"Safe" (but slower) charge method for AGM batteries?
It is fine to charge 12V AGM batteries with constant voltage of 13.6V or so. The exact voltage may depend somewhat on the specific battery model, and temperature of the battery. At higher temperatures, you will definitely need to reduce the voltage. Otherwise the battery life will definitely be reduced. This type of charging is often referred to as "float" charging, even though in one of your comments you said that it is "not float mode." I get what you are trying to say. Another term for it is "standby mode" or "standby use."

Here is a chart for lifeline AGM batteries which I found Here.


One thing to consider if you are using a power supply is that there should probably be a current limit also. Once discharged, an AGM battery may accept very large currents if you apply 13.6V. What will the supply do? Ideally it would transition to constant current mode until the voltage reaches 13.6V then transition to constant voltage. But it may also shut down, or overheat or who knows what. So, you need to attend to that detail.

Will the AGM battery be fully charged by this process? It may take a long time to actually get to 100% charge. Or it may never get there. But by carefully setting the CV voltage point, it should get very close.

If you are unable to adjust for battery temperature, the safest thing would be to use a slightly lower "float" voltage. Also, read up on your specific battery. This chart is for one brand of batteries. Others may be slightly different.