Battery in the trunk - is a ground wire needed?

[Ian]

This is a timely discussion, as I am rewiring my car with a stand alone ecu with the battery in the trunk.  It seems as if there are 3 strategies:

1) All the power from the battery being distributed from a bulkhead distribution fitting. Advantage is packaging and weight, disadvantage is feeding unwanted current into delicate electronics.

 

2)  Run 2 power lines to the firewall, and keep the delicate stuff (ecu, radio) on a lead separated from  the heavy draw components (fans, starter, etc). Advantage is protection of the delicate electronics. Disadvantage is weight and complexity of a second power line running to the front of the car.

 

3) Tee off the main bulkhead distribution point to feed delicate electronics.  Advantage is better packaging and weight without running a second power line to the firewall.  Disadvantage is that you will probably still get unwanted current into the leg with the delicate electronics.  Is there any way to run option #3 while protecting the elctronics?

 

Thanks for the electrical edification!

 

[PhilC]

13 minutes ago, Ian said:

This is a timely discussion, as I am rewiring my car with a stand alone ecu with the battery in the trunk.  It seems as if there are 3 strategies:

1) All the power from the battery being distributed from a bulkhead distribution fitting. Advantage is packaging and weight, disadvantage is feeding unwanted current into delicate electronics.

 

2)  Run 2 power lines to the firewall, and keep the delicate stuff (ecu, radio) on a lead separated from  the heavy draw components (fans, starter, etc). Advantage is protection of the delicate electronics. Disadvantage is weight and complexity of a second power line running to the front of the car.

 

3) Tee off the main bulkhead distribution point to feed delicate electronics.  Advantage is better packaging and weight without running a second power line to the firewall.  Disadvantage is that you will probably still get unwanted current into the leg with the delicate electronics.  Is there any way to run option #3 while protecting the elctronics?

 

Thanks for the electrical edification!

 

As per jimk’s post, you should have one thick and one thin from the battery to the bulkhead.

[slowbert]

As the ground/power wires to the engine compartment get bigger, there is a smaller voltage drop during starting.  In theory, an enormous cable will have such little voltage drop to be insignificant.  This is your option 1 or 3.  Both options require a big enough wire that the power drop is "not that much" during starting.  I bought 25 feet of cable and had less than 5 feet left with my trunk-to-engine bay routing.

 

Also, remember that your return path needs to be calculated in the voltage drop (so multiply the distance by 2).  If you include the 'extra' ground, the calculation is easier than guessing what the return path would be through the chassis.  Note that the limiting factor is probably *not* the resistance/current flow capability of the chassis - the limiting factor is probably the current capacity/resistance of the *connections* in the circuit - which are next to impossible to calculate/measure. 

 

 

Alternatively, you can have two power/ground paths: one dedicated to starting and other high-current draws (lights), and a second path for electronics that are sensitive to voltage drops.  This is option 2.

 

The tradeoff is whether options 1/3 are less weight/cost than option 2.

[Mark92131]

Here's my setup.  The car originally had the battery mounted in the trunk, but I never liked giving up trunk space, so I decided to go Odyssey 680 and a combination Power Distribution and Relay Box under the back seat.  I had the 1/0 Power cable from the original build, so I ran it up the driver's side from the back seat, through the firewall and to a busbar endpoint at the normal battery position.  I cut off the cable I didn't need, used it to make a negative cable and put new lugs on both cables, now grounded on the lug for the rear seat belt.  I also ran a thick braided cable from the chassis frame rail to the motor and a smaller braided cable from the motor to a firewall mounted busbar to have a common grounding point for all the sensors.  I ran a switched 12V line from fuse #5 to a busbar under the seat to feed the relays and a separate busbar for grounding the relays directly from the negative side of the battery.  I set up one of the relays for an electric fan for my Behr 320i radiator and then ran 3 harnesses to the front (electric fan, O2 Sensor and ECU) and one to the back (fuel pump).  The ECU harness powers a Microsquirt controlling ignition (wasted spark) on the S14 motor.

 

So I didn't like the position of the bus bar I added for the negative trigger feed for the Fan relay.  Basically the temperature switch in the bottom of the 320i Behr radiator and the manual switch in the center console will ground the relay and power the fan.  I also needed 12V power from the relays controlling the MicroSquirt ECU and the O2 sensor, so I added two more bus bars and moved everything up under the dash along with the MicroSquirt ECU as well.

 

I purchased a Deutsch 29 Pin bulkhead connector to handle the wiring between the engine compartment and the MicroSquirt ECU.  The Deutsch 29 connector only has four, 12 AWG pins, so I'm going to use 2 pins to power the 2 banks of injectors (1 & 3 and 2 & 4).  I originally had 12V+ Switched power off the Main Relay running to a 2 post junction block under the dash, but I decided to switch it up and ordered a Blue Sea ST Blade ATO/ATC Fuse Block instead.  This will allow me to add a 5 Amp fuse for the injector power feeds and and a 2 Amp fuse for the ECU.  The other 2, 12 AWG Pins will be used to power the EDIS-4 Module and the Bosch PWM Idle Control Valve.  All the other connections required to feed the ECU from the motor will be 14 AWG with the exception of the oil pressure and water temperature gauge feeds which are 20 AWG.

 

Probably overkill, but turned out pretty nice.

 

Mark92131

 

 

[Ian]

Mark, that is a very nice installation!  Thanks for the pics.  

[dlacey]

On 11/25/2022 at 1:21 AM, Ian said:

 It seems as if there are 3 strategies:

 

I think theres a simpler option, which the OEMs often use:

Run the heavy gauge cable direct from the trunk-mounted battery to the starter motor. Why? Because its only the starter which draws 100+amps, what the heavy cable is for, and having as little junctions/points-of-resistance ensures the starter gets maximum volts. Then, from the starter you can run lighter gauge  wires to power ECUs, Fans, HiFi, rest-of-car. In this configuration theres no need for any bulkhead fitting/distribution block/BusBar. I have this configuration on my '02 turbo, as i wanted to avoid 'irreversible' changes to wiring loom and engine bay, and i needed the trunk mounted battery to create space for an AC compressor.

Edited November 26, 2022 by dlacey

[Son of Marty]

Dlacey, is right on, the fewer junctions before the starter is the way to go and the way almost all stock cars (not Nascar) are wired.

[Greg Mierz]

Simple is the best. I put my battery in the trunk over 40 years ago. 00 welding cable directly to starter. 00 ground cable to the shoulder bolt receptacle big 8mm one. Multiple grounds from motor to chassis. It has been perfect for this long time. Run a large gauge wire from starter terminal to alternator, and from there to chassis. The large red wire going into buss bar is from 85 amp 318i unit.  Originally had bus bar next to radiator worker fine, gave me a 12V source under hood and ground to chassis. A few years ago I redid the buss bar and fuses to these photos. Car is early 69 6 fuse with relayed headlights and fuses since the early 80's , electric fan.

[Lorin]

Since there is all this talk about relocating batteries to the trunk in a car not designed for it I feel it would be wise to make sure everyone is aware of the need to vent a lead acid battery. 
 

A conventional lead acid battery expels hydrogen and that isn’t a problem in an engine bay that is exposed to the atmosphere. 
 

When you put that same battery in an enclosed space like a trunk the gasses don’t have an exit path and can build up. 
 

A trunk full of hydrogen can ruin your whole day. 
 

Lead acid batteries that are designed for enclosed installation always have a provision for a vent line that needs to be routed outside the car and that design needs to be incorporated into the 2002. 
 

Some other battery types that are permanently sealed do not emit gasses and are safe to use in enclosed environments like a 2002 trunk. 

 

 

Tldr: It’s ok to drive Like your ass is on fire but try to avoid Actually driving with your ass on fire. 

[cda951]

On 11/25/2022 at 4:19 AM, dlacey said:

I think theres a simpler option, which the OEMs often use:

Run the heavy gauge cable direct from the trunk-mounted battery to the starter motor. Why? Because its only the starter which draws 100+amps, what the heavy cable is for, and having as little junctions/points-of-resistance ensures the starter gets maximum volts. Then, from the starter you can run lighter gauge  wires to power ECUs, Fans, HiFi, rest-of-car. In this configuration theres no need for any bulkhead fitting/distribution block/BusBar. I have this configuration on my '02 turbo, as i wanted to avoid 'irreversible' changes to wiring loom and engine bay, and i needed the trunk mounted battery to create space for an AC compressor.

 I agree with this logic, though I like having the single large B+ junction in the engine bay like all modern BMWs have. It is much better to jump start the engine from there instead of removing the back seat or taking all the crap out of your trunk to access the battery and try to get jumper cables on. Plus, I work on cars for a living, so I like the ability to connect a power supply while doing diagnosis---this is much more likely on a modern car than on an old 02, but if you have EFI and need to do a software/firmware update, etc, a good power supply is key.

 

I do have an '81 Alfa Romeo GTV6 which I love, but it goes too far with the amount of exposed junction points in the engine bay, they were all corroded when I got the car from the junkyard  . If there is only one main junction, it is very easy to keep track of and keep it clean and tight, can weather-seal it if need be.

[John76]

18 minutes ago, Lorin said:

It’s ok to drive Like your ass is on fire but try to avoid Actually driving with your ass on fire.

Especially if you are driving the Hindenburg (with a non-vented battery in the trunk) ☹️

[jimk]

1 hour ago, Lorin said:

A conventional lead acid battery expels hydrogen and that isn’t a problem in an engine bay that is exposed to the atmosphere. 

What about the catalyst in the maintenance free lead acid batteries the combines the O2 and H2 to form water instead of venting it.  No water is added these days.

[jimk]

59 minutes ago, cda951 said:

It is much better to jump start the engine from there instead of removing the back seat or taking all the crap out of your trunk to access the battery and try to get jumper cables on.

Use an E30 cable and jump from up front.  Also can disconnect from the lug on the firewall when needed.

[jimk]

51 minutes ago, John76 said:

Especially if you are driving the Hindenburg (with a non-vented battery in the trunk) ☹️

What concentration of H2 is necessary to become an explosive atmosphere?

[Son of Marty]

48 minutes ago, jimk said:

What concentration of H2 is necessary to become an explosive atmosphere?

4 to74% is the explosive range of hydrogen, there's not near enough H2 in a standard size 02 battery to reach 4% of the volume of the trunk.