Difficult Shunt Location

[The Brockerts]

I'm just trying to get better control of my batteries by installing something like a BMV-712 Smart Battery monitor. I've attached a diagram of the basic GND environment.
I've got what I see as a standard setup in my 1990 Californian. Two battery banks, that consist of 2 8D's each. Port bank starts the Port engine(3208) and Generator. Starboard bank starts the Starboard engine(3208) and is the house Bank. I have a Victron Multiplus connected to the house bank and monitor that bank(somewhat) from the Victron Multiplus II via MK3 module connected to a Windows PC. I say "somewhat" because it does not see all the usage going out directly to the panel. I don't have a single GND I can get a shunt connected to and any place I connect up to will include the 3208 Starter current going through it. It looks like I need a 1000 amp Shunt because of the 3208's.
I"m looking for someway to better monitor these 2 banks.

 

[SteveK]

Where does all house load get the negative, the ground bar or from the Multiplus?

 

[The Brockerts]

All house loads get the negative from the ground bar

 

[l00smarble]

Victron offers a 1000a Smartshunt so that is an option. if you want the round display of the BMV you can get a 1000a shunt that woudl work with BMV (Victron part SHU102050200).

However....if it were my boat I would consider reconfiguring it so that both engines start from the same bank and the house bank is dedicated to house loads including inverter. If all AGM you can still add an emergency parallel switch to crank engines from house battery or parallel for cranking. I say consider because I have very limited information.

I HATE when battery banks are house and also start an engine. The voltage drop during cranking can crash out systems and potentially be damaging to electronics. Plus I feel engine batteries should be on standby for starting only.

If you did this then next battery replacement you would be in a better position to consider LiFePO4 as well.

Either way the negatives are going to have to be reworked somehow for the house battery to move all loads/charging to a bus and put the shunt between battery and bus. No magic way to get around this if you want to measure all current and keep track of amp-hours.

 

[SteveK]

All house loads get the negative from the ground bar

I don't disagree with @l00smarble
but if you want to keep the setup and add a shunt, I would move both start negatived to the ground bar, remove one battery negative and connect across to other bank. Now you have one negative going to ground bar and everything coming off it.
Cut/Insert the shunt into that one negative. I would also move multiplus negative before shunt at or after the last post going to ground bar.

 

[The Brockerts]

The current systems has a parallel switch already installed on the dash and it works as I've tested it. I've thought about having 1 8D for each engine as the start battery, and make the other 2 in parallel for the house bank. This seems like the easiest way with the fewest changes

 

[l00smarble]

The current systems has a parallel switch already installed on the dash and it works as I've tested it. I've thought about having 1 8D for each engine as the start battery, and make the other 2 in parallel for the house bank. This seems like the easiest way with the fewest changes

or make 3x 8D the house bank and start both engines off 1x 8D. You would dramatically increase your useable house bank.

Again I'm working with very limited information so I can't say for sure this is a good idea. If starting often at cold temps etc you may need two batteries for the old CAT. I'm sure they want 1,000 CCA. One really good 8D may work but devil in details.

 

[SteveK]

The current systems has a parallel switch already installed on the dash and it works as I've tested it. I've thought about having 1 8D for each engine as the start battery, and make the other 2 in parallel for the house bank. This seems like the easiest way with the fewest changes

what @l00smarble said, your parallels switch makes the positive parallel. That can remain, just keep it on the house positive only. It is the negative you want to align with the shunt for other than start loads.
Pick one battery for start only, move the positive from the both starters away from other loads and route only to the starters. The shunt will only report other loads.

 

[l00smarble]

If you just want to do the bare minimum to get the 1000a shunt in there:

 

[The Brockerts]

Thanks for all the good ideas, I'll mull it around and come up with a plan for the changes