Circuit Help

[nemesis_567]

Hello, I have the following setup:


It's a 12V circuit.
So on the domestic battery side I have a load of worst possible scenario 850w.
The alternator produces 120A at max.
The domestic battery is a deep cycle 180Ah.
The starter is 90Ah regular battery. Both are lead acid.
This is the cytrix in the diagram:
https://www.victronenergy.com/upload/documents/Datasheet-Cyrix-ct-120A-230A-EN.pdf

They are generally separated across 2m.

Mostly I just want to know what kind of AWG cables I need to handle all this properly. And is there anything else I should consider?

[Manul]

Around AWG0 - AWG1 perhaps. From a safety standpoint wires must be thick (and short) enough to ensure that during short circuit fuse blows and blows before the wires overheat. Too thin wires makes a fuse essentially non functional, because most of the power dissipation will happen in the wires. Also it depends how much voltage drop you tolerate and allow in your circuit.

Don't know what else to say, it is not clear to me what that circuit is supposed to be.

[strawberry]

AWG(american wire gauge) is weird system.
cross-section depend on wire length, awg1 good for 1m and 120A but less good for 20m long wire
both battery could be better be permanently connected in parallel

[nemesis_567]

The circuit is for a car battery with a backup battery.
The backup battery drives most non-essential systems like amp and other utilities.
The main battery starts the vehicle and drives important things like lights, dashboard etc...
I don't have space to keep both batteries together, so one has to go to the trunk.

The splitting instead of connecting in parallel is there so that the secondary battery will only ever charge if the main battery is good, since that's the priority. Once it goes above a certain voltage, it starts charging the secondary one.
If in the event of a main battery failure I need to start the car, I can use the secondary battery, by closing the circuit described in the diagram, which connects the secondary battery to the main circuit allowing me to start the car.

Hope that's clear.
AWG0/1 is very hard to use in the vehicle, since it is massive!

The total distance is around 2-3 meters front to back. Negative goes directly to chassis.

[Manul]

There are plenty of cars which have battery in the trunk by design. So maybe just go to an autoshop or a car service shop and ask what gauge wire is typically used. They might even have it and sell it to you. If you want to be able to start a car from your trunk battery, the wire needs to be quite thick, starter current is very high.

[rstofer]

Similar commercial products are available at places like WestMarine.com

https://www.westmarine.com/battery-combiners

Marine cable is a lot more flexible than building wire

https://www.westmarine.com/buy/ancor--battery-cable-by-the-foot--P0233718?recordNum=1

My Google fu isn't turning up any crimp-on lugs but they are available.

Here is the ampacity/size data but do the calculation based on resistance to find the voltage drop.  Particularly on longer runs.  I'd have to think about using the 'Engine Room' values versus the non-engine-room.  It probably makes sense when considering the under hood temperatures.

https://www.bluesea.com/support/reference/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart

[rstofer]

Amazon carries a lot of battery jumper cables that are up to 25' long.  The problem is, you don't know if you're getting the highly flexible wire or just something thrown together with building wire.  You can pretty much pick the gauge you want.

Ignore the ampere rating on jumper cables, that rating is for loads lasting mere seconds.  Use the table I linked above for ampacity and resistance.

[BrokenYugo]

Amazon carries a lot of battery jumper cables that are up to 25' long.  The problem is, you don't know if you're getting the highly flexible wire or just something thrown together with building wire.  You can pretty much pick the gauge you want.

Most jumper cables these days are copper coated aluminum.

I suggest a local place selling welding cable by the foot. Or go the old low buck race car route and get the cable out of a junkyard BMW or other rear battery car.

In the US most auto parts stores have a rack of terminated heavy gauge cables at various lengths for custom battery/alternator installations.

[nemesis_567]

Thank you all for the suggestions.

My concern is not the starting as that's a very very rare quick thing.

My concerns are:

I have no clue how many amps the secondary battery can draw from the main battery to charge.
The amplifier is 1000w, with 4 70w speakers connected, so I'm just being cautious and considering 40 amps for it at 12V margin(probably half that).
There is a 50w compressor cooler.
There are a few other utilities that take maybe 2-3 more amps at 12V.

I'm worried first and foremost for how much current the battery itself can pull. And then the rest because it's consistent.

[anda3243]

Is it not possible to calculate this? An empty but healthy battery will have maybe 8 or 10 V. An alternator will usually produce 14 V to charge the battery.
So there is a 4 to 6 V potential.

The typical battery internal resistance is 0.02 ohm. (I looked this up on google)

Using the formula U=R/I and solving for current I=U/R.
And using the above values I get
Imin = 4/0.02 = 200 A
Imax = 6/0.02 = 300 A

I am assuming that the resistance in the cables is 0. This is not true, the resistance in the wires to this battery will drop the current some. 
But this would be the instantaneous current, as the battery charges the voltage difference decreases, and thus the charging current decreases.
Also the alternator might not be able to support this high current, so the voltage over the primary battery and alternator will fall when connected to the empty secondary battery.
It was mentioned that the alternator can give 120A. I assume that this is at 4000 rpm or something like that engine speed, not at idle.
So if the empty battery is connected to the primary battery and alternator, and the motor is being revved so the alternator can produce 120A, then nearly all the 120A will charge the secondary battery.
But at this rate of charge you will fully charge your battery in probably around than 1 hour. (180 Ah rating for battery as stated earlier). And you will likely only pull the full 120 A the first minute or so, after that the voltage difference decreases and the charging current falls with it. And it is likely only the full 120A if you are revving the engine at the optimal speed for the alternator to produce that current.

I think this becomes an issue of duty cycle and heating of the wire as well.
Can you measure what wire size is coming out from the alternator? Then use the same gauge wire, maybe a tad larger if you want to feel safe, to connect your secondary battery?
Probably the people at Bosch or whoever built your alternator have figured out what wire gauge is suitable.

Ok, but what happens if just the two batteries are connected together in parallel? One empty, one full.
I found this link. https://www.powerstream.com/voltage-sag-during-engine-starting.htm
I read it as the battery can provide around 150A, but then the voltage drops to around 10V. A healthy battery is 12.8 V when fully charged, so 2.8 V drop. I think that this current can only be this high for the first couple of seconds. Then as the voltage difference decreases, the current also decreases. I doubt that the same gauge as jumper cables would be necessary, but probably more than AWG32 .
I found an interesting article that might help https://www.itacanet.org/eng/elec/battery/battery.pdf

I was writing as I was thinking. I hope this helps.
 

[nemesis_567]

Thank you. Your post was very insightful. I will digest it all and also check the cable from the alternator and report back.