It works by supplying a set voltage until the current drops and then steps the voltage up to the next level. The fact that your battery charger slacks off on the current does not indicate that the charger is at max capacity or that it is getting hot enough to affect output, it indicates that at the voltage it is programmed to supply that is the amperage that the battery can accept. It is more important to watch the voltage at which the battery is being charged, watching the amperage is secondary.

It is very common that the amperage acceptance rate drops as the battery both warms up and it absorbs the initial charge.

Unless your batteries are deeply discharged you would not want to be pumping a lot of power into it.