wiring dual underhood batteries

Thank you oldspark for discussing this with me. I am definitely using the two batteries in parallel I do not have a 24v system!!

LOL! I assumed - or hoped! - that that was obvious. (A big bad oops if it wasn't.) I was too lazy to edit the pic. (Actually I just used what was already there (on 12volt).) My preference is always to connect parallel batteries ONLY when charging, else when required in parallel - eg, for winching or cranking. But that's because I have a charge light and use that for all the smarts (ie, "smart isolator") - just add a relay. And because I always want a cranking battery no matter how much I've flattened the other battery...

Do you see any reason why I should have two runs of 1/0 wire to the trunk to minimize any voltage drop? Right now my plan is only one run of 1/0 to the trunk.

FYI I will be using an Alumapro cap5 v2 mounted very near the amplifiers as well.

Yes - because that will HALVE the voltage drop (along the +12V cable). But it depends what your requirements are.... But you may understand why the second battery is usually mounted near the amplifier. And why many will double up on the ground connections because they are shorter (battery to gnd; amp to gnd; chassis is gnd in between). 3500 W RMS - let's say 350A at max output. A 180A alternator at max output. So 350-180 = 170A from batteries; say 90A each @ 0.004 Ohms internal resistance (assumed; fully charged) hence 0.36 voltage drop at terminals hence a system voltage of say 12.8-.36 = 12.45 Volts. 12.45V with 350A thru 1/0 = ~0.1 Ohms per 1000' = 35V per 1000' = 0.35V drop over 10'. So 12.45-.35 = 12.1V at amp with 3.5kW output & 180A alternator... And so the calcs continue.... But for 1/0G (if 0.1 Oms resistance per 1000'), that's ~350A x 0.1/1000 = 0.035V drop per foot.

So...one run of 1/0 will have 0.35 volt drop over 10' which is minimal. And my Alumapro cap5 will help keep voltage up during periods of high amplifier draw. I should be good then, right? Thank you very much!

A 1V drop (at the amp) with a 5F cap will take about 14 milliseconds. (So in 1 second, power at 70Hz will have dropped about 20%.) A small battery instead - say 1.2AH or 7AH - might perform much better than that cap.

Ok, so now I am back to mounting a third much smaller battery in the trunk near the amplifiers. I am ok with this. Since I am using XS Power batteries the smallest they make is the D680 which is 20ah. Would I then connect all batteries at the negative terminal including a chassis ground for each battery? Use one run of 1/0 or two runs of 1/0? Thanks again for your input!

Why not instead move your 2nd battery to the trunk? It has no benefit in the engine bay.

oldspark wrote: Why not instead move your 2nd battery to the trunk? It has no benefit in the engine bay.

You are right, I could do that. I have two reasons for mounting the second battery in the engine compartment. 1. Weight!!!! My subwoofer weights 42lbs, the enclosure 60-80lbs and amplifiers 20-30 lbs. That is nearly 150lbs combined. The D3400 weights 46lbs. So total weight is nearly 200lbs. By mounting the secondary battery under the hood I could help reduce the additional weight being added in the trunk. 2. Both batteries should be charged and discharging equally. Although I have not done any testing that proves this true or not and to what extent. It appears that there are advantages and disadvantages of both mounting options. Which one is ultimately best?