howdy

i have an 83.5 westy in western wisconsin.  it's been cold, but the van has
started every time.  once its running and i turn on a few of the electrical
gadgets like the heater fans and headlights, the electrical system seems to
overload.  a symptom of this is if I turn all the doohickeys on at once, the
radio will turn off.  also my analog clock slows or even stops in these
conditions.  checking the volts at the battery while the engine was running
read around 13.

is this the sign of a weak battery?
are there other things I'm missing?
is this normal?

thanks in advance,
aaron in ec