Your Stryd uses a lithium-ion battery. Lithium-ion batteries are great, and used in many devices such as phones, and have some limitations.
If you want to make yours last longer, here are some things to keep in mind:
Never leave your Stryd for multiple months without charging! Lithium-ion chemistry prefers partial discharge to deep discharge, so it's best to avoid taking the battery all the way down to zero. Since lithium-ion chemistry does not have a "memory", you do not harm the battery pack with a partial discharge. If the voltage of a lithium-ion cell drops below a certain level, it's ruined. You can avoid problems if you place your Stryd on the charger once it starts flashing.
Long-Term Storage of Stryd. Fully charger your Stryd and then do a hard reset with the Stryd app on your phone. This will put Stryd in a mode where it cannot be accidentally activated and the power is switched off completely. Put Stryd in a drawer or on your shoe. Once you are ready to run, place Stryd on the charger to reactivate it.
Lithium-ion batteries age. They only last a limited number of years, even if they are sitting on a shelf unused. Do not "avoid using" the battery with the thought that the battery pack will last many more years. It won't. You can avoid problems if you place your Stryd on the charger once it starts flashing.