If you are using a mod that takes more than one battery, always use the exact same batteries (for example, a pair of Sony VTC5s) and keep them married, i.e. use them in pairs and always together — it’s a safe practice. This will ensure that the cells get the same number of charges/discharges and help delay imbalances in capacity and performance
Batteries are only good up to a certain amount of cycles (full charge to full discharge). Lithium-ion batteries will start losing capacity and strength when used extensively for longer periods of time. If you sense that your battery takes less time to discharge, just replace it. Even if you haven’t noticed something out of the ordinary, replace your batteries after six months to a year, depending on how often you use them.
When replacing old batteries, always recycle them instead of throwing them away. A lithium-ion cell can easily short-circuit in the trash can, which is a serious hazard risk. On top of that, recycling batteries is good for the environment. Keep an eye for battery disposal boxes and follow safety instructions when recycling your old batteries.