Are you talking about the main negative wires coming out of each BMS? There should be 3 of them on each of the overkill BMS's. Most people connect those to the negative bus bar of the entire DC system. That bus bar is often shared with the negatives from the charger, inverter, etc. Systems designs vary, but they should be routed to where ever all your negative's come together.
I'm talking about the 2 black wires coming from the bms in the picture. I have an MPP solar all in one system. And I have to batteries I made so both have 2 black wires coming out of the bms. My question where or what is the best place to hook them up too? The 2 batteries will be in parallel, so I'll have 1 negative on one battery going to my system and the other battery will have the positive going to the system.
The 2 black wires coming out of your BMS are essentially your battery negative. Some people will route them to a terminal on the battery itself and then use a separate wire to the system, but you can wire those 2 BMS leads direct to the negative side of your system if the BMS leads are long enough. The Smart shunt is a great place to attach to so that any load (besides the BMS) will be measured by the shunt.
So I shouldn't connect the bms to the shunt?
Yes, you can connect the BMS to the shunt, but typically the shunt would be connected "downstream" of the BMS (connected to the black wires), so the shunt consumption isn't being measured. If you are only going to have one battery, I guess you could install the shunt between the battery negative terminal and the BMS, but that's not the common approach. I don't think it would cause any issues that way, but I'm not sure. The BMS draws very little power, so I wouldn't worry about that aspect.
Yes the shunt will be between my MPP solar and the bms. I have 2 batteries hooking up to my system, so 2 BMW. I still can hook both bms black wires all to the shunt?
