Under normal operating conditions with proper HVAC system wiring, the internal rechargeable lithium-ion battery in a Nest Thermostat E should last indefinitely (often 10 to 15 years) because it automatically trickle-charges from the low-voltage 24V furnace wires. However, if installed without a dedicated C-wire in a home with power-stealing limitations, the battery may drain completely every few weeks or months, requiring manual micro-USB recharging. Verification: Check the device battery voltage status in your settings menu to ensure it maintains a healthy continuous charge above 3.7 volts.
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