Most portable power supplies come with lithium-ion batteries. You might not use the batteries continuously, but they are still likely to wear and tear. These batteries usually do not have a very long lifespan. They will probably last for a maximum of two to three years at full capacity even if you use them sparingly. You will begin to notice a dip in the performance of the device, and that is when you will know that the batteries have to be replaced. Do not wait for the batteries to stop working completely.