The trickle charger on the Mutliplus is not a true charger, it does not have a set charge voltage. It is a small circuit that is connected directly to the DC input header within the Multiplus and it contains a current limiter and a diode. The diode is to prevent the starter battery from charging the domestic battery if the domestic battery has the lower voltage. See the diagram below. This means that the trickle charge function is always active, therefore if you have solar connected to your domestic batteries then when the solar charger increases the domestic battery voltage, then the trickle charger passes some of that charge to the starter battery.
NOTE: for the trickle charger to work, the starter battery and the domestic battery need a common negative to complete the circuit. If you have a shunt fitted on the domestic batteries then the starter battery MUST be connected to the “System” / “Load” side of the shunt.
The diode in the trickle charger has a voltage drop of around 0.3V before any trickle charge current will flow, increasing to around 0.6V when the trickle charge current is higher. When both the domestic and starter batteries are lead acid this means that the trickle charge function really only works when the domestic battery has some charge taking place lifting the domestic battery above 13.1V when the starter battery is full at 12.8V (for a 12V system).
The trickle charge function is not recommended if the domestic battery is lithium and the starter battery is lead acid due to the higher voltage of lithium batteries. With a lithium battery having a working voltage range of 13.1-13.5V (for a 12V system) means that the trickle charge will permanently pass a current to the starter battery even if the starter battery is full. Additionally, the trickle charger is not controlled by the Mutliplus and will not shut down if DVCC tells the Mutliplus to stop discharging the domestic battery. With some BMSs such as the VE Bus BMS (V1 or V2) and Smart BMS CL 12/100 which do not isolate loads but control the Multiplus via DVCC or ATC/ATD input wires, this means that the trickle charge would continue to drain the domestic battery after a load disconnect had occurred.
IF your system has the capability to provide the energy for a permanent 13.2V storage charge on the starter battery AND has positive disconnection of the DC supply to the Multiplus (Lynx BMS or batteries with built in BMS disconnection), then the trickle charger may be used with a lithium domestic battery and lead acid starter battery.
The trickle charger must not be used if the starter battery is lithium because it has no control over the charging voltage or current.
The graphs above illustrates how the trickle charger works showing 1 day starting at midnight. The domestic battery is being discharged and the voltage has fallen to 12.7V, below the starter battery at 12.9V which remains full after the previous days engine run. At this stage there is no trickle charging and the starter battery voltage is slowly falling. At 07:00 solar charging of the domestic battery starts and the domestic battery voltage increases. When the domestic battery voltage reaches 13.2V the starter battery voltage starts to increase (0.3V drop due to the diode). After this point the starter battery voltage increases and follows the domestic battery voltage. When the domestic battery voltage reaches 14.4V, the starter battery voltage is at 13.8V, at this point there is a higher trickle charge current and the diode is dropping 0.6V. Between 12:00 and 17:00hrs the air conditioning is turned on using all the solar power along with some load on the domestic batteries, trickle charging stops and the starter battery voltage decays. The air conditioning is turned off at 17:00 and solar charging returns and the starter battery also receives some charge with solar charging ending at 19:00.