3x multiplus II 10kva in three phase operation.
3x SmartSolar MPPT RS 450|200 solar chargers.
3x 48v batteries (15kWh each) wired in parallel.
From what I’ve read it’s recommended to keep the cable distance between the batteries to inverters/chargers all the same, it even says so in the victron manual:
“When wiring, make sure all inverter/chargers have the same cable length. Also, the solar chargers need to have approximately the same cable length. And the same for the batteries.”
What confuses me is that just shortly after the manual gives this example for bigger installations where the battery distances differ:
My original plan was to wire all 3 batteries together like this:
2 bus bars in parallel next to each other.
same length cables from all the batteries to the bus bars.
battery minus connects to the left most side of the minus bus bar.
batter plus connects to the right most side of the positive bus bar.
That way the location of the inverters and chargers doesn’t matter since the distance to the battery is always the same, basically something like the reverse of this:
my concern is that if i do the wiring like it says in the victron manual(large installations) and one phase is being used more or one solar charger produces more it will lead to imbalanced batteries due to different distances.
is that a valid concern or am i worrying too much?
The concept in the top picture is more practical for a larger installation such as you are planning. This keeps the maximum current in the bus-bars down - they are used more for ‘equalising’ the charge and load between the batteries.
Your 2nd concept would load on section of the bus-bars with the total current, about 900A peak. ( this concept is ok for a smaller system,but for larger systems it leads to much bigger bus-bars which are not strictly necessary)
If one phase is more heavily loaded, then the bus-bar will handle the difference in current, and with lithium batteries, the slight difference in total resistance to each battery does not matter so much. This is because the cycle charge efficiency (Energy out/energy in) is usually better than 96%, c/f 60% for lead acid.
The main concern with larger systems is that the battery must be able to handle the peak load current. at 15kWh each battery, assuming 0.5C max discharge current, then you have only 150A per battery, this is insufficient for the inverters you are using.
The point with the same cable length is, that the voltage drop across the calbes should have the same value. Just imagine, you have three different cable length and the voltage drop when either charging or discharging is different and one battery has 1V more then another. Then the System will handle the batteries different. The battery with the lowest voltage, while discharing, will shut down earlier then the other with shorter cables because the voltage drop is lower.
That’s why you should keep tha cable at the same length and same thickness.
In larger installation, the nearest battery often has the same cable length, but the cable is just winded somewhere in the cable duct.
The peak load current shouldn’t be an issue because at a later point i plan to add another set of 3x15kwh batteries for a total of 90kwh which should be more than enough.
I also considered mounting way oversized aluminum profiles on the wall that i can get for cheap, but i read many times that aluminum can be dangerous due to corrosion issues and some say you have to keep checking the connections because they get loose over time (and knowing myself i will forget to do that).
So in the end it does not matter if i just distribute the batteries evenly along the (2,5m long) bus bar?
If you are using large section aluminium bars, then yes, the batteries can be evenly distributed along the bars, as can the inverter loads and pv chargers. Try to keep each pair of batteries symmetrical about the inverter connection point, but this is not essential.
Stainless steel bolts and spring washers should be used for bolting the connections. This will help minimise the corrosion, there are also anti corrosion pastes available in the marine market if corrosion is likely to be a big issue.
All systems should have an electrical tightness check every 6 - 12 months.
