wiring 2 battery's opinions on grounding

[Stryker]

Ok guys so i've got the extra battery for my trunk and have read countless threads about wiring. i want to know how i should do this.
mainly has to do with ground wire.
should i run a wire from the front batt to the back batt and then ground to chassis? or should i just ground the rear battery at a good ground in the trunk? i have a unibody car so there is very minimal welds like a subframe car. i can do either way just wanted to hear what you all think would be best and why. i have alot of 0 Ga wire in my possession so thats not a worry. i also have all the necessary other things. just lemme know what you think and why.
thx

[money-b]

oh...i've always beeen curious about this....how does it work...i mean does the battery charge at all? and is it in parallel or series?

[Stryker]

parallel, ya it charges and you usually need a bigger alt. series would equal 24 volts and poof there goes your ride in a blaze of glory i.e. fire

[shawn k]

Just do a solid ground in the trunk my friend. Save your cable. It's true that steel is not nearly as good of a conductor as copper, but there is a LOT of steel to allow good current flow. In addition, I would suggest doing the "big three" as well (if you havn't already that is).

[Stryker]

Just do a solid ground in the trunk my friend. Save your cable. It's true that steel is not nearly as good of a conductor as copper, but there is a LOT of steel to allow good current flow. In addition, I would suggest doing the "big three" as well (if you havn't already that is).

big 3...it's been done for a while now. thx tho. I'm leaning towards the back ground just cause the wire's are diff color and and look dorky under the hood.

[gbody805]

As long as the body gives a ground path. You should be fine.
Just make sure you ground to a solid body panel. Something that is not just spot welded or glued in but an actual solid part of the body.

[ttocs]

no isolator?

[Stryker]

no isolator, not needed as far as I know. I want power from both of the battery's simultaneously.

[gbody805]

I thought if the batteries were the same, you wouldn't have a problem.
Mix-matched would eat at each other till they died.
Not sure how long it would take but it would happen sooner or later.

[ttocs]

that is what I understand as well. If you want to run dual batts you should buy them both at the same time or the charge/discharge characteristics would cause one to kill the other.

[Stryker]

same type sorta. one optima yellow and a Deka 9a31 for the back both are AGM the guy at the warehouse where i bought the Deka said thats bunk so i took his word plus what i've read. gonna replace the yellow top with a like Deka battery next month. plus both have warranty's. not too concerned with the batt. end of things but the ground. do i? or don't i? run a rope from front. or just ground at the back?
wanna know about that. both work but apparently running a wir from the front is the best way for least resistance. $$ for extra wire tho.

[The Golden One]

the only time you can get away without an isolator is if both battery's are exactly the same and they have to both be the same age if not then you will have charging problems. also your right about a unibody car sometimes not having good conductivity trough the frame i relocated the battery in my mustang to the trunk and i sometimes have to get boosted off. i know when i get subframe's it will help it out im going out side to look in my bmw to see if they ground the battery to the frame or some other way as it has a factory trunk mounted battery.

[gbody805]

Would you be able to test the ohms between the front firewall and the spot chosen for the ground in the rear?

Would this give you a true reading for this situation?

Just thinking out loud.

[gridracer]

If you use an isolator you will always have power to start your car if you run down the front battery playing the stereo with the car not running. With car running with an isolator the stereo will still use both batteries I thought.

[The Golden One]

that would'nt be a bad idea to test the resistance of the body as gbody stated. i would assume you could test the resistance against a 1/0 wire that would equal the same distance to see which would work better just dont know if a meter is precise enough. oh the bmw is grounded on the frame in the trunk but they did weld a plate with a stud in it then the wire is bolted down with a nut.

[Stryker]

ahhh fudge.... ok I'm on the edge here with the differing battery's theory.
every thread i search on google some joe has a different experience.
I'm gonna return the Deka 9a31 which was 250.00CAD and get 2 of the deka 9a34's which are 350.00 for both so both battery's are the same etc. etc. one in front, one in back no isolator needed cause if i run 2 battery's down low enough that they won't crank the engine i've been hangin in a parking lot too long. , ran in paralel. my new alt. can certainly handle the load. combined the 2 batts provide more power which i don think is all that much of a worry

[gridracer]

Ask Zak at AAMP about batteries and isolators for a good answer he is the guy to talk to.

[ttocs]

have you ever had a dead remote that needed 2 or 4 batts and only had 1? I have and put it in there. It would work for a while but nomally the next couple of weeks that one good one was just as dead as the bad one, and sometimes even started to leak. Now remember that this is just the disharge cycle that is different with those two, with your situation you will be concerned with how they charge dofferently as well.

If you have one batt at 13v(old one) and the new one comes in at 13.5, what is going to happen? The two will balance out to a voltage somewhere inbetween the two but what happens to that is going on internally? Is the good battery acting as a charger for the bad one or is the bad one just making a load to drag the good one down? IT is both and they both lead to a set of bad batteries.

Now you are not only putting to batteries that do not match manf times, but that are two completely differenty batts manf by two different companies, hell they are even designed and built differently.... I would have to recomend an isolator or buying a 2nd battery at the same time to replace them both.

Now for grounds, well you are right you will get a better transfer if you run the wire but that will of course cost more. But if you are gonna do it, do it right and do it once and be done with it.

[shawn k]

that would'nt be a bad idea to test the resistance of the body as gbody stated. i would assume you could test the resistance against a 1/0 wire that would equal the same distance to see which would work better just dont know if a meter is precise enough. oh the bmw is grounded on the frame in the trunk but they did weld a plate with a stud in it then the wire is bolted down with a nut.

Actually this reading would not provide enough information. When testing either the 1/0 or the unibody for resistance each should read damn near 0 ohms. This is all fine and good, but the concern comes in when current flow is the goal. Since resistance is measured by the meter applying a "tiny" amount of current through the conductor, this doesn't tell us just how much current flow capability is available. That being said, the vehicles chassis can provide excellent current flow (hundreds of amps) simply because of the shear volume of metal.

[oldschoolfan]

First, if you are concerned about the resistance from a point of metal at the rear of the car to the battery ground, check that point with a meter. It won't be 100% accurate because you will need to make leads that will reach from that point in the rear to your battery negative terminal. I would make the leads of equal length so the meter would end up in the middle of the car. I don't really think it would matter if one lead were longer than the other, I just like symmetry. Anywoo, you are going to see that resistance to ground wont be an issue. Unless you pick a completely isolated bracket but the chances of that are really pretty slim. Pick a thick part that appears to be structural that you have access to both sides of, even if the other side has to be outside of the car. It is best to have a through fastener with a nut on the other end as opposed to a self tapping fastener. This way you can really secure that wire without worry of it vibrating loose. If the other end of the fastener ends up outside don't worry about it, just seal it well to protect it from the weather. Use stainless fasteners as well. They don't conduct as well as copper or brass but will hold up well in the automotive environment. To finish this point, you want the shortest ground possible. You don't need to spend all that money for 20 feet of ground wire when your car is made of metal. I promise that the volume of steel in the body of the car will more than make up for the low resistance of that length of copper wire. In my race car I had a single rear mounted battery with a 1 foot ground cable to the body. It never failed to turn over that 500 horsepower V8.

The batteries. An isolator is nice but not necessary. The isolator will, if wired correctly give you the ability to kill one battery while listening to your stereo with the car not running without killing your starting battery. But then once you do start the car your charging system is going to be working overtime to try and recharge that discharged battery. I just read up on this so it is fresh in my head. There is a disadvantage to using an isolator in that they use diodes to limit the flow of electricity in one direction only. This is at the cost of about .7 volts of charge current lost to each battery from the alternator. This is all the time so your alternator is going to be working harder just because of the lost efficiency through the isolator. Just a fact of life for that bit of control.

Now, the batteries will not drain each other. What will happen is if one battery is not as healthy as the other, as in age or sulfated plates, the weaker battery will drain power from the other until they balance out. The one with the higher voltage will go down as it charges the other and the one with the lower voltage will come up as it is charged by the one with the higher voltage until they meet in the middle. Again, if one of these batteries is not as healthy as the other it will lose it's charge faster than the other therefore so will the stronger battery lose it's charge to maintain that balance of equal voltage. The point is you want two healthy batteries.

Similar types of batteries would be ideal. Size equality is not as important as type. As in standard lead acid, gel cell, deep cycle or agm. All three are different but the same. What is different is that all have an ideal voltage for charging and this is different for all of them. They also discharge differently. For this reason it would be best to have the same style so they can at least charge at the same rate and discharge at the same rate. This is not necessary but will help them to last as long as possible and perform their best.

Read up on them it is very interesting stuff.

[shawn k]

have you ever had a dead remote that needed 2 or 4 batts and only had 1? I have and put it in there. It would work for a while but nomally the next couple of weeks that one good one was just as dead as the bad one, and sometimes even started to leak. Now remember that this is just the disharge cycle that is different with those two, with your situation you will be concerned with how they charge dofferently as well.

If you have one batt at 13v(old one) and the new one comes in at 13.5, what is going to happen? The two will balance out to a voltage somewhere inbetween the two but what happens to that is going on internally? Is the good battery acting as a charger for the bad one or is the bad one just making a load to drag the good one down? IT is both and they both lead to a set of bad batteries.

Now you are not only putting to batteries that do not match manf times, but that are two completely differenty batts manf by two different companies, hell they are even designed and built differently.... I would have to recomend an isolator or buying a 2nd battery at the same time to replace them both..

YES

Now for grounds, well you are right you will get a better transfer if you run the wire but that will of course cost more. But if you are gonna do it, do it right and do it once and be done with it.

NO

[The Golden One]

your right about the meter it would most likely read 0 but one thing about unibody construction is some cars are joined buy epoxy and spot welding and then there's paint in between that also causes resistance so all these things cause enough resistance to where the current wont be enough to maintain a proper charge.

[shawn k]

your right about the meter it would most likely read 0 but one thing about unibody construction is some cars are joined buy epoxy and spot welding and then there's paint in between that also causes resistance so all these things cause enough resistance to where the current wont be enough to maintain a proper charge.

Yes indeed, but that's also why I (and others) have been saying it's important to choose a "good" spot for a ground. You're right, but it's already been brought up.

[ttocs]

I am not sure how anyone on here can argue that a new, fresh run of high current wire will no do better then the frame especially when you consider unibody contruction.

[oldschoolfan]

your right about the meter it would most likely read 0 but one thing about unibody construction is some cars are joined buy epoxy and spot welding and then there's paint in between that also causes resistance so all these things cause enough resistance to where the current wont be enough to maintain a proper charge.

I agree but I don't. You are right they use these but the epoxy is mostly to seal two pieces of metal together not to hold them together structurally. Yes they do use spot welds also. What they also do which is most advantageous is try to use as few dies and pieces of metal as possible. Therefore we get the advantage of very large pieces of metal that run almost the entire length of the vehicle. The engineers do have electrical continuity it mind when they build these things because every peripheral device has to be grounded. No, most of these peripheral items don't use near the current we want to, but the continuity is there and designed in.