Rechargeable lithium-ion batteries are commonly used in everyday life. When these batteries are connected together to form a battery pack, differences between individual cells can cause problems. This can lead to the battery pack shutting off early and make it hard to estimate how much charge it has left. A new method has been developed to identify the most important cells in a battery pack and accurately estimate its state of charge. Testing on a ten-cell battery pack showed that this method successfully identified the key cells and improved the accuracy of state of charge estimates.