(ET) Bad luck? bad lot? bad owner?

[tbamc <tbamc gbta net>]

I bought 6 new US 2200 batteries in the summer of 2004, when I got my 
E-12.  They don't get much below 50% SOC before being recharged, I watch 
water levels carefully, check full pack voltage fairly frequently 
(during and after charging),  occasionally check individual battery 
voltage, and don't have any accessory use that would unbalance the 
pack.  I charge the pack before winter storage, and usually check it 
mid-winter to be sure it's ok.

Last year when brought out of storage I found one battery at just over 4 
volts.  Couldn't charge it with a 6v charger and presumed a cell was 
gone, so replaced it.  Thought it just unlucky.

Same story this year.  Found one battery at about 4v, 6-8 weeks ago.  
Got a replacement battery last week.  After installing it, found ANOTHER 
battery at 4v.  All others 6.1 to 6.3.  I'm fairly certain I checked 
each battery 8 weeks ago, when I found the bad one, and the 'newer' bad 
one was ok, at that time.  In addition, during that 8 weeks, the bad 
battery was removed from the E-T.

So, 3 bad batteries in a pack that's been used for only 4 summers.  Is 
there something I'm doing, to cause this?  Do I need to look for a fault 
in the machine?

BTW, FWIW when I bought a new US2200 recently the dealer mentioned that 
the more recent US  Batttery (plate?) technology is now very similar to 
Trojan's.

Thanks.

Thon