The energy it has stored escapes gradually: this is called 'self-discharging'. If it remains completely discharged for a long period of time, irreparable damage can occur due to chemical reactions inside the battery. Refer to your battery's user manual for information on what to do if you do not intend to use it for a period of time. Recommendations vary depending on the type of battery.
More and more people are switching to electric bikes for a lot of practical reasons. It provides convenience because you do not have to wait in traffic or look for the perfect parking spot. That saves you time and money for gas. As for health benefits, you can squeeze in some exercise and manually pedal your way to work then switch to the electric motor when you get tired.
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