It seems that there is a mode of battery failure in which the battery will show the full 12.6+ volts of a full charge, and even be able to deliver a small amount of current for a short time before collapsing completely, only to show full charge moments after the load is removed.
I had the battery on my TT fail in this way. Meter checking voltage only showed full charge. But it would only keep a couple house lights on for a few minutes. Took it to one of the auto stores and their volt meter also showed full charge, but a load test revealed the truth: it had almost no capacity to deliver.
I wound up having to replace that battery. I still don't know how, exactly, it came to be in that state or what I could have done to prevent it.
