Just FYI, the above statements that for a fixed charging
voltage the current
will be governed only by the battery resistance is not correct. The current does not reduce due to change in resistance.
The overall battery resistance is very nearly constant and does not necessarily increase with charge, in fact it is more likely to decrease.
Looking at it in closer detail, what you have is a charging
source, say 15.5 volts. You have a circuit resistance (cables, battery resistance and charger
source resistance) that is nearly constant. And you have a battery that is charging.
If the battery has a charge of 12.5 volts you only have 15.5 minus 12.5 = 3 volts pushing charge current into the battery so the current is 3 / resistance, not 15.5 / resistance.
In this formula the battery voltage increases as the battery charges so 15.5 - 14 volts for example, means only 1.5 volts is available to force charge in. THIS is the reason for charging current dropping off.
What is confusing is since you can only measure the voltage on the battery terminals, you can't measure the actual charge level. This terminal voltage measurement shows the total of the actual charge voltage PLUS the voltage drop across the internal resistance. The actual charge level is what you would see on the terminals after the battery has been off charge for 24 hours.