lithium and agm batteries: a warning !

Yes, there is a battery combiner - here is the one I have. Like Marsella said, it is installed in the battery compartment, right between the service batteries and the positive bus bar.

now I also understand why Leon insists on moving the alternator to the service battery. the battery combiner may separate the alternator from the service battery, making recharging the service battery when the service battery goes down impossible with the engine. still , in my boat solar panels and generator will charge it.

The battery combiner will still work as advertised whether the alternator is connected to the start battery or the service bank even with lithium service batteries. There are four potential problems though that arise with lithiums installed. First, the lithiums can charge at a much higher rate than AGM’s and they can overload and overheat the alternator. Second, the lithiums charge profile for lithiums is different than AGM’s. Third, if, you remove the combiner and separate the two banks, you need to manage the charging in a different way than the simple combiner. This is usually accomplished with some version of a Dc-Dc charger between the banks - it is a one way switch and can be set for the correct charge profile. Fourth, the lithiums have a BMS, either internal or external which controls things like charging and if the BMS decides to cut off charging while the alternator is producing a significant amp load, you can get a “load dump” which can ruin the alternator. 

So, you can leave the alternator on the start battery and charge the lithium bank with a Dc-Dc - this protects the alternator but limits your service bank charging to 30-50 amps and it also limits charging of the start battery to only when the engine is running. Or you can move the alternator to the service bank and charge the start battery with a Dc-Dc from the service bank. In this setup you will now have start battery charging anytime the service bank is being charged. The big drawbacks are that the alternator is now not being protected from overdrawing overheating from the lithium bank and if the BMS decides to shut off charging, the alternator and DC load bus is not protected from a “load dump” which can damage the alternator and other electronic devices. The most common way of solving this is through an external alternator regulator controlled by the BMS. The regulator keeps it from overheating and the BMS will shut down the alternator properly through the regulator without getting a “load dump.”

in practice, what does it mean to move the alternator onto the service battery: deploy 200A cables from the engine room to the service battery?

As Marsella said, this cable is already in place if you elect to modify. A cable runs from the alternator already to the combiner right next to the battery. It could then be connected either to a Dc-Dc charger to the service positive bus bar or directly to the service bus bar depending on how you wish to construct your system.