(ET) One cell frozen in the whole pack

[Charlie <medievalist gmail com>]

When I was plowing yesterday, the I-5 was sluggish at first, and never
really got up to the usual level of power.  I also noticed a strange
sulfuric eggy smell at times.

When I put the charger on, it was unusually loud.  So I checked all
the batteries and I found that one of them had a single cell frozen
solid!  The other two were fine, as were the other five batteries.

I was trying to be careful about not letting the batteries get
discharged, to prevent them from freezing.  In the fall I carefully
checked every cell with a good quality temperature-compensated
hydrometer and they all looked good.  The charger and electronics are
the simpler late model type, the tractor's a 1973 I-5.

Does anybody know what happened here?  Is the battery irreparably
damaged?  What happens when I charge the pack with one cell frozen?

--Charlie