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?

  • 1
    \$\begingroup\$ I don't think AGM is supposed to bubble noticeably. Flooded batteries will bubble a bit during charging. I don't remember actually HEARING the bubbles, though. But I have seen them. Maybe I had the cap off, or maybe I could see them through the translucent case. I don't remember. \$\endgroup\$
    – user57037
    Mar 22, 2019 at 6:17

1 Answer 1


The symptoms are bad and you should have stopped immediately.

Based on max charge energy of 20Ah or 260Wh on a full battery there be electrolyte loss, but hard for me to estimate how much. Although your capacity is 260Ah or 3.4kWh that is only 8% overage, when it cannot store more charge ! It boils electrolyte.

  • Normal maintenance test is to measure specific gravity if possible.
    • Next plot discharge rate V vs time for say C/10 constant load Amps to get capacity from specs for Voc vs SoC. (V open cct vs State of Charge).
    • But do not charge and load at the same time unless it is at float V

To me, it indicates the charger was not so smart at detecting a weak single cell and over-charged that weak cell causing H2 outgassing from a weak cell which appears to have been sulphated.

From the web I found this;

The 8 stages are:
1. Desulphation
2. Soft Start
3. Bulk
4. Absorption
5. Analyze
6. Recondition
7. Float
8. Pulse

What historical performance and age can you tell us?

From my experience on pulse desulphation designs that work, they got the sequence wrong.

I would have designed it to quickly sequence as follows 2,5,3,5,7,5 then if dV/dt steps too much with pulses, that indicates high ESR from sulphation and then continue with 2 until it improves applying 7 and 2 simultaneously.

Write to Mfg and complain with your test results and answer to above.

Although I don't know your battery specs, similar ones are below enter image description here

enter image description here

  • \$\begingroup\$ The batteries are only a few months old and hardly used. They have been well maintained and have never been discharged to less than 12V. The bubbling issue is not occurring with the 15A charger. I suspect that the issue is with the charger rather than the batteries. \$\endgroup\$
    – Guru Josh
    Mar 22, 2019 at 3:36
  • 1
    \$\begingroup\$ I agree.......... unless there were incorrect settings for battery type?? \$\endgroup\$ Mar 22, 2019 at 4:28
  • 1
    \$\begingroup\$ To the OP: Do you have a way to measure (even roughly) the battery capacity? Make sure it has not declined dramatically? \$\endgroup\$
    – user57037
    Mar 22, 2019 at 6:18
  • \$\begingroup\$ @SunnyskyguyEE75 The charger is exclusively for 12V batteries. The AGM light was illuminated on the charger indicating that the battery type was correctly identified. \$\endgroup\$
    – Guru Josh
    Mar 22, 2019 at 10:09
  • \$\begingroup\$ @mkeith The battery bank has been on a weekly scheduled charge for 2 hours and its resting voltage is 13.1V. That’s the only information I have that relates to battery capacity. How can battery health and useable capacity be accurately measured? \$\endgroup\$
    – Guru Josh
    Mar 22, 2019 at 10:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.