On Jun 12, 2022, at 12:33 AM, Bill Kinney <cruisingconsulting@...> wrote:

Porter,

The need for a regular return to absorption is less important for AGM than it is for Flooded batteries.  With a flooded battery the acid can stratify over time.  The more concentrated acid sinks to the bottom of the cell, and lighter, less concentrated, acid floats on top. Bringing the battery to absorption voltage where it 'gases" for a bit mixes the acid again. Since the acid in an AGM is not a free flowing liquid this isn't an issue and they are fine holding (more or less) indefinitely at the manufacturers recommended float voltage.

BUT, if solar is your only charge source, you aren't really "floating" but cycling--a little bit--every day.  If you have Victron MPPT controllers, their default setting for "variable absorption" will work well as the only charging source.  They look at the system voltage when they first wake up in the AM, and estimate the battery SOC from that, then pick an absorption time to match.  I don't know that you can do much better than that unless you have a Venus system and can use the DVCC function.  Certainly the risk of damaging overcharging is minimal.   If you force the MPPT to stick to the float voltage only, you risk a slow walk-down of the SOC of the batteries with each night, or a string of cloudy days.

The amount of actual charging that can occur at the float voltage is small. I think you are better off topping them up every day and keeping the batteries full instead of letting them drift at a lower SOC that you have less control over. 

All this assumes you have some significant power draw.  If everything is off, and the normal power overnight power draw is just a couple of A-hrs, then it matters less.

Bill Kinney
SM160, Harmonie
Le Marin, Martinique