wiring dual underhood batteries

I plan on wiring two D3400's in my engine compartment. One battery on the left fender (driver's side) and one on the right (passenger side). I have put alot of thought in this and I do not want a secondary battery in my trunk. My vehicle came equipped with one battery on the driver's side. I am powering 3500wrms. I have a 180 DC Power alternator. I am using all 1/0 power wire and ground wire. Which of these two diagrams that I drew up would be better, ie. hold voltage and supply amperage to my trunk mounted amplifiers? Thank you!

BTW as per forum rules : 2008 Hyundai Elantra

I should also add that I will be listening to the stereo with the car on only. No need for relays and solenoids.

TECHNICALLY it doesn't matter. Notice and heed that word. NO connection is perfect. I would personally use the second one, simply because it is one less connection (and really... it's three less connections - terminal one to wire, terminal two to wire, and and two terminals to a vibration prone alternator output. Too many additional places for error and resistance.) between battery #2 (the starting battery, as you have it labeled) and the amplifiers. Also, forget the "jumper" on the negative terminals altogether, whichever digram you choose. Proper, solid connections to the chassis will do all that you need them to do. Also, you are completely correct, there *IS* no need for relays or solenoids, but really, there never is! :)

I like simplicity and minimal connections! Thanks for your comment about this.

I agree. #2. Your problem will be the voltage drop from the front to the back. And the heavy inter-battery +12V connections make the alternator connection irrelevant (normally the alternator is to one battery only). Of course, being permanently connected means a (rated) 5 year life and no protection if you leave the amp on.

oldspark wrote: I agree. #2. Your problem will be the voltage drop from the front to the back. And the heavy inter-battery +12V connections make the alternator connection irrelevant (normally the alternator is to one battery only). Of course, being permanently connected means a (rated) 5 year life and no protection if you leave the amp on.

I do not know what you mean by a rated 5 year life? The will not be a voltage drop front to back but possibly an amperage drop. I put a decent amount of thought into this. A rear mounted battery is exposed to a different temperature than an engine compartment mounted battery which affects charging rates and discharge rates. And there is also a charging amperage drop between the front and rear battery due to wire resistance. So I figured that I would give up a couple amperes due to wire resistance to have battery longevity. Plus I just have a better piece of mind adding a second battery under the hood versus in the trunk. Is my thinking correct?

edouble101 wrote: There will not be a voltage drop front to back but possibly an amperage drop..... And there is also a charging amperage drop between the front and rear battery due to wire resistance...

No - the current is the same! There is a voltage drop, not a current drop. Remember - current is the SAME through a series circuit. The voltage drop along the cable is V = IR where I is the current through the cable's resistance R. The rated or design life of that battery is 10 years. Hence - in simple terms - two such batteries in parallel halves the design life to 5 years. That's using its rated life as a reliability figure. But wiring two batteries in parallel doubles the probability of failure where that prob of failure is for self induced failure during normal use. That's simple reliability theory - if an interconnected system doesn't increase reliability, it reduces it. Since a failed parallel battery will fail the other battery, it's a negative effect (a good battery won't hold up a bad battery). There are other factors - like for normal cranking, the parallel arrangement will extend life (because the current drain and discharge depth is lower)... But suffice to say, if one battery goes down, so does the other. EG - Two new "10 year" D3400s in parallel. One fails after 3 months (due to manufacturing fault - say a collapsed cell or 2) which fails the other. Hence after 3 months (or 3 months and 1 week) you have 2 dead batteries. Warranty claim? Maybe for the first, but good luck for the 2nd.... And for some brands, good luck for the first!   

oldspark wrote: edouble101 wrote: There will not be a voltage drop front to back but possibly an amperage drop..... And there is also a charging amperage drop between the front and rear battery due to wire resistance... No - the current is the same! There is a voltage drop, not a current drop. Remember - current is the SAME through a series circuit. The voltage drop along the cable is V = IR where I is the current through the cable's resistance R. The rated or design life of that battery is 10 years. Hence - in simple terms - two such batteries in parallel halves the design life to 5 years. That's using its rated life as a reliability figure. But wiring two batteries in parallel doubles the probability of failure where that prob of failure is for self induced failure during normal use. That's simple reliability theory - if an interconnected system doesn't increase reliability, it reduces it. Since a failed parallel battery will fail the other battery, it's a negative effect (a good battery won't hold up a bad battery). There are other factors - like for normal cranking, the parallel arrangement will extend life (because the current drain and discharge depth is lower)... But suffice to say, if one battery goes down, so does the other. EG - Two new "10 year" D3400s in parallel. One fails after 3 months (due to manufacturing fault - say a collapsed cell or 2) which fails the other. Hence after 3 months (or 3 months and 1 week) you have 2 dead batteries. Warranty claim? Maybe for the first, but good luck for the 2nd.... And for some brands, good luck for the first!

Gotcha. When looking over wire gauge charts I normally see amperage lost over a specific distance. I rarely read about voltage lost. Tricky.

edouble101 wrote: When looking over wire gauge charts I normally see amperage lost over a specific distance. I rarely read about voltage lost.

I suspect you misinterpret.... Amperage is never lost thru a wire or component. But the longer the wire, the higher the resistance, hence less current travels through it. (Ohms Law: V = I x R. The voltage V across something is equal to the current through it times its total resistance.)    The thinner a water pipe, the less water travels through it, but the water is not "lost" along the way. (A smaller pipe diameter is increase Resistance; Current is the water flow (quantity or rate); Voltage is the pressure drop.

edouble101 wrote: Tricky.

DEFINITELY!! Not difficult - merely tricky & confusing (and may be difficult to grasp). Thereafter it's not difficult - though I will probably NEVER say it is NEVER tricky - I still get tricked/confused when I know better.... PS - WRT paralleling 2 batteries, you have minimised any inequality except for their interconnection (see pic below) by having batteries of the same type & batch and age and history and temperature.... I compliment you on agreeing that trunk & engine bay difference mean temperature differences hence they re NOT the same battery (wrt behavior). Mind you, there are many that say it is fine to parallel batteries - eg - see jmelton86's "Not only is it done all of the time, with mismatched batteries alot of the time" in the12volt's 2 batteries (though no mention is made of their reliability or thermal runaway for AGMs etc...). I'll stick to my simple reliability and interaction statement above. I'll let others disprove it and counter (IMHO) the expert claims and findings. As top the final detail - the interconnection to ensure equal loading and charging (sorry - I don't know the author or web source): Or from the above link: (The 2nd "Parallel" version is better - it ensures equal current through BOTH batteries.)