Battery in the trunk
 

So. I want to add a second battery, and put it in the trunk, for for my amps. I was wondering if I run both positive and negative leads to the back battery... or do I just run the positive, and just ground the back battery to the body.
Tis' probably a noob question... but i need to know. :p
Thanks.

First and foremost, THANK YOU for not asking about a capacitor. They have their place, even in a car audio power system, but stopping lights from dimming is NOT it.

Now to the question at hand.

What are you running for amps? I'm assuming you want to add a second battery to help prevent the lights from dimming as the bass hits. Before you add another battery try using a good battery and the wiring I've described below. I recommend Odyssey and there are a few re-brands that can be found for decent prices. If you're still having problems with the lights dimming it might be time for a second battery but that would be a rare occurrence and typically only happens when you're no longer a noob. ie, you're much farther into the hobby and using some SERIOUS power.

I run positive and negative to the amps, from the battery. AND positive and negative from the alternator to the battery. Done. No ridiculous big 3 bull shit that adds complexity, adds connections and uses a fender or whatever as a part of the ground system.

The explanation, before you get your flame thrower.

Odyssey's arranged glass mat technology is also used by Optima, but in a different arrangement that diminishes it's abilities. Odyssey's have the highest short circuit current and lowest internal resistance of any battery I know of. Certainly any in a reasonable price range.

Batteries and alternators act as a single unit when the car is running, in much the same way a compressor and reservoir tank work together. If you remove the battery from the system the car will still run, but there will be voltage spikes and drops of over 30% outside the normal 14.4v. Before you flame me, go try it. I've tested it personally.

Next, the body is held together with spot welds and adhesive. Grounding anything, using as much current as an automotive amp, to the trunk will cause voltage drops. Yes you can take a meter and find exactly 0 ohms between the trunk floor and the negative battery terminal. You can also find 0 ohms on 30 feet of phone cable. I will never use either to ground something using that much current. And before anyone decides to mention it, superposition theory is based on the system having zero voltage drops from one position to the other.

Actually there will be voltage drops , I work as an avionics tech , electrons is what I do all day errday !
A continuity test only proves point a and b work and that there is no short if you go to ground ... We ground 12v dc on airframes . Cars aren't a big deal .. So the fact that you are gonna run a negative from the battery is really dumb, no offense, unless you got monster Earthquake amps pushing well over 200Amps, I pushed my car to 150 amps with stock battery and that's when my lights starting dimming


I-Tapatalk

yeah when I ran two batteries I grounded the battery in the trunk to multiple grounding locations within <3ft. Also had all 3 amplifiers grounded separately. Possibly overkill but my amplifiers totaled about 230 amps, and killed my first alternator before I added the battery.

That was back in my first car when I LOVED boom.

There will be voltage drops, so you agree. Then you call me dumb. Then you compare the riveted together body/frame of an air plane to the spot welded and glued body of a car. Maybe you should get some sleep, read both of our posts again and come back when you can form a coherent thought.

It's cool that your an avionics technician though. Where do you work? While we're sharing, I make my living as an electronics technician now but was a marine engineering technician for a bit over a decade before this.



Hmm, I wasn't trying to say it wouldn't work, though I can see how that was what I communiticated. Of course grounding to the body will work, just not as effectively.

Lol glued together? Haha your grounding to the chassis not a bumper..
Welding is just as effective as riveting when it comes to current flow


I-Tapatalk

Not as effectively as what? Not all alternators have a bolt for the battery to ground to and when they do the battery still has multiple grounding locations in your engine bay. Not to mention the fact that the shorter your ground cable is the better, meaning having a cable running from the back to the front is not as good as simply grounding in the back.

Also when grounding in the trunk im not just drilling a hole though the floor pan and grounding it there, im running it through existing bolts, I like using seatbelt bolts at the bottom of the seats, some of my installers like using other points, but its not just to sheetmetal.

Another point, if you are running the battery positive to the alternator and the alternator doesnt have 2 separate positive outputs it generally a good idea to use a battery isolator to keep the two batteries separate.

I have used a kit like this before. It worked great.

[ame="http://www.amazon.com/Battery-Isolation-Voltage-Sensitive-Relay/dp/B00400D6WU"]Dual Battery Isolation Kit with Voltage Sensitive Relay (VSR) : Amazon.com : Automotive[/ame]

Grounding to the body of a spot welded unibody car is less effective then using a properly sized wire.

All alternators are grounded through their body. Simply removing one mounting nut or bolt, slipping a ring terminal under it and putting it back together gives you an alternator ground.

I'm really not sure why multiple grounding points in the engine bay matters. Having a ground path that's as short as possible and has as few connections as possible does matter though.



Clearly we agree on this point.

And just as clearly we disagree on this point. The body of the car can act as a path to ground but it is not the power supplies ground. Also this method increases the number of connections.

We also agree on this. If you do choice to ground in the trunk a seat belt bolt or strut bolt is often the best option. Though some have argued that the seat belt bolt could cause a safety issue.

I agree on the battery isolator, but I'm confused as to how we circled back to two batteries. When I wrote about running a wire from the battery positive to the alternator B+ I was talking about doing this with the main battery of the car, not adding a second battery.

The entire thread is about adding a second battery. When did we get off of that???

In my first post I tried to direct the op to using a better battery and better wiring as that will most often be plenty. A second battery ( or more ) can be useful, but you shouldn't need them until the setup is getting extreme.

Okay wow. :p ... I will be using about 70 amps, I just wanted to make sure not to put too much stress on my stock, front battery. I will be running 0 gauge... but, you both do agree, I should run both positive and negative from the front battery to the back one? Ooorrr will 70 amps be fine using just stock battery? I just don't want too much stress on my alternator..

70 amps you'll be fine everything stock.

x2