I've read a few threads which imply that the VW factory fit split charge system for a second battery is a cheap solution that simply connects the second/starter battery together once the engine is running. I've just fitted an ammeter to the leisure battery so I've been able to actually monitor what's going on when the vehicle is running.
We have a solar panel which means that the leisure battery is effectively at 100% charge most of the time. The starter, on the other hand, is rarely above 80% charge because that's how the bluemotion system works. Therefore, when the van is running, one of two things are happening:
1) Van is braking (or alternator voltage is high for some other reason) - charge is flowing from the alternator to both batteries. It was up to about 10amps for the journey we just did.
2) Alternator not kicking in - leisure battery voltage is now higher than starter so current flows from leisure battery to starter. Also about 10amps for last journey.
What this means in practice is that 10amps is constantly swinging in and out of the leisure battery depending on the alternator voltage and the leisure battery could end the journey with less charge than it started with!
This is clearly pretty nuts and probably not ideal for the leisure battery. The ideal solution would obviously be a DC-DC charger (I didn't fit this when installing the solar panel as I couldn't find one which coped with a high enough solar panel voltage) but I might simply disconnect the factory charge input and see if I can rely on solar alone.
We have a solar panel which means that the leisure battery is effectively at 100% charge most of the time. The starter, on the other hand, is rarely above 80% charge because that's how the bluemotion system works. Therefore, when the van is running, one of two things are happening:
1) Van is braking (or alternator voltage is high for some other reason) - charge is flowing from the alternator to both batteries. It was up to about 10amps for the journey we just did.
2) Alternator not kicking in - leisure battery voltage is now higher than starter so current flows from leisure battery to starter. Also about 10amps for last journey.
What this means in practice is that 10amps is constantly swinging in and out of the leisure battery depending on the alternator voltage and the leisure battery could end the journey with less charge than it started with!
This is clearly pretty nuts and probably not ideal for the leisure battery. The ideal solution would obviously be a DC-DC charger (I didn't fit this when installing the solar panel as I couldn't find one which coped with a high enough solar panel voltage) but I might simply disconnect the factory charge input and see if I can rely on solar alone.
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