Just some random thoughts. This area is probably open to further refinement over time. But IMO it is generally OK to float at 13.5 for long periods. If there is going to be a very very long period of inactivity, then a storage voltage with a further reduced voltage is a good idea.
Here is a quote from a quite reputable source on this topic.
Float voltage and potential overcharging:
Lithium-ion batteries do not have a non-damaging way to dissipate excess charging. It is important not to overcharge lithium-ion batteries. It will cause breakdown of electrolyte.
You should also consider tolerances in charger's voltage and keep any float voltage safely below full charge voltage of lithium-ion cell. For LFP cell this full charge voltage is 3.40v to 3.43v. You should not set charger float voltage above about 3.38v per cell to assure there is some safety margin.
Also consider cell balancing will not likely be perfect with some cells having slightly higher and slightly lower voltage.
LFP cathode material is very rugged compared to NMC and NCA cathode material used for EV's. The avoidance of full charge state often touted in EV's EMC and NCA lithium-ion cells does not apply as much to LFP batteries. There is a minor additional decomposition acceleration rate of electrolyte at higher cell voltage, but it is very small. Higher cell temperature is more damaging to electrolyte then holding a high SoC for LFP cells. Holding high SoC at high cell temperature does have a significant electrolyte degradation rate acceleration.
The above recommended 3.38 vpc for a 12 volt battery is 13.52. And that is providing for additional margin for poor voltage control on various chargers as well as slight cell imbalance that affects total pack voltage. In other words, one cell might be held over the recommended 3.38 if there is pack cell imbalance even though the total is 13.5. At 13.5 all cells should be within .004 or less IMO.
Keep in mind lithium "float" for a boat or camper is not a float at all as the term was sometimes used in the past. Its actually just another lower absorption voltage. So we essentially have a higher absorption voltage (14-14.4) to decrease charge time, especially near the top 10%, but the higher absorption voltage is a useful to aid in cell balance due to the higher voltage "uncovering" cell split and thus providing an opportunity to apply a corrective balance.
It may be useful to start discussing or thinking about "float" in terms of "lower absorption". It may help with visualizing what's happening with something like a Victron Inverter charger, IP22, IP43, most Xantrex, most Magnum (but not all) inverter chargers or chargers that have absorption and float (lower absorption) settings.
That is...when sitting at the lower absorption/float, the charge source is set to maintain that float voltage of the entire DC circuit, battery and all. And a good inverter/charger or charger will maintain tight control of that set float/lower absorption voltage despite having additional ships loads that may come and go and may be fairly hefty. For instance, if a good inverter charger has run through a complete cycle and is not holding the entire DC circuit at 13.50 volts and there are no additional loads (other than slight battery self discharge and its own component consumption) and then 3 DC fans and many DC lights are turned on then the voltage in that DC circuit will dip slightly, be detected by the charge source, and current output raised so there is no longer a voltage dip, maintaining ~13.50. The opposite happens when those loads are suddenly turned off.
So if you have a high-quality charge source that holds stable voltage (which most of us should have) then IMO you can use 13.5 for float if you essentially want to be "ready" to use the boat or are using the boat on a routine basis. Or if you want to maintain a high level of readiness and capacity even sitting at the dock for things like power outages, storms etc to power things like bilge pumps, refrigerators, Wi-Fi, Starlink to maximize possible run time. But those are subjective decisions.
Many battery manufacturers actually float at 13.8v. I wouldn't. But I am not too concerned with 13.5. Does it have to be 13.5? Not at all. But at 13.5 you can be confidant of SOC being at 98-99%, especially as those ships loads come and go. At 13.5 its just a little capacity "taken off the top". Every tenth volt lower than that, especially in the 13.2 to 13.3 range, its more difficult to decide what your actual reserve capacity is for the power out scenario.
One thing to note, some Victron components such as the MPPT have the ability to set absorption and float voltage to 2 decimals and do a great job of voltage control to that second decimal. In those cases you could set 13.45 and really split hairs for your liking.
As a side note...that quote above came from RCinFLA on the DIY Solar forums. This is the thread and its post number 20
One of my BMSs seems to be disconnecting both the charging and discharging MOSFETs when it hits 100% but what I find super interesting is that disconnected from the inverter, voltage after 2.5 hours has fallen only from 3.425 to 3.4v. Why are we even talking about float voltages of 3.35, 3.375...
diysolarforum.com
I encourage you guys to click on his profile and just pick out random posts by him. He is a working EE in this field and appears to be of exceptional talent. He generates many of his own documents that are just excellent. Much of it is well above my head.
