Victron Battery Monitor BMV-712 SMART (Bluetooth) BMV 712 Lithium Battery AGM

The BMV-712 'Smart' is Victron's high precision multi-function battery monitor with built-in Bluetooth. It essentially acts as a battery 'fuel gauge' for & anyone & needing to keep a check on the performance of their battery or battery bank and can monitor the voltage of a 2nd battery. The information displayed by the BMV-712 includes voltage, current draw, Ah consumed, % charge level, 'time-to-go' and it features a programmable alarm and in-built relay to trigger external equipment under an alarm condition (note: information displayed for 2nd battery is voltage only).
The shunt is fitted in-line with the leisure/house battery's negative cable, the starter/auxiliary battery's negative cable is connected to the shunt's output and fused +ve supplies are taken  from each battery into the shunt's 2 inputs. Once the battery monitor is panel-mounted in a suitable location (with or without the supplied square bezel) it is connected to the shunt's PCB using a single RJ12 data/power cable and programmed using the on-board menu.
Built-in Bluetooth allows you to display many of the monitor' features via any Android or Apple device or laptop and most settings of the monitor can be configured with the VictronConnect app
As an alternative to monitoring the voltage of a 2nd battery  the BMV-712 shunt's 2nd/auxiliary input can be used for the following:
Temperature monitoring of a single battery/bank using an optional temperature sensor. Battery capacity reduces as temperature increases so the sensor measurement can be fed into the monitor to adjust displayed capacity based on temperature.
Mid-point voltage monitoring of one bank of batteries connected in series (or banks of series-connected batteries joined in parallel). This allows detection of a bad cell or a bad battery which can potentially destroy the other connected batteries by over-charging. The voltages either side of the mid-point are compared and the deviation displayed in % and V, with a programmable alarm providing a warning that the deviation is too great.