Abstract: A system includes a vehicle having a hybrid power train. The hybrid power train has a battery pack with a number of cells. The system further includes a heat transfer device including a shape memory alloy (SMA). The SMA in a low temperature position provides a first heat transfer environment to the battery pack, and in a high temperature position provides a second heat transfer environment to the battery pack. The first heat transfer environment provides a first heat transfer amount to the battery pack, and the second heat transfer environment provides a second heat transfer amount to the battery pack. The second heat transfer amount is greater than the first heat transfer amount.