OK, here's a challenge...

I accidentally left the headlights on in my '83 and drained the battery.

Jumped it, got it home, trickle-charged it, all good. This has happened in years past.

But this time is different: ever since, if I use my headlights during my commute (30 min each way, mostly highway) I notice I drain my battery to the point of needing a trickle-charge or jump within a week.

If I don't use my lights, car seems fine.

I've checked the alternator, am getting a steady 13.8/13.9 V @ 2000 rpm, 13.4 V at idle.

I've checked the battery, which is admittedly probably too old -- my understanding is that if I draw half of the CCA for 15 seconds at ambient the voltage from 12.6V should not be more than 3 V or to 9.6V -- mine drops to 8.34V. But if it were just an old battery, why would draining it have caused what I'm seeing? And why would my car still do fine when I'm running without headlights?

Any thoughts? Next steps to troubleshoot etc? (I don't want to stick a new battery in if something is shorting and will kill it too!)

-K