voltage begins to matter with LFPs when they get close to full. As long as they are still absorbing, they don't give balance issues. Keeping the end-of-charge voltage on the low side is a great way of hiding
cell balance issues. It also places less stress on the cells, but charging
is not all about voltage: it is also a matter of how long you hold it (absorb) before stopping.
Some boats make short occasional use of the engine
only and can get away with somewhat approximate setups on the basis that stopping the engine
stops the charge.
Those who motor
frequently and for long periods need a way to stop charging
. Using a "low voltage" like some have done is not ok, unless it is so low that it essentially fails to charge and then what is the point...
I mentioned earlier already had Next Step charge controllers, so I adjusted them to get 14.0V absorption for 30 minutes and 13.35V "float" (which is too low to charge LFPs and just high enough to prevent discharging them, so it terminates the charge). The owner lives aboard a lot and frequently motors short distances, occasionally longer ones. 14.0V gives decent charging efficiency with shorter engine runs and 30 minutes is not enough to get the bank to the end of the current
taper, which is good.
That boat cycles batteries quite significantly, a bit like what you describe. If it didn't, I would have reduced the absorption time and voltage.
One issue with alternator controllers is reliability
. Balmars are particularly bad on this front - my experience based on all the blown ones I have seen here - and they are essentially unrepairable due the circuit board being cast in silicone.
Others seem to do significantly better while being simpler and you can put new MOSFETs in them if/when they blow. A good alternator controller should have no heatsink fins and run cold. If it creates enough heat to need them, it means its design is poor and/or intended to be as cheap
as possible to manufacture.
If you are not intensely focused on fast alternator charging, you can sometimes opt for charging through splitting diodes. Those will commonly drop between 0.9V at high current
and 0.4V at low current. As a result, they tend to also limit alternator output: e.g. losing 0.9V @ 120A would mean a charging voltage of 14.3-0.9=13.4V, which is way too low to actually push 120A, so the system settles for a lower current and lesser voltage drop, which keeps the alternator cooler. When the current tapers near the end of the charge, the voltage drop becomes closer to 0.4V and the charging voltage rises to 14.3-0.4=13.9V, which is not bad.
If you motor
occasionally and not for hours or more in a row, it can work out. Otherwise you still need a way to terminate the charge and using external controllers is a practical way of achieving that. Another one is interrupting the field circuit (alternator off), or disconnecting the LFP bank from the isolating diodes - provided there is a start battery
left to act as a remaining load.
Some alternators (like the Mitsubishi found on all the new Volvo
engines) have external voltage sensing. This opens additional avenues. The options are too numerous to be fully discussed here, but many were here in the past if you take the time to go back and read.
You need to start by assessing what you have got. This includes how your alternators actually behave, what is their regulation voltage, where they take their voltage reference from and how you get the current to the batteries.
Multihulls can greatly benefit from isolating diodes anyway, because you can split each alternator 3-way: port start, starboard start and house bank. This provides very simple redundancy and small DC/DC boost chargers can top up the starting batteries from the lithium voltage up.
Junsi cell loggers are toys really. Their voltage reference is inaccurate and they load the cells unevenly, so leaving them connected is out of the question.