Sizzling Battery

About a month ago I installed a leisure battery, dual battery relay, second fuse panel and a 2 bank trickle charger. I wired a separate 120v outlet branched before the breaker so that I can kill the inside outlets but still plug in and run the charger. It's a 2amp trickle, 8amp low battery and a 12amp extra low cycle function. I was in the garage with the door closed installing a new battery hold down and it was super quiet. I thought it was raining out side because it sounded like that faint sound of rain hitting the roof. Looked out, no rain. I stuck my head in the engine bay next to the battery and it had a sizzle and bubbling sound. I listened to the starting battery and it did the same thing. I looked at the trickle charger and it was indeed on in the 2amp position. Forgot to tell you that the charger is automatic. There are no buttons on it just lights indicating what position it is in.

I'm not sure if that is a normal sound for a charging battery. Should they sizzle/bubble when they charge? They have been running fine for a month now and they very well could have done this from the start but I have never been in this situation where it was dead quiet.

The one thing that I can say about the cycling/trickle charger is that the batteries have never been in better condition. They are holding a charge longer and longer.

My dad keeps a battery tender on his panel truck since he hardly ever drives it. It makes the exact same sound you are describing. He has been using it for about 2 years and never had any troubles with it. I think that's just a normal sound for them to make.

Here is the response I got from the manufacture.
" It's definitely normal with flooded batteries. It's called gassing... basically excess current is breaking down the water into oxygen and hydrogen
(electrolysis). The gas forms bubbles which gurgle up to the top. It happens once the battery reaches 80-90% full. Finish voltage depends on the battery. For floodeds the finish voltage should be about 2.4V to 2.46V per cell when the battery temperature is at 80° F, increase voltage 0.028V for every 10° below 80° and decrease by 0.028V for every 10° above 80°. Equalization charges go up to 2.583V to 2.6v per cell. Gel cells typically don't like going over 2.35V per cell or they will vent. You should expect voltages of of 13 to 14.24 while in the charge mode."

I monitored the voltage and the external temperatures of both batteries when they got to the final step. The volts were 13.65 and the external temps were around 78 degrees with an outside temp of 54. I timed the cycle and they only made the sound for 12 minutes at the end of the cycle. Everything seems to be on target with what the manufacturer said. I'll post if something changes.