Very much reminds me of the battery that came with our new van. Never showed a voltage much above 11.8v on the Sarjent charger unit and I just assumed that was normal. Did seem to start OK, but failed in a spectacular fashion after 12 months as we were about to go on a ferry. Got push started, spent the night crapping myself that it wouldn't start in the morning ( which, thankfully, it did). I was convinced that it couldn't possibly be the battery because it was so new. Had fuses blow when it refused to start, so again convinced myself it was down to something other than the battery. Anyway, died on a permanent basis rather fortuitously in the car park of a French hypermarket where there was an entire aisle of replacement batteries. No more than a 10 minute job to replace. Immediate 12.4v, and 2 years on not a hint of a problem. Moral: never assume something works properly just because its new........