Don't worry about ripple, batteries can handle it. Do worry about providing clean power; unfiltered outputs (and inputs -- it goes in both directions!) can do wacky things to attached and nearby hardware (including the circuit itself).
Usual circuit is to put a big cap at the output, absorbing the ripple. The battery may have a low impedance still (so that the ripple current is shared in parallel between them), who cares. Or add another L (and maybe a C) to filter it completely.
I guess you already have a current mode controller, which is the important part. You'll wrap that with a voltage error amp (so the current setpoint is driven by the voltage error), and that's basically all you need for a CC-CV battery charger. 4.2V and a few amperes is really all you need per Li ion cell. Very easily charged chemistry!
In CC mode, the V amp saturates, delivering maximum current -- without blowing up anything, it just sits there happily at design-maximum output, no more, no less. In CV mode, the output voltage settles down to the setpoint and current is gradually throttled down (gradually due to the compensation R+C across the error amp).
Bonus, you can change the setpoints/maximums and use it for general bench use.
At this power level, it shouldn't really be necessary to use multiple phases as mentioned above, but if you're charging multi-cell packs, it gets attractive. Consider designing a PCB module for the constant current stage (current error amp, PWM modulator*, output switch, filter), building a few, then wiring them in parallel, driven by a common volt amp so they all track the same current.
*Bonus if the clock signals are phase shifted, so you get the benefit of multiphase, but this isn't strictly necessary. There's still some advantage from using modest size modules in parallel, versus one large ungainly module.
Tim