electrical problem

I mean after the the voltage got to 13.5 the converter stopped charging. After a couple hours the battery voltage settled at 12.6. Then over a time (days-week) the battery voltage went down but the converter never started maintaining or charging the batteries again. I also tried the converter on (1) 12 volt battery same results. Answer to question. I don't have a problem replacing one battery at a time. The shop that I bought the replacement battery from and that tested my other batteries, didn't even try to sell me new ones even after I brought up the same question. Thank you all for the information. I think I see the purchase of a Progressive Dynamics Converter in my immediate future.