The voltage that your charger puts out is only loosely related to the voltage at the battery terminal.
As you state there are several separate issues there.
First off, and most important, is the distinction between
the voltage of the battery itself:
A. isolated and **at rest**. Which can take 24+hours for real accuracy after high C-rate activity whether charge or discharge; usually an hour is enough for rough
versus
B. voltage while being charged, at the posts, the data point used to track the progress of the charge cycle from low SoC up to the CV setpoint (Bulk stage)
The relationship table between SoC% and this 'latter B measure' will vary enormously depending on the C-rate for a given specific cell.
While the relationship between SoC% and 'A the former' is more useful for a given bank of a given age,
it does vary enormously between different LI chemistries, even if all are the same nominal voltage
and will still vary widely by brand/model and wear-level age, even within the same chemistry.
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Now we come to the distinction between the source charger's setpoint / VOC / unloaded voltage
versus the voltage of the charging circuit, as soon as it is closed with the target load, in this case a depleted battery.
This is "negotiated" between those two, ends up somewhere between the setpoint and A above, depending (again) on the C-rate.
The resistance of the wiring is only involved with the (hopefully very slight, within 1-2%) difference between "B" and now "C", voltage at the charger output terminals.
Better quality chargers have a battery V sensor circuit, separate from the fat power leads. BMS and RC hobby balance chargers use the outermost pair of the balance leads.
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Finally, the delta between the charge termination voltage setpoint "B" and "A"
This is a property of the specific chemistry, and LFP's gap is much larger than that of LiPo or the other li-ion at nominal 3.6-3.7Vpc
So after charging the latter to say 4.15Vpc for longevity, the resting Full voltage will be quite close to that
while, as I stated above, whether LFP is charged up to 3.65V (stressful, too high) or 3.45V (good) the bank V will then "settle down" to resting Full voltage at 3.33 - 3.35Vpc.
Any higher than that does **not** represent any significantly higher actual stored energy or capacity utilisation, it is just "surface charge" dissipated by even the tiniest load, maybe 0.001% of capacity.
Note when testing over time to detect any calendar life self-discharge, while isolated in storage, the starting point should not be at a high SoC, but somewhere around the 50% point.
>While you may need to be sure you have a full charge every night before you start cooking, etc., when parked between trips, you actually need to let the SOC drop to around 50% between trips. Mentally, this is HARD!
As are all paradigm shifts, overriding long ingrained habits
Once you have gotten intimate with living in your rig off grid for long periods, observed your usage patterns in various conditions, things get more intuitive.
When you know your consumption will not bring SoC much below half way by the next charge cycle, then only charging to 80-90% still leaves a nice cushion for exigencies.
The exception is attempting to go solar-only, especially with an undersized bank. At any time, insolation may become poor, possibly for days at a time, while there is no need for a run into town when the alternator could be used to top up.
Having a quiet little inverter genset, in effect energy on demand, takes all the pressure off.
This lets the owner-designer start small, minimalistic and frugal, both on panel wattage and bank storage
and only expand either/both as proven necessary by observed experience.
