Internet enabled body area networks (BANs) will form a core part of future Ambient Assisted Living (AAL) technology. In BAN based AAL applications, due to the dynamic nature of human behavior, the off-body BAN channel can be prone to deep fading phenomenon caused by body shadowing and multipath fading. This emphasizes the necessity of a power control mechanism, especially in AAL applications where battery replacement is difficult or undesirable. To that end, we present a novel practical adaptive power control protocol based on the knowledge of the channel deviation to simultaneously prolong the lifetime of wearable devices and reduce outage probability. The proposed scheme is both flexible and relatively simple to implement on hardware platforms with constrained resources making it inherently suitable for AAL applications. We also profile its performance against traditional, optimal and other existing power control schemes. It is demonstrated that not only does the outage probability reduce significantly, but the proposed algorithm also saves between 20-60% average energy consumption compared to the competing schemes.