More alternator fun

[probedude]

'Automotive' lead acid batteries do not like to be deep cycled.  Deep
cycling would be say below 50% depth of charge.  You'll probably only get
20-40 cycles from an automotive battery with that regime.

  Deep cycle or golf kart batteries are designed to give many more cycles to
a much greater depth of discharge (hundreds of cycles down to 20% DOC) but
they are not able to give burst high current power - which is what an
automotive batt is designed to do.

You said your battery was fairly new?  It doesn't sound like you've depleted
it too many times.