[RE-wrenches] LFP Inter-Battery Communication

[Jason Szumlanski]

The advantages of closed loop communication between LFP batteries and
inverter systems are pretty obvious. But when you are operating in an open
loop because you are working with a legacy inverter that does not support
battery communication, how important is it for batteries to be wired
together?

I started contemplating this today when I ran into a Pytes stack of 20
E-Box 48100R (5kWh) batteries. Whoever installed them wired all 20 in a
single communication daisy chain. Come to find out, this version of the
battery only supports 8 or 16 in a communications network. The bank is
connected to a couple of XW+ inverters, so it is operating based on
voltage. The owner must have had to use an unconventional method to turn
the batteries on, because typically you would only press the switch on the
master battery. In this case, that doesn't work because the communication
chain is broken at some point.

The reason for my visit was the battery is sitting at 43 volts because the
generator didn't start and the low battery disconnect was set to 44 volts,
well below the 49 volts recommended by Pytes. Side note:  I could not set
the low battery cut out higher than 48V with these inverters.

I was there to get these jump started with a Chargeverter. I rigged it up
and got things going, but various batteries were alarming out. Ultimately I
just disconnected the communication cables on all the batteries and they
all started charging from the Chargeverter which was connected to the
common bus. Awesome!

So back to the original question... Are the batteries really doing anything
in terms of balancing when they are connected in a communications daisy
chain when they are not enclosed loop communications with an inverter? How
important is the communication anyway? And in this case, assuming that the
maximum batteries in a communication chain is eight, I would probably be
better off setting up four separate communications with four master
batteries in groups of five. Again, I'm not sure how important that is in
terms of balancing. What if I had different quantities per communication
group? They're not talking to each other anyway!

Not to muddy up this thread, but I will say that the Chargeverter saved the
day. Within 20 minutes, I was able to get the inverters turned on and the
SCP to help me identify what went wrong. Then I turned on the inverters and
got them charging the batteries simultaneously with the Chargeverter and
solar. It really just needed a jump start! I had rigged it up in a way that
I could easily do this and then safely disconnect the Chargeverter while
the rest of the system continued to charge the batteries. It was my first
experience with this little gem, and I'm happy to have it in my toolbox now.

Tomorrow I am going back to restart the charging process and hopefully get
the batteries up to 100%. I also plan to check the firmware, which I am
almost positive is mismatched in this stack of batteries. All of my
off-grid systems are a boat ride away, so I'm going to pretend it's a
weekend and I'm not working. Haha.


Jason Szumlanski
Florida Solar Design Group
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