Keeping a portable pegged with high cpu usage for long transcodes could heat the battery up which could in turn reduce the batteries life.
BUT given how tech in machines has moved on you won't notice a difference unless your providing the thing with no air circulation an literally cooking the thing.
These days batteries will last 3-5 years no matter what you do, just use the computer for what you bought it for (anything else and it is pointless owning it) and don't worry about it, it'll tell you when the battery needs replacing and then you get apple to replace it.
Thanks for the answers. It heats up a little bit when using handbreak. And the CPU is then at around 95%. But it cools down almost instantly after the encoding. I have the computer on my desk next to an open window so it is constantly getting cold air. (around 34°F outside). I just wanted to be sure that handbreak does not shorten the battery life.
Thanks