This is my plan for wiring up a 180Ah LiFePO4 battery. It has an isolation relay between the main start battery and the LiFePO4. Load will be Dometic fridge and a water pump primarily so small. The smart shunt 300A is so I can get battery state of charge. I’m curious about what people have done with heavy gauge ground wires coming off the (in the pic) Blue AUX battery. In my Mercedes Metris van it’s pretty beefy. By my measurements, it’s 1 gauge. The wiring recommended for the Oprion XS is 4-6ga. I’ll be using 4ga.
So for basics, this both sounds like theres a factory installed Aux battery already there? Which probably is some lead acid, likely an AGM? So you want to exchange the AGM with an LFP and exchange the isolation relay with an Orion?
1AWG isnt really big, but surely depends on where you’re coming from. And since its your main negative wire for the Aux battery i wouldnt downsize it. Any additional cross section means less resistance in the system, which is crucial for 12V systems.You want to minimise voltage drop on cables.
Dont confuse cable cross section recommendations for the main negative cable and for the Orion, those are two different things. The Orion XS indeed does not need that big of a negative wire, since its negative cable only carries the delta between input and output current.
Youd have to ask Mercedes as to why they chose that cross section, but most likely to minimise voltage drop
If its ok or not depends also on the cable length. Use the victron toolkit app to get an idea of voltage drop and necessary cross section, or the wiring unlimited book
I would continue to use 1AWG for the main connections, so Aux Negative ↔ Shunt and Shunt ↔ Vehicle Body, the other negative connections you can size as needed with the above tools
Thanks! That’s very helpful. Yes, there’s already an AUX AGM battery in there. Trying to increase the usable Ah. The AUX ground wire is only 12” or so long. I should be able to run the existing ground wire to the smart shunt and then make a very short 1ga wire to go to the chassis.
I’m now wondering if the 300A shunt I bought is OK. Most people seem to run the 500A version. I figured it was ok given the 50A Orion XS?
I am now very glad I bought a crimper that will handle north of 3/0 wire!
If the Aux. Battery has duties like running a winch or assisting the start battery then it might try to draw more than 300A. This might also exceed the output of your new Li battery, causing its bms to shut it down
However, if no such heavy loads/duties exist on the system then the 300A shunt is probably OK. Your fridge, lighting, pump etc will be nowhere near that much and the Orion XS doesn’t go past 50A.
Agree with the advice to retain 1ga for the shunt to ground connections and use smaller cables as appropriate to ground the Orion and loads.
Thanks for the advice! It sounds like three conditions need to be met for the isolation relay to close
Engine Running: The most primary condition is that the engine must be running, allowing the alternator to produce charge voltage.
Voltage Threshold: The alternator must be providing a sufficient voltage, typically indicated when the main battery voltage (or the voltage recognized by the BCM N82 module) exceeds 13.5V.
Time Delay: There is often a short time delay after the engine starts before the relay closes to ensure the main starter battery is prioritized for recharging first.
This should mean that there will be very minimal loads on the LiFePO4 battery driven circuits. No winch
