Yes, that's the charger I remember. Thank you.
From the
Manual, page 12:
After pasting the above, I see you also read the manual. The problem is the time based absorption phase (I did not correctly recall the information earlier when I said Bulk phase). The charger will keep the voltage high even when the battery does not need a higher voltage. This is damaging to the battery and causes the off gassing that was discussed in another thread.
Again, this method of charging, with the timed absorption stage, is not bad
if the battery is deeply discharged down to 50-60% because it will take 5 to 6 hours to fully charge from that level. Except, if the battery is not ready to go into the float stage, it will not receive the voltage it needs. This is damaging to the battery. This was a good attempt to optimize charging 30 years ago, we're way past that now in understanding batteries and charging protocols.
Good modern 3-stage battery chargers change stages based on the battery's ability to accept a charge. Briefly and simply, a modern charger will start with a high current charge at a low voltage. As the battery is charged the voltage will rise and at a predetermined voltage level the charger will maintain that voltage level as the current (amps) gradually decreases until is it only able to supply a small amount of current, when it will shift to a low current low voltage float level. If the battery is deeply discharged, the charger will take 5+ hours to go through those stages, if the battery is not deeply discharged the charger will only take a few minutes to a few (less than 5) hours to cycle through the stages. And that's the problem, with a timed charger, like the Charles 5000 series, there is too much time where the charger's output is out of sync with the battery's need. That causes the battery to seldom be charged at the correct stage and to often be charged at the incorrect stage.
The other and less critical problem with the Charles charger is the fixed settings. AGMs, Gels, and FLA batteries from different manufacturers have different "ideal" charging voltages. Some AGM manufacturers may want a 14.6v absorption, some may want a 14.2v absorption voltage. The charger can't be fined tuned to the manufacturer's recommendation, the Charles uses a best guess voltage.
If you are dead set on AGM batteries, West Marine has a clearance sale on Mastervolt AGM batteries at very attractive prices. While Mastervolt generally produces good equipment, the amphour capacities of there AGM batteries are on the low side for batteries of the same size.