"Note that periodic fast charging can help restore usable capacity after periods of constant slow charging after deep discharge cycles."
Yeah, but a recommendation doesn't help much if I don't know the basis of why it's recommended.
Given that the recommendation doesn't seem to make sense, what I'm trying to understand is if my use case differs from the the use case assumed by the recommendation.
In order to get maximum cycle life from your Firefly batteries they suggest a “minimum” charge current. Your solar cannot provide this, so it will be up to your shore charger or alternator to provide the beneficial high charge current. This high charge-charge current is also a major part of the "Firefly Restoration Charge" protocol..
Firefly is not the only AGM maker to suggest high charge currents being beneficial to longevity, Odyssey recommends .4C min, Lifeline .2C min (higher is preferred) etc.. Fast charging does make a difference in longevity and is quite critical with Firefly, especially in reversing the effects of PCoC abuse when doing a restoration charge
.. The bolded portion of post 12 explains it.
"If deep cycling, ideally once or twice per week, the batteries should be charged at a current of 0.4C or more (40 Amps for a 100 Ah battery), or at least 25A per battery.
If you don’t wish to charge the Firefly batteries as they recommend, eg; sufficient charge current, all charge sources eg; solar, wind, alternator, shore charger, are temp compensated, programmed for proper float and absorption voltages then you may want to consider less expensive batteries or ones that prefer slow charging such as thick plate flooded batteries. Start building the infrastructure now, (alternator/regulator, shore charger, solar etc.) with the money you saved on the Firefly's, and when the deep-cycle flooded batteries die you'll be better prepared for AGM batteries.
Like with most AGM batteries the infrastructure to support them, shore charger, solar, alternator/regulator etc. is actually the most important aspect, that you can really control, in regards to longevity.