[wy_white_wolf]

...
The equalization charge will remove the sulfation that has built up on the plates due to the low voltage. The advice I need is, how long? I know testing the specific gravity of electrolyte solution tells you when to stop. There are many variables to the time required, including the size of the battery bank. One article I read suggested 10 or 15 hours might be required. Is this true? Does it need to be maintained for the full 15 hour? Or, should I do 5 hours and the test the electrolyte? Then do 4 more and test, etc.

I have never been in this situation before. Any suggestions or advice would be greatly appreciated.

You should check the SG hourly until they all reach the desired level. If one is lagging far behind the others you should apply an equalize charge to it individually to catch it up without having to damage the others. Depending on how bad they are sulfated you may never get some to equalize.

The 10.5 volt cutout on the inverter is to protect the inverter, not your batteries.

WWW

 

[wy_white_wolf]

Also what is your normal charge rate and equalization charge rate in comparison to the battery AH rating?

WWW

 

[wy_white_wolf]

The problem isn't/wasn't your inverter. The problem is a poorly designed system. At 12v that makes a 4356Ah battery bank. There is no way you could apply a 400+ amp charge rate to properly charge them. They sulfated over time because of that. You just never noticed the problem until recently. A partially sulfated battery will show voltage as fully charged until enough of the plates are covered that they cannot maintain voltage under load.

WWW