Thanks for posting TNriverjet. When I went looking for 6TMF batteries, the NAPA guy thought they could get batteries of same manufacturing date. I had told him not to gather batteries from different places of different ages because it's necessary to have similar batteries in a series-parallel system. My four new batteries are not only just a couple of weeks old but had consecutive serial numbers (beyond my expections). Try that at Walmart.
But from responses above we should have used different batteries, and our 6TMFs are crap. But they are consecutively numbered crap, and absolutely no mods had to be done to install them...and they are purty...
The real question is: why do the upper pairs of batteries on the FMTV's go bad first. I suspect it's the nature of the battery's ground reference of the 'upper' batteries. The charge voltage for that pair is referenced to chassis ground in the alternator but the actual physical negative terminal voltage of the upper batteries is whatever the charge voltage of the "lower" pair above chassis ground, as maintained by the alternator (also chassis ground referenced).
Some recording of voltage history across the upper batteries might be interesting. If the average voltage across the upper batteries is low enough to prevent full charging, that could account for the cells going high resistance.
Rotating batteries (swapping High position to low position and vice versa) would undoubtedly help, but who has done that routinely!? That will be on my maintenance plan in the future.
But I did get 7 years out of the old 6TMF's and their history was unknown.