THE BIT THEY DIDN'T PUT ON THE BOX.
The little silver pouch behind the smart price.
When a smart home thermostat battery fails out of warranty, official replacement kits or modules cost £35–£50. Reading the universal dimension code stamped on the silver cell allows you to source the identical battery from hobbyist RC drone suppliers for £5–£8.
SO WHAT'S THE CATCH?
Lithium cells require correct protection circuit, polarity and qualified handling.
“The interesting bit isn't the badge. It's what happens when you search for the part behind it.”
THE INTERNET DETECTIVE'S CHECKLIST
Look for complete part numbers and revisions, make sure you're comparing the same quantity, and check the exact model/application. That's especially important with electrical, braking, steering, battery and other safety-related components.