Here is a prototype for a secondary regulator:
View attachment 92698
I start with the battery at 13.5V (discharged). There are some bike loads R7. When the bike starts, the existing charging system puts out an initial 18A, but the voltage at V(in) is dragged down by the loads and battery. The big PMOS M1 is fully turned on, so based on its Rds, V(out) follows V(in). The 18A available from the alternator divides, about 13A goes into the battery I(R6), and the rest into R7.
The battery voltage is increasing. When it reaches 14.4V, the input to the TL431 exceeds 2.495V, and it turns on, pulling down on the base of Q1. That begins turning off M1 at about 13s in the simulation run (actual times are arbitrary). Note that D1 is being used a ~2v Zener.
As M1 turns off, V(in) rises to its max available 14.8V. Meanwhile, the current into the battery tapers to ~zero while the load current through R7 is maintained constant.
This is just the prototype, not shown is key switching to prevent battery discharge while the bike is parked, and an adjustable sensing network.