Battery in trunk

On another site, there has been considerable discussion on moving the battery from under the hood to the trunk. The main discussion is if a fuse is necc. and what size. anybody got any imput.

you could yank a bmw e30 batt cable from a junkyard. i believe the large section is unfused, but the smaller attatched body electronics lead has a fusible link in place.

the cars I have had with the battery in other places than other than under the hood.. never had a fuse. Granted, all of those cars are approaching 40 years old now..

Size of a suitable fuse is going to vary with application. I don't personally use a fuse, but it's nice to at least have a disconnect switch.

An often overlooked aspect of moving the battery to the trunk is alternator performance. Many alternators are capable of remote voltage sensing, and having a long cable between the battery and alternator often leads to a poorly laid out harness with excessive voltage drops between the alternator and it's loads. Basically, if the alternator senses voltage near the alternator itself, that ideal 14.2 volts might only be 12.5v or so after a few junction points, resulting in a poorly charged battery, dim lights, etc. If the system voltage is sensed away from the alternator and closer to the loads, the alternator will automatically raise it's voltage so that the correct 14.2 volts is maintained at the sensing point, ideally a centrally located load distribution center.

I have thought of moving the battery to the trunk but constantly wrestle with the added weight of the long cables.

What is better slightly better weight balance with another 20 lbs or less optimal balance but 20 lbs lighter?

I build a lot of hot rods with trunk mounted batteries, and I always fuse them with two 200 amp ANL style fuses in parallel. More than enough fuse for cranking over higher compression big blocks, but they will pop if something goes bad...

later, matt

If money is no object, I'd go with a small, gel celled battery like a Braille, in the stock location. No worries about added weight, or voltage reference, just a few pounds taken off the nose.

That being said, they do cost some decent coin, and this is Grassroots....

I'm in the midst of pondering this dilemma right now. I need to move mine to make room for front-mount intercooler plumbing. I need to see if a small battery in the stock location is going to work, or if I need to stick it in one of the storage bins behind the seat.

These guys mostly want to get the battery out of the engine compartment. They are worried about the main cable shorting, Hence the fuse. The starter load is the biggest problem. I think xfactors idea would work

PS: I am not doing this myself, just trying to be an adviser.

dean1484 wrote: I have thought of moving the battery to the trunk but constantly wrestle with the added weight of the long cables. What is better slightly better weight balance with another 20 lbs or less optimal balance but 20 lbs lighter?

I ran one cable to the trunk. The ground is about 10" long and grounded in the trunk. I didn't weigh it, but there's no way that 12v+ cable is 20lbs. It's only about 15' of 4g. The weight difference of REMOVING weight from the front and ADDING it to the back, in a reasonably light car like my Mk2 VW made a very noticeable difference to the balance of the car. No charging or starting issues so far. I did it about 3 weeks ago.

you aren't just taking 50 pounds off the nose- you are moving 50 pounds to the back of the car. .that's a 100 pound weight swing.

plus.. that cable can be laid low in the car. I know in my Fiats, the cable goes to the starter first and then is junctioned off

Right, the cable goes from the battery to the starter, just like when it was unde the hood. The only ground cable needed is from the battery to the frame. Of course the engine has to be grounded also.

Here is a great discussion on the technicial issues on moving the battery.

http://www.redpepperracing.com/forums/index.php?s=&showtopic=20659&view=findpost&p=357203

You'll never guess what car had the battery in the trunk from 1990-2005 :) The cable was unfused and went straight to the alternator.

Thinking about it, I have six cars with trunk- mounted batteries. Wow. The M5, two Miatas, my MG, the locost and the Mini.

Not the best pic, but... two 200 amp anls stacked for the starter, and a 125 amp to run the car.

For most 4 cylinder apps you could run a single 300 amp anl.

later, matt