There is no one size fits all best solution. There are several issues to consider & different possible ways of attenuating each issue. Depending on the priorities on the individual
boat, different answers may be best for different people.
Lithium batteries have a BMS that can disconnect them on short notice & if they are the only battery connected to a running alternator, the alternator gets blown out. This can be dealt with by using a special BMS with a pre trip warning that lets you kill the field to the alternator before the bank drops out, but not every
dock side wrench spinner who may
work on the
boat will understand that system. The more common option, is just to use a lead battery across the alternator.
Lithium batteries run at a slightly higher voltage than lead batteries, have different charge curve preferences, don’t like to sit full all the time, don’t like to cold charge, don’t like to be run hot, have a very high acceptance rate,. & really don’t like to be over charged. Since they are so different from lead batteries, putting them directly in parallel with lead is not a good idea. There are different ways that they can be isolated.
These days, most people seem to charge one bank directly, & use a dc to dc charger to feed the other. Be aware that a dc to dc converter is different from a dc to dc charger. Be aware that both come in isolated & non isolated ground configurations. If you don’t know the differences & why they exist, you are not the guy who should be setting up the system.
Another option is to use an old
school battery isolator that is simply a collection of really big diodes in a box with some big heat sinks & a few terminal posts to connect battery
cables to. This prevents back feed from one bank to another & lets bulk charging go to all banks at the same time, but does not give separate curves for each bank. The lead, being lower voltage, will often charge to full first. The lithium will still suck up lots of amps. The lack of separate charging curves makes this a poor choice if the
motor is run all the time. If the
motor is only used now & then, and a
solar system then tops things off, you can probably get away with it. In this configuration, if the BMS drops out, the lead is still effectively across the alternator & it doesn’t go pop. In this configuration, you can have separate solar controllers for each bank and each of them can have whatever curve that banks wants to see.
Alternators can only put out so much before they cry. A big lead bank will beat them up as much as a moderately sized lithium bank. Amps are amps. There are different ways of dealing with this. You can use an external
regulator that allows you to compensate for temperature, or lets you manually set a
current limit. You could also put a larger diameter drive pulley on the alternator so that it spins more slowly & doesn’t try to generate as much power. That is cheaper, but gives you reduced performance at low
RPM.
If I felt like writing a few more pages, I could give several other options, but I’m not in the mood to write that much at this time.
It all starts with the question - what does my battery manufacturer say that the proper way to charge there batteries is?