Alternator to MPPT regulator?

Hi,

I bought a new (used) sailboat last year and I’m going to equip it with LifePo4 domestic batteries, and five 100W solar panels to keep them charged. The charge will be regulated with a Victron SmartSolar MPPT 100/30 regulator, and I wonder if this unit also could solve another problem? The diesel engine has a traditional alternator that I believe can deliver 70 A. Initially, I planned to get an isolated DC to DC charger and let the engine battery charge the lithium batteries – but couldn’t the MPPT regulator be used instead?

I’m planning to use a relay to switch between solar power and alternator power for the regulator. When the engine is turned off the solar panels are charging and when it’s on the alternator is charging. This way I can charge the lithium batteries exactly the way they should be charged and at the same time I will limit the current from the alternator so it won’t burn up from overheating. The alternator will always be connected to the AGM engine battery so it can’t be destroyed if the regulator shuts off. If you wonder about the isolator it's already there and I can't see any harm in leaving it were it is.

The only disadvantage I can see is that I wont be able to use both the alternator and the solar panels to charge at the same time …

Anyone out there with some skills?

Someone on the forum has used a Victron MPPT controller successfully to charge using another battery instead of a panel. The controller has a current limit that actually works so it didn't kill itself with the low impedance battery on its input. It was a while ago but a search might turn it up. Definitely not manufacturer supported, if you do manage to kill the Victron you can kiss goodbye to any of the 5 year warranty that may be remaining.

The voltage available to the Victron on the PV input must be at least 5v over the domestic battery before it will attempt to charge the domestic battery.

Thanks for your reply! I didn't know about the 5v thing – that probably kills my idea ... I guess that someone charged a 12v battery with a 24v battery then? I will search for it!