Now to the glaring factual errors. The internal resistance of a battery or to be more correct the internal impedance, but lets keep it simple and stick to resistance is dependent on many factors some physical and some chemical. The simple physical factors that you are talking about are the bulk resistance of the conductors and the active materials in the battery and as you say are covered by Ohm's law and don't change much. They are only part of the total resistance, the resistance caused by the many chemical processes is variable and difficult to calculate. See powerstream.com/internal-resistance.htm or en.wikipedia.org/wiki/Internal_resistance for more details. The chemical processes are a large factor with lead acid batteries that limit extended large charge and discharge currents and result in the need for long absorb times.
Your statement "Makes no difference if charge is stored by moving Ions in an LFP battery or electrons in a Pb battery. Again it is Ohm's Law, not chemistry" just shows that you do not understand battery chemistry and how it impacts on the electrical characteristics of batteries. For starters, both LFP batteries and PB batteries have ions moving from the anode and cathode to and through the electrolyte and electrons moving from the anode and cathode through the external circuit to balance out the flow of ions. The chemical difference between the two battery types is that in LFP batteries lithium ions move from the electrodes into the electrolyte then move through the electrolyte to the opposite electrode and electrons moves through the circuit. With LA batteries a chemical reaction occurs at the electrodes which releases or consumes hydrogen ions, sulphate ions and water from the electrolyte.
As you increase the charge rate say from C/20 to C/10 you arrive at Absorb at 70% SOC and now will take a much longer time to saturate, You hit a wall with LFP at C/2 charge rate and you arrive at absorb at 50% SOC, and will now have a 2-hour Absorb time. At 1C it will take you 3 hours from fully discharged to fully charged. Is that because of the chemistry? No Sir. Its Ohm's Law because Pb has the same speed limits regulated by the INTERNAL RESISTANCE.
AbsorbTimes3.45V.jpgAbsorbTimes3.6V.jpg
No technically it is called Constant Power, which is limited by the flaky and unreliable source of power; the sun and clouds. But it is still referred to as CC because the circuit is identical in operation. A Solar CC will deliver as much current as the variable Power can provide up to CURRENT LIMIT. AC chargers do not have this problem as power is unlimited with no time constraint. If you replaced your Solar Panel with a 18 volt DC Power supply, the Controller would work exactly like an AC charger as I described above.
For LFP batteries, charging from solar is not the same as charging from a constant current charger. With a constant current charger we can charge at say ~C/4 (0.25C) and terminate the charge at 3.45 volts and have a predictable ~90%SOC. With solar we could have anything from ~90%SOC to ~99%SOC due to the variable nature of the power coming from the solar panels. If you are bottom balancing with no BMS this could have disastrous consequences.
Simon
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller
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