85 DL 270,000 miles

Weather is very cold this week
on monday at 23ºF it would crank for about 20 secs before the battery died
got a jump and it fired right up - check the ditributor cap later at work and the center post (which comes from the coil) was melted away on the inside, forcing the current to jump about 1/4" before it reaches the rotor - how did it even run?
i put a new rotor and dist cap on about 6 months ago
but later in the day it did start just fine with the old cap
so i put on a new cap and it started a little easier
tuesday, wednesday and thursday mornings were about 28-34ºF and it still started pretty easily
this morning at 18ºF was about 20-30 secs of cranking before battery wimped out
i do have a voltage gauge which shows that i continually generate just under the 16V orange area with no loads and just over half on the gauge with much load
the battery is about 2 and a half years old

what would cause distributor cap post to melt away?
could the damaged distibutor have caused the coil to burn itself out?
how do i check for a burned up coil?
Might I have a battery which was only sold to me by size and no also by CCA?
how many CCA is recommended for the B230F in the 240 (No A/C)?
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big rich in arkansas '62 544 132k, 85 244 256k, 95 855 142k http://richardwiebe.blogspot.com/