L2 charging doesn't follow a curve like DCFC, it will maintain constant charging speed up to around 100%, maybe tailing off slightly at 98-99% for cell balancing. So, utility costs shouldn't be a consideration. The logic is, DCFC generates heat from higher current (amps) and needs to slow the pace. At 90% or more, the charging speed drops to near the speed of AC charging. So with AC charging, you never have to slow down until you get to the cell balancing phase.
On lower powered DCFC, the charging doesn't typically slow until higher state of charge. The following chart applies to the original 60kWh packs in older Bolts, but the concept is similar with newer packs, but follows a curve rather than step downs. Notice the higher SOC at lower kW charging speeds before the step downs occur.
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When charged > 90%, regen will be limited for the first part of your journey.
Lots of articles advise not charging to 100%, that may or may not be as true for EVs as for laptops and other devices that lack thermal conditioning, but why risk it? If 80% is adequate for daily needs, use it. When planning a longer trip, charge to 100%, and if possible target finishing the charge as close to departure time as possible to avoid sitting at 100% for lengthy periods.
There may be headroom at the top, above 100%. But it is not accessible, and some who have dug in deep claim Bolt has very little room past 100%. It isn't like volume measurements, it is a voltage question. Pack voltages are kept within "safe" ranges so as to not stress cells.
The 80% suggestion for public charging is as much about slow DCFC past 80% as it is opening access for others. On trips, your strategy plays out better by making more stops and keeping SOC in the faster part of the charging curve. Charging from 80-100% on DCFC probably takes more time than 20-60% in most cases, so you will make better time avoiding >80% in most cases.