H38: What is this Xantrax device under my main DC panel

[Davidasailor26]

This is great stuff, man we're a bunch of nerds lol I just wrote 3,000 lines of code to automate a home mushroom growing environment and control it from my phone lol

Nice! Sounds like a fun project.

To clarify a few points, as is usual some confusion has crept in above

1) From the factory the boat came with two batteries in the house bank that are approx 6' of jumper cable apart from each other, and included that inaccessible class T fuse
2) I intend to add a 3rd battery IN THE MIDDLE (not at the end) but with the same lengths of jumpers by pulling the factory jumpers going to the far end battery back a bit and running them to the middle battery, then adding two new jumpers for the far battery
3) In the original factory config the positive jumper from Bat 2 to Bat 1 terminated on the Class T fuse, not the Bat 1 positive post
4) I intend to leave that positive jumper on the Class T fuse because I can't access it.

Thanks for the clarification. Gives me something else to play with in the circuit simulator later.

The other thing to throw into the mix on whether this makes a material difference is that I'm using LiFePO4 with internal BMS, whereas all the papers etc I could find were for some form of LA battery. Given that my batteries are effectively "smart" devices and that LiFePO4 itself doesn't suffer from the same issues as LA around less than 100% SOC, in my mind this further reduces the material impact of differences.

I don’t see how the internal BMS’s do anything to mitigate the imbalances that could occur between different batteries. The only way I see that happening is if the BMS’s communicate among themselves to check each others’ voltages and add some false internal resistance to maintain balance. The BMS’s are meant to protect the individual batteries from over current, over/under voltage, and imbalance between the internal cells, but I didn’t think balancing a whole bank was a feature they support. If anything lithium is more sensitive to imbalance than traditional LA since their discharge curve is so flat, so it takes a larger change in SOC to effect a small change in voltage. I’d add that my simulations above were all done for a traditional LA bank with traditional LA chargers and loads. The impact would likely look worse with the types of sustained high loads and charge amperages you often see with lithium setups.

In the end though, all I can do is all I can do. If I can access that Class T fuse and move the jumper to the Bat 1 battery post I will. If I can't, I will just have to live with it. If that means battery life goes from 20+ years to 10 years oh well.

I agree completely. The value of the discussion (for me anyway) is learning the effects and quantifying them as well as possible to get an idea for whether the issue is likely to cause negligible performance degradation or immediate issues. Then it becomes possible to make an informed decision about how much time and expense rewiring the bank is worthwhile.

 

[flightlead404]

I don’t see how the internal BMS’s do anything to mitigate the imbalances that could occur between different batteries.

You are correct, my post was confusingly written. They balance their own cells, but not individual batteries within a bank. I was thinking of the under/over voltage protection in terms of stopping the charging of one battery while continuing to charge others. I don't anticipate ever reaching 0% SOC but if I did, same protections apply.

If anything lithium is more sensitive to imbalance than traditional LA since their discharge curve is so flat, so it takes a larger change in SOC to effect a small change in voltage. I’d add that my simulations above were all done for a traditional LA bank with traditional LA chargers and loads. The impact would likely look worse with the types of sustained high loads and charge amperages you often see with lithium setups.

Interesting thought. I will bake on that.

I agree completely. The value of the discussion (for me anyway) is learning the effects and quantifying them as well as possible to get an idea for whether the issue is likely to cause negligible performance degradation or immediate issues. Then it becomes possible to make an informed decision about how much time and expense rewiring the bank is worthwhile.

Precisely.

 

[flightlead404]

I was able to remove part of the cabinetry under my nav station and gain reasonable access to the Xantrax Class T fuse. I removed the jumper from the #2 battery and with significant sweating, cursing, pleading, and jiggling, was able to gain enough length to relocate it to a MRBF on the house #1 battery. I relocated the box thruster from the positive terminal of the #1 battery to a stud on the panel under the nav station from the switched side of the house battery switch.

Diagrams and photos shortly