To Shunt or Not to Shunt

[BrendanK]

Hello! This is my first post here and I'm sure there will be many to come as I am just about to get started building my off-grid power system.
I will be installing in the main house:
2 EG4 6500EX inverters with 2 full 6-slot server rack battery banks with EG4 Lifepower 48V batteries (60kWh total)
This is based off of Will's video on his complete 48V off-grid power system.
These will be fed by 20 LG NeOn 395W panels in 2, 10 panel strings.
My question is about shunts. Are they necessary? What are the pros and cons to having one?

Also, semi-related question:
the guys at Signature Solar said that 2/0 gauge cable from battery bank to busbar would be more than suffice, but Will recommended 4/0 gauge for that application. What are people's thoughts on this?

THANKS!
B

 

[sunshine_eggo]

Given the batteries will communicate their SoC directly to the AiO, I can't see a reason for a shunt unless you with to monitor it via some other means.

 

[Adam De Lay]

Once you start having multiple batteries in play, it seems like the inverter just takes the average SOC of each combined battery to determine the overall SOC. A shunt will track and count each ah that passes through in each direction for a more accurate SOC measurement.

As far as the cable size, each inverter can charge/discharge 120a I believe. So you're at 240a if you run each inverter at max capacity, but you also wana give a little head-room. I would say 300a to be safe. Depending on the specific type and length of cable used, amp ratings will vary. I know I have 2/0 running with no issue.

 

[BrendanK]

Thanks, all! This is a huge help.

 

[ArtieKendall]

Once you start having multiple batteries in play, it seems like the inverter just takes the average SOC of each combined battery to determine the overall SOC. A shunt will track and count each ah that passes through in each direction for a more accurate SOC measurement.

Can you elaborate on this? If you only have one shunt, and it's presumably connected to the negative wire from the battery bank, how can it do anything but monitor the combined current in or out of the entire bank? So isn't that the same as taking the average SOC of the batteries?

 

[Adam De Lay]

Can you elaborate on this? If you only have one shunt, and it's presumably connected to the negative wire from the battery bank, how can it do anything but monitor the combined current in or out of the entire bank? So isn't that the same as taking the average SOC of the batteries?

It’s all about accuracy.

Let me preface by stating that I don’t have any server rack batteries or any comms with my inverters. I’m only talking based on what I’ve seen from other installations.

It seems like the inverter when communicating with a bank of batteries just averages all the different values from the batteries based on what whatever the battery says the SOC is at. I’ve seen where the battery SOC is wrong. So if a battery is wrong (cells out of balance, bank wired improperly, etc), and it’s just averaged in with all the other batteries SOCs, your overall SOC will be wrong.

The shunt will count in detail the amp hours that pass through. If you have one device that counts all the amp hours of the entire system (it’s not averaging), it knows right where the SOC is supposed to be.

 

[robby]

If your using closed loop communication then the Average SOC is passed onto the Inverter which is then controlling the charge cycle based on the SOC. Adding a Shunt really adds no value to this situation unless your trying to monitor the Charge and Discharge for some other reason.
If you were not using closed loop then a shunt is definitely the way to go.

 

[pollenface]

I choose no shunt. Don't need one. Added complication. Batteries usually go into float before midday so that's all the info I need.

 

[Crowz]

I prefer running a shunt on all of my stuff. That said I dont have my server rack batteries yet for my aio inverter but I will be adding one when I add them.

My other batteries do have shunts. It's nice to know whats really going on and you can spot problems instantly with the shunt that would be harder to "dig" without it.