I figured more people on this forum relocate the battery to their trunk, so i'd post here.

I have a monster of a battery, barely fits in the stock location. There's no way I can get a stock, or home-made or aftermarket battery hold down in there. So far it's "gravity" held I do want to hit the track next summer, so I figured i'd change that. I could buy a new smaller battery (then find some use for this 1-2 yr old battery ) or hit two birds with one stone and move it to the rear of the car.

I already wired the car with 1AWG wire for the stereo, so it'd be easy to move that wire to the battery. I don't carry a spare tire, so I could put it in the well there.

Question:

- How do I mount the battery in the spare tire well? Is there a kit? Can I just weld a plate there, and mount it as I see fit, as long as i'm using 2 x 3/8" studs to hold it down?

- Lead acid battery in the spare tire well. Legal, a no-no etc? Is it ok if I use one of those plastic battery boxes?

- Other concerns? Do I need a battery cutoff when using a battery in the rear, or is that just if i'm faster than xxxxx ?
I'm thinking i'd use a 180A circuit breaker back there right after the battery perhaps.

 

You need to have a disconnect switch no matter what your ET is to be NHRA legal. You will need to run the wire going to the alternator through the switch so it will kill the engine when its running when you turn the switch off.

BMR makes a kit for relocating the battery to the spare tire well but I don't think its made to be NHRA legal. The box needs to be sealed and vented if you go that route. Personally I would get a dry cell battery so I wouldn't have to worry about leakage and corrosion with or without a box....keep in mind even a dry cell technically needs to be vented to be legal. I mouted my moroso box on the passenger side of the trunk area and used a dry cell battery.

http://www.bmrfabrication.com/F3-misc.htm

 

As mentioned above, it needs to be in a sealed and vented box to pass tech. When I had my battery mounted in the rear years ago, I fabricated a rear firewall to isolate it from the driver's compartment. I now use a Taylor box which is sold at Mopac. Although it's not an air tight box, it does pass NHRA/IHRA tech inspections as long as the vent tube goes outside the car and it's cheaper than the Moroso box. The cheap red and white marine battery boxes are not NHRA/IHRA approved unless the battery is also mounted behind a firewall which isolates it from the driver's compartment.

Any battery not in the stock location must have a master cutoff at the rear. The switch can not be made of plastic or keyed. Off must be clearly marked. If installed as a push/pull, push must be off. When turned off, it must also kill a running engine. The switch must be wired into the positive wire.

I still have a couple of red top Optima batteries. One is in my UPS power supply for my computer I now use a 31 series tractor battery in my car which just fits in the Taylor box. It has a huge reserve capacity and I've never had to boost the car even on the weekend when my alternator died.

 

Ah, excellent, exactly what I needed to know.

I'll measure my battery and see what i've got here. I was looking at the summit or moroso sealed box, all $99 of it. I could fabricate some kind of bracket to hold it in place, then use a sealed box like that. What's the price on the Taylor one at mopac, do you remember?

I don't really want to have to buy a new sealed battery, kinda defeats the purpose of relocating the battery I already have...
I think there's already a hole in the bottom of the rear tire well, so I could run the hose down there.

WRT to the cutoff switch:
I was thinking i'd run the power from the battery into a 180A circuit breaker or something. Then run it into the engine compartment. The wire from the output of the alt, and the battery, would go to one side of a continuous duty solenoid, and the other side would go to the starter (which is where the power is drawn to go to ignition and everything else). Then I could wire up the "kill" switch to the solenoid.

When you say the master switch has to be at the "rear" you mean outside the car right? Like one of those big super obvious buttons on the bumper? Is there any less obvious legal way to do this? Under the bumper? Inside the gas fill flap?

 

I bought my Taylor box years ago when the exchange rate wasn't that great. I'd have to dig through a stack of receipts to see if I can find it. They're not that expensive. On Summit Racing, the Moroso box lists for $98 while the basic Taylor box without the wiring kit lists for $86 plus it's smaller than the Moroso box.

There is no rule for a sealed or non-sealed battery. Both need to be inside a sealed and vented box if inside the driver's compartment which in a third gen is everywhere except under the hood.

Why would you need a breaker? There isn't one from the battery to the starter now. I doubt a 180 amp breaker would be enough. A starter can easily draw 300 amps while cranking a cold engine.

The master switch at the rear does not have to be constantly accessible from the rear like on my race car. Search the forums as it's been discussed many times. You can mount the switch inside the car then use a removable rod through the back that can be pushed to turn off the switch. When driven on the street, you remove the rod to prevent anyone from turning off the switch. The switch needs to be accessible at the rear while at the race track as per the rulebook. If for any reason, a safety crew member needs to shut off your car, they shouldn't have to look to see where it's hidden. You could be shut off if there's a sudden fire on the start line or if you're on your roof after the finish line.

The trick to installing a master switch is the ability to kill the engine. Because of how the alternator feeds itself power, disconnecting the battery doesn't kill the engine because the alternator still feeds the electrical and ignition system which will keep the engine running. Normally a simple relay is used to provide power to the field windings. When the relay's power is interrupted, the field windings lose power and the alternator stops charging which kills the entire electrical system. Not quite as easy as it sounds but that's the basic idea. Single wire alternators are easier but not recommended.

I have the main power wire coming off the master switch to a continuous duty solenoid mounted inside the car. My main power switch on the dash turns on the solenoid which provides power to my fuse/relay panel to power up everything in the car. The main power wire on the alternator runs all the way back to the battery bypassing the master switch so even when the master switch at the rear is off, there's still power to the alternator main wire but it doesn't power anything else in the car.

When I turn off the main solenoid, it also kills power to the alternator field wire so all the power gets killed and the engine shuts off since the alternator output only feeds the battery and the master switch disconnects the battery from the rest of the car. If the alternator was connected at the main solenoid or on the output side of the master switch, it would self feed the electrical system and the engine wouldn't shut off.

I also use another relay to provide power to the ignition box from my ignition switch on the dash. That way I can turn on the main power and crank the engine over before turning on the ignition. I can kill the engine by turning off the master switch at the rear, turning off the main power switch on the dash or turning off the ignition. When I get back to the pits, I usually just turn off the ignition because I leave the main power on to record data from a vehicle reaction timer, play back my replay tach and to provide power to the water pump and fan to cool the engine down.

 

A slight change to the master switch rule. A master switch mounted at the rear is mandatory for vehicles running 9.99 in the 1/4, if running 135 mph or quicker or if the battery is not in the stock location. Since the majority of people who are not running quicker than 9.99 or 135 mph and want to move the battery to the rear, they still need to install a master switch or leave the battery under the hood until they do get into the 9's where they will have to install a master switch no matter what.

 

It was my understanding that any wire longer than 12" or so should have some sort of fusing. I'm not sure if that's a law or rule anywhere, that just seems to be electrical safety 101 sorta thing. If someone t-boned me it'd be fairly easy to slice that wire and short it to the body of the car, which would start a fire, do some welding or the battery would explode. This is assuming a minor t-boning, where a car fire is more important than the accident... A circuit breaker is slower acting than a fuse, so I think I could find something that would work. This is a small detail anyway...

The way I mentioned wiring up the kill switch should work though. Output of alternator and battery would go to one side of a solenoid, the other side goes to the starter and off to the ignition, field coil of alt etc. So if someone hits the kill switch, it disconnects the solenoid coil, the battery +ve and alternator output would be an open circuit. The power to the field of the alt, and ignition both come from the power distribution box, which comes from the hot side of the starter, which would have 0 voltage on it. That sound right?

That whole rear kill switch thing sounds pretty heinous though. That seems strange that if my battery is in the rear of the car, I need a disconnect, but if it's up front i'm fine. ??
I'll search and see if anyone has a suitable setup for me.

 

one custom made sealed vented out side battery box, holds big mercades diesel battery

 

Good lord, that's big enough to be another fuel cell!
You needed that much battery??

 

yup i can sit in the lanes with the fan on for a long time starts right up

 

That's why I also run a truck battery.

Here's what my rear looked like back in 2003 with a fabricated firewall. Because the battery and fuel cell were behind a firewall, a battery box wasn't required. I was sure I had a pic of the battery sitting down at the bottom in a battery box similar to the above photo but can't seem to find it.