I recently installed a Blue Sea digital voltmeter with a momentary battery selector toggle switch. My plan is to check the battery voltage when arriving at the boat, prior to turning on any loads, to check battery voltage/SOC. I'll keep in mind the elapsed time since the battery charger was last on or motor last run, to account for surface charge.
(I don't leave the shore power plugged in when not at the boat)
If this voltage is not 12.72V or slightly higher, after two or three days with no loads, the batteries are suffering from incomplete charging.
I believe this will give me an idea if I'm slowly discharging the batteries or if the 3/4 to 1 hour of run time the engine gets when I day sail keeps up with my amp draw while sailing.
3/4 to 1 hour is nowhere near sufficient time to
fully recharge a flooded battery. Shallow discharges suffer from upper end Coulombic efficiency issues and despite perhaps only being at 90% SOC it still takes many, many hours to get back to 100% SOC regardless of the charge amperage available.
I've considered the Balmar Smart Gauge, which I believe does the same thing but much more accurately. I just question if I need to spend the money on it considering my use of the boat.
I mostly day sail for 4 to 7 hours, 1 to 3 days a week. Average motoring time per sail is 3/4 to 1 hour. When sailing I use AP, Chartplotter, Depth, Speed, Wind, i70S and usually an iDevice is charging.
Don't discharge below 12.2V "
under an average house load" and charge as long as you reasonably can and then let the shore charger take care of it back at the dock.
I do plan to leave the shore power plugged in overnight once every 2 or 3 weeks to completely charge the batteries to 100%, especially if I spend a night or two on the hook.
If you don't want to plug into shore power then your best investment would be a 30W to 100W Solar array. 2-3 weeks between 100% SOC is a recipe for short battery life.
With the Balmar Smart Gauge costing about $50 more than my house bank of two GC2 batteries, I question the value added by installing one.
Based on your described use just plug her in and recharge after each use. Don't go below 12.2V, charge at the correct voltages temp compensated, equalize and maintain the bank and they can last 6-10+ years.
The Smartgauge and SG200 are excellent products, the SG200 actually adds state of health and other excellent features, and it is less money. Course if you have the opportunity to recharge after each use, & you don't dip below 12.2V very often, then there are other things you could spend money on.