Slow charging and sudden battery life loss

Wound up taking my computer to the Apple Store since they had an opening. They will spend quite a bit of time giving you their disclaimer that they won't do any repairs for you since you've installed 3rd party parts into the system - ignore this. The battery replacement is a ridiculously easy job, and you don't want to be paying them to undo a couple screws anyway.

Anyhow, they ran a test that confirmed the problem was with the battery. It took maybe five minutes, and the same test also verified that everything else was in good working order (logic board, MagSafe port, charging adapter, etc). I also wound up getting my replacement battery on warranty from iFixit later that day, and the early returns on this new battery are encouraging so far.

Lastly, I want to add that I learned it's supposed to take about 4 hours for your MBP to fully charge from empty. The first 0% to 80% is done via "fast charging", and is supposed to take about 2 hours. The last 80% to 100% is done via "trickle charging", and is supposed to take another 2 hours. The whole point of this trickle charging is to avoid damaging the battery with overcharge once it's near capacity, and is a logical explanation why you'll have much lower wattage when charging during those conditions. I've verified this with constant observation using CoconutBattery, and my MBP now takes 4 hours or less to charge.

Hopefully this info helps everybody who came to this thread with a similar issue. Don't assume it's the logic board right away - that's a PRICEY part, and a hard fix, if I'm not mistaken. The Apple Store won't charge you anything to diagnose the problem, so take advantage of that if you can.