Jeremy,
From your last post, it sounds like your "problem" that you are referring to, is a low water level in your house batteries only. If so, you can not blame that or the alternator failure, on a bad cell in one of your cranking batteries.

The water level in your house batteries should be checked every month because some batteries need to be refilled that often. If you are finding your house batteries "very low" on water you are not checking them often enough.

Gerald  

Hi Gerald,
You are right. I have not checked the house batteries often enough.  They need topping up more frequently than the house battery bank on my sailboat.  I am going to install a system that will top up the batteries automatically.
However, in mitigation, one of the starting batteries did have a dead cell,  Interstate replaced both 31 MHD 's today under warranty.  I am heading to Ottawa Canada tomorrow.  It will be interesting to see how the voltmeter behaves.
Could one dead cell in a starting battery cause the alternator to overcharge the house batteries??  

Jeremy.
One dead cell in a cranking battery can not cause the 16V overcharging of your batteries that you reported earlier. However the 16V overcharging could be the reason that the battery failed.

The voltage regulator in the alternator is the only thing that prevents the alternator from overcharging, and a bad cell in one cranking battery can not cause it to fail and overcharge. There is a possibility that a shorted battery can require so many amps that the alternator will overheat and fail, but that is very difficult to prove. It gets down to the old question what happened first, like the chicken and the egg, but that is only on a shorted battery.

Gerald

Good morning Gerald.
   You are probably right.   I have not been checking the batteries as often as I should. I had no idea the 6 volt batteries required such frequent attention . I have a similar set up on my sailboat and these 6x6volt batteries need topping up monthly.....and even then only a little.    The coach batteries are operating in a hotter environment to cause the electrolite to evaporate so fast.
Thank you for your input. Invaluable as always.   Jeremy