An HVAC controller may include a first sub-assembly and a second sub-assembly releasably securable to the first sub-assembly. The first sub-assembly may include a housing having the back and a cover, a printed circuit board at least substantially enclosed by the housing, a battery compartment formed in the back of the housing for receiving one or more batteries, and a display exposed for viewing through an aperture in the cover of the housing for viewing the display by a user. The printed circuit board may include one or more battery terminals that extend from the printed circuit board through the back of the housing and into the battery compartment. In some cases, the battery compartment may define a cup-shaped reservoir that may be configured to collect leakage from one or more of the batteries.