Adding new battery assistance

[nemesis_567]

Hello, I am trying to do a project.

I want to add a new battery to my car so I run some additional utilities when needed.

So my alternator produces up to 90 amps. Everything is 12V.

I have purchased a battery isolator capable of handling 120 amps(Victron Energy CYR010120011)

Now my concern is cabling because one of the batteries will be in the boot. So the thing is what is the maximum current that this second battery will be able to draw from the alternator so I can decide what is the ideal cable cross section. I could just get the AWG0  but that would be a pain to carry all the way back.

I hope this is the right place to ask this, if not let me know and I will do what is necessary.

Thank you for any assistance and sorry for any stupid questions(I am a learning newbie)

[bdunham7]

That depends a lot on the characteristics of your additional battery and exactly how things are connected up and then how you use them.  It is possible, although not likely, that the current could exceed 90 amps if the auxiliary battery were discharged since it could draw current from the other battery as well as the alternator.  So what sort of auxiliary battery are you going to use?

[nemesis_567]

Lead acid between 70-90Ah, but it's not fixed in stone. I was thinking something like a Tudor tc700.

As for the rest, I have an amp and default stuff connected to the main battery(amp can pull up to 40-45A). I'd like to connect this one to the secondary battery in the future.

[bdunham7]

So using a starting battery gives you the worst-case scenario because they have the lowest internal impedances.  Just as a rough guess, I would say it would be possible but unusual to have currents exceeding 60A for a short time (a few minutes at most).  You don't need to worry about voltage drop, just burning something up.

In my experience, an AWG4 (~5mm diameter) wire with high-temp insulation and a 60A to 100A fuse would probably work out just fine.  To be ultra-robust, an AWG2 (~7.5mm diameter) wire with a 100A fuse should be more than good enough.  You might want to have a fuse at both ends.

[nemesis_567]

What battery would you recommend to be extra safe if I wanted to connect the amplifier(up to 45A) to the auxiliary battery and use AWG 4? Also cable runs around 4.5 meters from the battery in the front to the boot location.

[bdunham7]

A deep discharge battery (typically a marine application) would be better suited for this sort of thing, but I have no idea what types you have available. 

[nemesis_567]

These are far more expensive. Is there a place where I can consult the maximum amps a given battery will pull? I am not finding that information.

[bdunham7]

These are far more expensive. Is there a place where I can consult the maximum amps a given battery will pull? I am not finding that information.

I've never seen it specified, just discussed and occasionally tested.  If your system (amplifier) can draw 45 amps, then perhaps the more robust cable is appropriate anyway.  However, the 4 gauge wire is likely fine a long as you don't use cable with cheap PVC insulation that melts at a low temperature.

[nemesis_567]

Ok this is my thought process. Let me know if is stupid or wrong.

So currently at max volume I have two speakers at 17.5V RMS and two at 15.5RMS average resulting in less than 20A(since speakers are 4Ohm, measuring the most demanding frequency V wise) output plus whatever the amp itself consumes. So the 45A is quite a reserved estimation.

Since alternator is 90A(assuming best case), and the vehicle has some consumption by itself, I am making these assumptions at 12v:

Visor: 2A
Lights(4*55w): 18.4+-
Radio(since sending signal to amp): 4A maybe
Blinkers inner lights: 4A maybe
Autopi: 1A
Dashcam: 1A

Total 30+- (AND I STILL HAVE TO CONSIDER: Engine Fan + AC)

I am conservately estimating I only have around 5A reserve power to further use without having to upgrade alternator(having a 65A max draw for a safe margin), so I was thinking maybe I can add a fridge as well(2-3 more amps) and that'd be as much as I can have without compromising the vehicle.

Since to aux battery I'd connect the amp and maybe 3 more amps(fridge), I wouldn't have that equipment driving too many amps from the alternator.

All that said...

If I add a fuse I can be certain that the worst that will happen is a fuse replacement right?
The 4 AWG can handle 135   for chassis wiring(I think not my case) and 60 for power transmission(I assume my case since it's 5m distance).