I use a Mercedes Sprinter 907 as a campervan. When I stay off-grid in the same place for several days and have to lock and unlock it frequently, I have the problem that the starter battery is heavily loaded due to the power consumption of the control units.
For emergencies, I am therefore looking for a solution to charge the starter battery from the leisure battery, even without shore power. I would probably need this only 2–3 times per year. My idea was to run a Victron IP65 charger from the inverter and use it to charge the starter battery via the jump-start terminal in the engine compartment. I am aware, that this would be quite inefficient, but for my use case this is not really relevant.
However, I am not sure whether this could risk damaging anything.
Briefly about the setup: The Sprinter has the standard starter battery, which is an AGM battery, plus a 210 Ah lithium battery as the leisure battery. A Votronic VBCS charges the leisure battery via the alternator and when connected to shore power. When shore power is connected, it also charges the starter battery with 5 A.
In addition, I have two Victron MPPTs for solar charging, a SmartShunt, and a BatteryProtect, in case that is relevant. I use a Smart Inverter 12/2000 as the inverter.
I am particularly unsure regarding the VBCS, since it also has a connection to both the starter battery and the leisure battery.
It will work but you have to disable VBCS by grounding the sw input to prevent a loop. From manual:
2. Der Sw-Eingang wird auf GND gelegt: - Das Gerät ist komplett deaktiviert und lädt nicht - LED „Solar" und „Power" blitzen gleichzeitig alle 5 Sekunden auf.
I have seen this paragraph in the manual but have not thought, that it would relevant for my use case. I assume, that the problem would be, that the VBCS recharges the leisure battery from the current provided by the Blue Smart. But is this really the case as long as the motor is not started and the D+ signal ist not active? And would the LiFePo4 really take current, when it is fully charged, what it probably will be. Probably the MPPT from my roof solar will also provide some voltage.
My VBCS is mounted below the driver seat and I would rather avoid having to demount ist, but when I have to I will try this.
By far the simplest solution is to use a battery maintainer/trickle charger like the one I’ll link below. I am in the US but in the UK there are a few different options for these. In the US Ablemail sells their units on ebay due to difficulties in exporting these kind of electronics. Its a very simple unit. We used to have a manufacturer of these in the US but the closed down a few years ago.
I wanted to avoid having to remove the seat and install some permanent hardware for a problem I will have very seldom. Especially as I do not want to do this myself knowing my both left hand, as we say here in Germany . But the longer I think about it your suggestion would probably be the best solution. Im am not sure, but I think Victron also has a solution for this, I know that Votronic has: https://www.votronic.de/en/standby-charger-2/
I am also thinking about replacing the starter battery with the Bulltron LiFePo4 starter battery, which should mitigate the problem somewhat.
I bought the van as it is, so I would have to disassemble it to be sure. But it is from a respectable manufacturer (HRZ) und looking into the list of manufacturer options it has the option EK1, which should provide the D+ signal under the seat. So I am pretty sure, that it is used for the VBCS. I could probably test this by powering the Blue Smart from shore power and monitoring the leisure battery voltage, especially if it is not fully charged.
. But the longer I think about it your suggestion would probably be the best solution. Im am not sure, but I think Victron also has a solution for this, I know that Votronic has: