Hello David, I'm sorry I'm just seeing this now and wasn't involved while you still had your old batteries. I had four T-105's in my sailboat for years. I suspect premature cell failure and usually it's on a single cell. That's the beauty of using 6V (3-cell) instead of 12V (6-cell), you can better pinpoint the problem cell and only need to replace one of four batteries. I would have told you to do what I eventually did and was very surprised by: Run some kind of decent load like 10-20 amps, then go into the battery compartment and measure each individual battery's voltage AT THE TERMINALS, not the cable clamp/connector. The loaded batteries should show around 6.1, 6.2 Volts. I was amazed when one of mine was like 5.3 Volts, all the others ok. And I bet even within that battery it's only one of the three cells that's bad, but our resolution is 3 cells. And replacing with a single new T-105 everything was like new.
The reason you need to measure at the terminals themselves is the other likely problem would have been a high resistance connection; if you measure two batteries 6.2V and 6.2V then when you measure across all the wiring you better see 12.4V. If not it's then easy to make a few more measurements to narrow down where the bad connection = voltage drop is.
