brake to throttle at mid-corner allows for weight transfer to the rear to get good grip and build speed on corner exit. There are also other techniques that can be used in the Boxster to get though corners fast but this is the basic approach.

Conversely, a 911 has an inherent rear weight bias, so braking to get weight on the front of the car so it turns in properly is key but the rear weight bias allows for early and substantial use of the throttle in the middle and exit phases of the corner without inducing oversteer. So a 911 typically enters corners slower but can be fast though the middle of the corner and blasts out of corners with all of that weight on the rear wheels.

But at the end of the day, balance wins out. For equal power and tires, the mid-engine layout will typically produce faster corners than a rear engine layout which is why the latest Porsche 911RSR is a mid-engine design.

Why the Porsche 911 RSR Had to Go Mid-Engine


As explained above, there isn't much benefit in moving weight to the rear, but there is benefit in moving weight lower in the car. But you'll have to move a lot of weight lower in order to notice any real difference in handling.

Talk about fast hands... check out this 914, similar weight distribution, on ice...

https://youtu.be/Zcgu4xedAjs