The biggest problem is higher internal heat degrades batteries, whether that is input or output. Output side you can say has a wider pipeline in the design of a battery. Input side (charging) is more restrictive and the battery resists high input current naturally. Analogy, output of a battery is like a fire hose, where input side into a battery in charging is like using an air compressor to fill a soda can that only has a pin hole. Both still generate heat the higher the current in or out, just input current is much lower.
Max CDR output, say 20amps with an LG HG2, is enough to overload most residential electrical circuits in a house as most Breakers/Fuses are rated at 15 to 20 amps. That 15 to 20amps has to be distributed between your clocks, tvs, lights, etc. Only 2 to 3 common circuits in a house's/apartment's wiring is for the Stove, Microwave (inside the residence), and for the AC Unit/Heat Pump/Air Handler Motors (HVAC circuit outside most times), which can be 30amps. To charge at those currents for theoretical discussion, you'd not be able to have any other electrical device connected to the circuit you are charging a single HG2 battery in that instance or you would pop breakers/fuses, the circuit doing the charging, and potentially the main breaker all other breakers are connected to which is most times 50amps (2 trees of 25amps each). Some batteries if you read their data sheets have a max input charge of 4A, but at that current and the battery natural resistance to power input, a battery can easily reach 80C even higher, where full output, again saying a 20A CDR full discharge the battery will reach again 80C though at 5X the current compared to the input side. Both input and output, if the batteries reach that 80C internal temperature, they degrade, and degrade very rapidly due to that amount of thermal abuse alone. 18650, suggested daily charging 500mah (1/2A) is suggested, rapid charge 1A max, 2A can really tax them, 26650 gentle charge at 1A, rapid at 2A, full 4A would tax the 26650 as bad as a 18650 at 2A.
As posted in the earlier thread, suggested charge rates are for safety and battery longevity overall, just physics and technology limits of current batteries.
