Getting only 90 volts provided to your house is a clear violation of Edenorte's service agreement with you. I know, this is the DR and it isn't easy to get what you pay for, but I would file a complaint with Protecom.
I have tried everything from pure EDTA, which is the main active ingredient in Battery B'agra, to Batter B'agra itself, and it has not extended the life of any brand of batteries that I have owned. EDTA is a chemical that reportedly reduces the amount of sulphation on the battery plates. Sulphation normally happens when batteries sit around unused in a less than fully charged state. In your situation, getting only 12 hours of power every day means your batteries are either being discharged, charged, or simply catching a quick "breather" until they are used again. The damage that occurs when batteries are used in that manner is the loss of material from the plates that falls to the bottom of the battery and EDTA/B'agra will not repair that kind of problem. I will say that batteries that have remained as good as new for almost 4 years is quite remarkable, so whatever you are doing is working for you and those Trojan Black batteries.
I was told that Battery B'agra also contains some sulphuric acid. I have no idea if this is true since it was told to me by a competitor of the Battery B'agra company. EDTA is a weak organic acid, but Sulphuric acid should never be added to batteries unless there was spillage of the electrolyte.