I plugged a 20A, 8 stage charger into my 2 x 130Ah 12V AGM battery bank and immediately I could hear electrolyte flowing and bubbling inside the battery. The battery bank was full when I plugged the charger in. I was just testing it out when I discovered this unusual behaviour that I have not seen before with other chargers. I usually use a 15A smart charger which charges silently. After an hour the charger had only reached stage 4, the unit was extremely hot and the bubbling sound had escalated. I turned it off but am concerned that this may have overcharged and caused damage to my batteries.
Is this normal behaviour or is it something to be concerned about?
If so, what is the likely cause and what is the likelihood that damage has been caused to my batteries?