New battery meter meets existing solar setup

[LiftyK]

Your advice please. Today I helped a friend fit a NASA BM1 Mini battery monitor to his leisure battery. He has a small solar panel charging his battery and it is connected to the shunt correctly, as are other loads. My brain is struggling to understand the relevance, if any, of a link from leisure battery negative to starter battery nrgative. Should this affect the meter operation?

The reason I ask is because the meter
1-displays voltage around above 13V, as the solar panel is operating. Good.
2-displays a charge current around 2A when a mains charger is operating through the shunt, but the mains charger has a light to say the battery is charged. Perhaps the solar voltage is confusing it.
3-puzzlingly, I’ve not been able to produce a discharge current from cabin lights, VHF radio and plotter all being turned on. Perhaps the solar panel is feeding the loads. I’m surprised, given the weak sunlight crossing the modest panel at a shallow angle

Is the strap between the leisure and starter battery negatives ok?

Do folks ignore the indicated battery voltage until the sun goes down and the solar panels stop contributing?

 

[PaulRainbow]

One end of the shunt connects to the domestic battery negative, nothing else connect to this end of the shunt, or the battery.

Connect the other end of the shunt to a busbar. Every negative connection goes to this busbar, including the engine battery negative and the negative cable from the engine. If the engine cables are not connected in this way the alternator charging isn't "seen" by the battery monitor.