diff --git a/ecobee/auth.go b/ecobee/auth.go
index f5da038..70f798c 100644
--- a/ecobee/auth.go
+++ b/ecobee/auth.go
@@ -31,17 +31,40 @@ import (
 var Scopes = []string{"smartRead", "smartWrite"}
 
 type tokenSource struct {
-	token               oauth2.Token
-	cacheFile, clientID string
+	token     oauth2.Token
+	clientID  string
+	authCache AuthCache
+}
+
+// AuthCache encapsulates persistent cache for OAuth2 token.
+type AuthCache interface {
+	LoadTokenData() ([]byte, error)
+	SaveTokenData(data []byte) error
+}
+
+type fileAuthCache struct {
+	fileName string
+}
+
+func (p fileAuthCache) LoadTokenData() ([]byte, error) {
+	return ioutil.ReadFile(p.fileName)
+}
+
+func (p fileAuthCache) SaveTokenData(data []byte) error {
+	return ioutil.WriteFile(p.fileName, data, 0777)
 }
 
 func TokenSource(clientID, cacheFile string) oauth2.TokenSource {
-	return oauth2.ReuseTokenSource(nil, newTokenSource(clientID, cacheFile))
+	return TokenSourceWithAuthCache(clientID, fileAuthCache{cacheFile})
+}
+
+func TokenSourceWithAuthCache(clientID string, authCache AuthCache) oauth2.TokenSource {
+	return oauth2.ReuseTokenSource(nil, newTokenSource(clientID, authCache))
 }
 
-func newTokenSource(clientID, cacheFile string) *tokenSource {
-	file, err := ioutil.ReadFile(cacheFile)
-	ets := tokenSource{clientID: clientID, cacheFile: cacheFile}
+func newTokenSource(clientID string, authCache AuthCache) *tokenSource {
+	file, err := authCache.LoadTokenData()
+	ets := tokenSource{clientID: clientID, authCache: authCache}
 	if err != nil {
 		// no file, corrupted, or other problem: just start with an
 		// empty token.
@@ -56,8 +79,7 @@ func (ts *tokenSource) save() error {
 	if err != nil {
 		return err
 	}
-	err = ioutil.WriteFile(ts.cacheFile, d, 0777)
-	return err
+	return ts.authCache.SaveTokenData(d)
 }
 
 func (ts *tokenSource) firstAuth() error {
@@ -205,7 +227,15 @@ type Client struct {
 // Application Key.
 // (https://www.ecobee.com/consumerportal/index.html#/dev)
 func NewClient(clientID, authCache string) *Client {
+	return NewClientWithAuthCache(clientID, fileAuthCache{authCache})
+}
+
+// NewClientWithAuthCache creates a Ecobee API client for the specific clientID
+// (Application Key) and cusom authentication token cache.
+// Use the Ecobee Developer Portal to create the Application Key.
+// (https://www.ecobee.com/consumerportal/index.html#/dev)
+func NewClientWithAuthCache(clientID string, authCache AuthCache) *Client {
 	return &Client{oauth2.NewClient(
 		context.Background(),
-		TokenSource(clientID, authCache))}
+		TokenSourceWithAuthCache(clientID, authCache))}
 }