A 3 stage converter can take up to 3 days on campground/generator power to recharge a depleted deep discharge battery.
The reason is as the stages step down charge rate to prevent boiling of the battery, the amperage available drops into the milli-amp range.
Here is a graph showing why that happens.
In "BULK" stage 1 - a depleted battery has plenty of surface available to take electrons and can absorb them at a high rate (upwards of 20 amps at 14.4 volts). If the charger maintained this high rate, the available surface area would clog up as it takes time and power for those electrons to be pushed deeper in the thick lead plate of a deep discharge battery. BULK can only be maintained to about 50% capacity (about 4 hours of charging) or the excess amperage would be converted to heat and the water/electrolyte would be boiled from the battery. As the "surface" voltage increases, the converter detects this and steps the charge rate down to "ABSORPTION" Stage 2.
In Stage 2 (ABSORBTION), the voltage drops to about 13.6 volts and amperage drops to about 2 amps. This is the NORMAL mode of your converter. It will charge the remaining 50% and run all your 12 volt DC camper items. It can take upwards of 24 hours to charge the battery to 90% of capacity depending on how much other demand is on the converter (furnace etc).
If the RV is in an essentially "No Load" condition and the battery is above 90% of rated capacity, the converter will switch to stage 3 or "FLOAT" mode. In FLOAT, the output voltage drops to 13.2 voltage and will trickle charge (about 500 milliamps to the battery plus camper parasite draw) to replace the remaining 10% of capacity. Any demand on the RV will automatically switch the converter back to stage 3 and the voltage will ramp back up to 13.6 volts.
To return a 12 volt deeply discharged battery to 100% can take 3-5 days on external power depending on RV DC usage at the time.
Discharging a deep discharge battery below 50% will result permanent damage to the plates and permanently reduce the maximum capacity of the battery.
Recharging when the battery drops to about 80% capacity. (The "GOOD" light just goes out is 50%!), will result in the best battery life of these expensive batteries.
Lou and Laura with Bella - German Short Hair Pointer
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HAM CALLSIGN - KC3FFW