I remember that this is an issue with those older 2011 MacBook Pros. I can't honestly remember if there are any great solutions to this, and while it wasn't too bad at the time (generally they would just hold their charge), it is probably worse now that users are playing back more HD content. If I remember correctly, those machines can only decode a single HD video with GPU acceleration, so the other will likely be using the CPU (which will drain the battery faster). I think that the 2012 models were the first to have decent GPU accelerated video decoding available to them.
