Hey guys, long ago in this section (ELectrical: batteries, etc.) is a detailed discussion as to how "fast" you can charge your batteries. This comes from the Amp-hour law published first between 1920 and 1930 in an industry regognized text book and has subsequently been published and verified several times since then.
You can safely charge ANY lead-acid battery designed to perform proper duty on your boat
WITHOUT excessive gassing or temperature rise at a current equal to the number of Amp-hours "missing" from your battery. Even then, the values may be vastly exceeded for short periods of time.
This is true for gel-cell, AGM
and flooded-cell (yes, they are all still lead-acid chemistry). Just try to find an authoritative source from a battery designer
electrochemist stating than one should only charge at 25% of the Amp-hour rating of a battery. This is a myth that I read in sailing magazines and forums
like this one that keeps getting passed on like a wispy rumour.
In some cases the charge voltage will be equal to 15 Volts to initially do this (assuming that you have a current source capable of delivering such amounts). The three-step charging regimen was developed as a simple practical way as a first approximation to achieving the Amp-hour Law charge regimen.
Back to Leighton's problem: Keep in mind that if your battery has lost
significant capacity due to sulphation it will not charge accept much current and the voltage will quickly rise to 14.6V WITHOUT recoverning its lost
capacity. It may need to be equalized. There could be other problems as well yet this is my first cut at a determination. If the battery is properly equalized so that its internal resistance is low again and that it will charge accept a large current when discharged then you will discover that using the 14.6V (measured AT the battery) acceptance voltage will go a loooong way towards keeping that battery in good shape. Never let that acceptance voltage go below 14.4 V @ 20 deg C else you will be back in your present situation eventually.