The Model EWT-BC is an optional wall charger for the Model EWT-3900 Wireless Thermostat
The Model EWT-BC is an optional wall charger for the Model EWT-3900 Wireless Thermostat For more information: http://www.ewccontrols.com/acrobat/090375a0236.pdf

The EWT-BC is the black module to recharge the EWT-3900 thermostats.
THE BLACK CABLE IS NOT INCLUDED WITH REPLACEMENT CHARGING HEAD. REUSE THE CABLE THAT CAME WITH YOUR ORIGINAL THERMOSTAT PURCHASE.