MacBook – charging the Macbook Pro (85w) with the Macbook Air (45w) charger

The fact that your computer shuts down at 50% remaining charge and also that your MagSafe LED remains orange makes me think that either the replacement battery isn't an official Apple part, that it is defective/DOA, or that your logic boards SMC is corrupted and needs to be reset.

If the battery is indeed new and Apple original than proceed with an SMC reset:

http://support.apple.com/kb/ht3964

This will tell the SMC chip to reset all power states and reboot so that it forgets the battery and MagSafe adapter momentarily and then rediscovers them.

Hope that helps

Unless you have had the battery replaced in the last few years it’s most likely dead. You can use a couple of different apps(Battery Health, Coconut Battery), or even the built in System Info screen to see the charge cycles and condition of the MacBook's battery.

I helped a friend with almost the same identical problem on a system of the same vintage, and it was the battery that was dead. No matter how many times he reset the SMC, it never helped.

As a side note, Apple recommends that you cycle the battery fully at lest once a month to avoid potential problems that can occur form charging the battery to soon in a battery discharge cycle. So if you are always plugging into power when you have only 30% of your battery drained, you could be shortening the life of the battery.