Smoothing rectifier voltage

[MtnMan]

Hi, new to forum.
I have a boat motor with a rectifier regulator that I want to use to charge a 100Ah lithium iron phosphate battery that is in the boat. The Li battery has a DCDC charger dedicated to it.
I tried simply connecting the motor output to the DCDC alternator input but it doesn't work. I suspect the DCDC doesn't like the voltage ripple.
I placed a 12Ah AGM between the motor output and the DCDC and it works but the DCDC wants to dump 24A into the Li battery and the motor puts 8A into the AGM. The DCDC cycles on and off as the AGM charges from the motor up to 13.4v then the DCDC dumps 24A into the Li until the AGM hits 12.4v the the motor charges the AGM up to 13.4v and the cycle repeats. This heavy cycling for such a small battery will cause it to fail pretty quickly I suspect.

Can I replace the rectifier regulator with just a full wave rectifier and something to smooth the ripple? What components?
The DCDC will accept up to 32v DC in both the alternator and solar inputs.

The boat motor stator output is 28-30v AC 100w.