question

Assuming it's a normal road vehicle, the starter battery is grounded already. So any metal point of body/chassis can be connected to the load side of the shunt. If there's high resistance giving poor readings, replace that with a wire direct to the negative of the starter battery.

Thanks, well its a Land Rover Defender so kind of a normal road vehicle ;-). I installed like the top diagram now. My readings are fine in the BMV app but I don't understand why that starter battery is losing power so quickly. I have a power draw in the car of 0,023A (dashboard and alarm) so that's about 0,55Ah a day but the starter battery (50Ah Optima red top) is losing 0,1V a day which is around 2,5Ah. Batteries are separated by a CTEK 250se. Maybe the shunt needs a separate ground not connected to the starter battery?

Defender... Sold mine a few years back. Miss it a lot.

50Ah sounds rather small. Was about right for my old SIII on petrol though.

Did you measure the current from the starter battery? Did it change with the BMV connected?

We're you aware of the voltage drop before?

Just wondering if the battery is on its way out.

Hehehe nice. Battery is brand new (820CCA) but have a 125Ah Victron Super Cycle household battery I can add to the starter just in case. Checked the battery disconnected and seems fine.Yes the current is measured from the starter battery, didn't really change when i connected the earth from the BMV shunt. It's weird, maybe there is a relais that switches on later when the alarm is engaged? Anyway. Completely disconnected the BMV and CTEK now and will check voltage again for a day or so to see what's happening. Never measured voltage before so don't know. Thanks for your replies

Voltage on lead batteries does drop/stabilize after going off charge. Then drops from internal leakage. However voltage only gives a very rough indication of SOC which is why a BMV is needed.

Configured correctly it will give you the most accurate measurement of SOC.