diff --git a/healthd/Android.bp b/healthd/Android.bp
index 65eaedd..b3de9c4 100644
--- a/healthd/Android.bp
+++ b/healthd/Android.bp
@@ -243,17 +243,44 @@
 
 cc_test {
     name: "libhealthd_charger_test",
-    srcs: ["AnimationParser_test.cpp"],
-    shared_libs: [
-        "liblog",
-        "libbase",
-        "libcutils",
+    defaults: ["charger_defaults"],
+    srcs: [
+        "AnimationParser_test.cpp",
+        "healthd_mode_charger_test.cpp"
     ],
     static_libs: [
-        "libhealthd_charger",
+        "libgmock",
     ],
     test_suites: [
         "general-tests",
         "device-tests",
     ],
+    data: [
+        ":libhealthd_charger_test_data",
+    ],
+    require_root: true,
+}
+
+// /system/etc/res/images/charger/battery_fail.png
+prebuilt_etc {
+    name: "system_core_charger_res_images_battery_fail.png",
+    src: "images/battery_fail.png",
+    relative_install_path: "res/images/charger",
+    filename: "battery_fail.png",
+}
+
+// /system/etc/res/images/charger/battery_scale.png
+prebuilt_etc {
+    name: "system_core_charger_res_images_battery_scale.png",
+    src: "images/battery_scale.png",
+    relative_install_path: "res/images/charger",
+    filename: "battery_scale.png",
+}
+
+phony {
+    name: "charger_res_images",
+    required: [
+        "system_core_charger_res_images_battery_fail.png",
+        "system_core_charger_res_images_battery_scale.png",
+    ],
 }
diff --git a/healthd/Android.mk b/healthd/Android.mk
deleted file mode 100644
index 4b09cf8..0000000
--- a/healthd/Android.mk
+++ /dev/null
@@ -1,36 +0,0 @@
-# Copyright 2013 The Android Open Source Project
-
-LOCAL_PATH := $(call my-dir)
-
-ifeq ($(strip $(BOARD_CHARGER_NO_UI)),true)
-LOCAL_CHARGER_NO_UI := true
-endif
-
-### charger_res_images ###
-ifneq ($(strip $(LOCAL_CHARGER_NO_UI)),true)
-define _add-charger-image
-include $$(CLEAR_VARS)
-LOCAL_MODULE := system_core_charger_res_images_$(notdir $(1))
-LOCAL_MODULE_STEM := $(notdir $(1))
-_img_modules += $$(LOCAL_MODULE)
-LOCAL_SRC_FILES := $1
-LOCAL_MODULE_TAGS := optional
-LOCAL_MODULE_CLASS := ETC
-LOCAL_MODULE_PATH := $$(TARGET_ROOT_OUT)/res/images/charger
-include $$(BUILD_PREBUILT)
-endef
-
-_img_modules :=
-_images :=
-$(foreach _img, $(call find-subdir-subdir-files, "images", "*.png"), \
-  $(eval $(call _add-charger-image,$(_img))))
-
-include $(CLEAR_VARS)
-LOCAL_MODULE := charger_res_images
-LOCAL_MODULE_TAGS := optional
-LOCAL_REQUIRED_MODULES := $(_img_modules)
-include $(BUILD_PHONY_PACKAGE)
-
-_add-charger-image :=
-_img_modules :=
-endif # LOCAL_CHARGER_NO_UI
diff --git a/healthd/animation.h b/healthd/animation.h
index d02d7a7..c2d5f1c 100644
--- a/healthd/animation.h
+++ b/healthd/animation.h
@@ -18,6 +18,7 @@
 #define HEALTHD_ANIMATION_H
 
 #include <inttypes.h>
+
 #include <string>
 
 class GRSurface;
@@ -52,20 +53,11 @@
     // - When treating paths as relative paths, it adds ".png" suffix.
     // - When treating paths as absolute paths, it doesn't add the suffix. Hence, the suffix
     //   is added here.
-    void set_resource_root(const std::string& root) {
-        if (!animation_file.empty()) {
-            animation_file = root + animation_file + ".png";
-        }
-        if (!fail_file.empty()) {
-            fail_file = root + fail_file + ".png";
-        }
-        if (!text_clock.font_file.empty()) {
-            text_clock.font_file = root + text_clock.font_file + ".png";
-        }
-        if (!text_percent.font_file.empty()) {
-            text_percent.font_file = root + text_percent.font_file + ".png";
-        }
-    }
+    // If |backup_root| is provided, additionally check if file under |root| is accessbile or not.
+    // If not accessbile, use |backup_root| instead.
+    // Require that |root| starts and ends with "/". If |backup_root| is provided, require that
+    // |backup_root| starts and ends with "/".
+    void set_resource_root(const std::string& root, const std::string& backup_root = "");
 
     std::string animation_file;
     std::string fail_file;
diff --git a/healthd/healthd_mode_charger.cpp b/healthd/healthd_mode_charger.cpp
index 386ba1a..04a99a3 100644
--- a/healthd/healthd_mode_charger.cpp
+++ b/healthd/healthd_mode_charger.cpp
@@ -33,7 +33,9 @@
 #include <optional>
 
 #include <android-base/file.h>
+#include <android-base/logging.h>
 #include <android-base/macros.h>
+#include <android-base/strings.h>
 
 #include <linux/netlink.h>
 #include <sys/socket.h>
@@ -58,6 +60,7 @@
 #include <health2impl/Health.h>
 #include <healthd/healthd.h>
 
+using std::string_literals::operator""s;
 using namespace android;
 using android::hardware::Return;
 using android::hardware::health::GetHealthServiceOrDefault;
@@ -103,7 +106,13 @@
 
 namespace android {
 
-// Resources in /product/etc/res overrides resources in /res.
+// Legacy animation resources are loaded from this directory.
+static constexpr const char* legacy_animation_root = "/res/images/";
+
+// Built-in animation resources are loaded from this directory.
+static constexpr const char* system_animation_root = "/system/etc/res/images/";
+
+// Resources in /product/etc/res overrides resources in /res and /system/etc/res.
 // If the device is using the Generic System Image (GSI), resources may exist in
 // both paths.
 static constexpr const char* product_animation_desc_path =
@@ -625,6 +634,12 @@
         batt_anim_.set_resource_root(product_animation_root);
     } else if (base::ReadFileToString(animation_desc_path, &content)) {
         parse_success = parse_animation_desc(content, &batt_anim_);
+        // Fallback resources always exist in system_animation_root. On legacy devices with an old
+        // ramdisk image, resources may be overridden under root. For example,
+        // /res/images/charger/battery_fail.png may not be the same as
+        // system/core/healthd/images/battery_fail.png in the source tree, but is a device-specific
+        // image. Hence, load from /res, and fall back to /system/etc/res.
+        batt_anim_.set_resource_root(legacy_animation_root, system_animation_root);
     } else {
         LOGW("Could not open animation description at %s\n", animation_desc_path);
         parse_success = false;
@@ -633,13 +648,13 @@
     if (!parse_success) {
         LOGW("Could not parse animation description. Using default animation.\n");
         batt_anim_ = BASE_ANIMATION;
-        batt_anim_.animation_file.assign("charger/battery_scale");
+        batt_anim_.animation_file.assign(system_animation_root + "charger/battery_scale.png"s);
         InitDefaultAnimationFrames();
         batt_anim_.frames = owned_frames_.data();
         batt_anim_.num_frames = owned_frames_.size();
     }
     if (batt_anim_.fail_file.empty()) {
-        batt_anim_.fail_file.assign("charger/battery_fail");
+        batt_anim_.fail_file.assign(system_animation_root + "charger/battery_fail.png"s);
     }
 
     LOGV("Animation Description:\n");
@@ -678,10 +693,11 @@
 
     InitAnimation();
 
-    ret = res_create_display_surface(batt_anim_.fail_file.c_str(), &surf_unknown_);
+    ret = CreateDisplaySurface(batt_anim_.fail_file.c_str(), &surf_unknown_);
     if (ret < 0) {
         LOGE("Cannot load custom battery_fail image. Reverting to built in: %d\n", ret);
-        ret = res_create_display_surface("charger/battery_fail", &surf_unknown_);
+        ret = CreateDisplaySurface((system_animation_root + "charger/battery_fail.png"s).c_str(),
+                                   &surf_unknown_);
         if (ret < 0) {
             LOGE("Cannot load built in battery_fail image\n");
             surf_unknown_ = NULL;
@@ -692,8 +708,8 @@
     int scale_count;
     int scale_fps;  // Not in use (charger/battery_scale doesn't have FPS text
                     // chunk). We are using hard-coded frame.disp_time instead.
-    ret = res_create_multi_display_surface(batt_anim_.animation_file.c_str(), &scale_count,
-                                           &scale_fps, &scale_frames);
+    ret = CreateMultiDisplaySurface(batt_anim_.animation_file.c_str(), &scale_count, &scale_fps,
+                                    &scale_frames);
     if (ret < 0) {
         LOGE("Cannot load battery_scale image\n");
         batt_anim_.num_frames = 0;
@@ -722,6 +738,43 @@
     boot_min_cap_ = config->boot_min_cap;
 }
 
+int Charger::CreateDisplaySurface(const std::string& name, GRSurface** surface) {
+    return res_create_display_surface(name.c_str(), surface);
+}
+
+int Charger::CreateMultiDisplaySurface(const std::string& name, int* frames, int* fps,
+                                       GRSurface*** surface) {
+    return res_create_multi_display_surface(name.c_str(), frames, fps, surface);
+}
+
+void set_resource_root_for(const std::string& root, const std::string& backup_root,
+                           std::string* value) {
+    if (value->empty()) {
+        return;
+    }
+
+    std::string new_value = root + *value + ".png";
+    // If |backup_root| is provided, additionally check whether the file under |root| is
+    // accessible or not. If not accessible, fallback to file under |backup_root|.
+    if (!backup_root.empty() && access(new_value.data(), F_OK) == -1) {
+        new_value = backup_root + *value + ".png";
+    }
+
+    *value = new_value;
+}
+
+void animation::set_resource_root(const std::string& root, const std::string& backup_root) {
+    CHECK(android::base::StartsWith(root, "/") && android::base::EndsWith(root, "/"))
+            << "animation root " << root << " must start and end with /";
+    CHECK(backup_root.empty() || (android::base::StartsWith(backup_root, "/") &&
+                                  android::base::EndsWith(backup_root, "/")))
+            << "animation backup root " << backup_root << " must start and end with /";
+    set_resource_root_for(root, backup_root, &animation_file);
+    set_resource_root_for(root, backup_root, &fail_file);
+    set_resource_root_for(root, backup_root, &text_clock.font_file);
+    set_resource_root_for(root, backup_root, &text_percent.font_file);
+}
+
 }  // namespace android
 
 int healthd_charger_main(int argc, char** argv) {
diff --git a/healthd/healthd_mode_charger.h b/healthd/healthd_mode_charger.h
index 6e569ee..6f9ae8c 100644
--- a/healthd/healthd_mode_charger.h
+++ b/healthd/healthd_mode_charger.h
@@ -53,6 +53,11 @@
     // HalHealthLoop overrides
     void OnHealthInfoChanged(const HealthInfo_2_1& health_info) override;
 
+    // Allowed to be mocked for testing.
+    virtual int CreateDisplaySurface(const std::string& name, GRSurface** surface);
+    virtual int CreateMultiDisplaySurface(const std::string& name, int* frames, int* fps,
+                                          GRSurface*** surface);
+
   private:
     void InitDefaultAnimationFrames();
     void UpdateScreenState(int64_t now);
diff --git a/healthd/healthd_mode_charger_test.cpp b/healthd/healthd_mode_charger_test.cpp
new file mode 100644
index 0000000..f444f66
--- /dev/null
+++ b/healthd/healthd_mode_charger_test.cpp
@@ -0,0 +1,181 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <sysexits.h>
+#include <unistd.h>
+
+#include <iostream>
+#include <string>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/strings.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include <health/utils.h>
+
+#include "healthd_mode_charger.h"
+
+using android::hardware::Return;
+using android::hardware::health::InitHealthdConfig;
+using std::string_literals::operator""s;
+using testing::_;
+using testing::Invoke;
+using testing::NiceMock;
+using testing::StrEq;
+using testing::Test;
+
+namespace android {
+
+// A replacement to ASSERT_* to be used in a forked process. When the condition is not met,
+// print a gtest message, then exit abnormally.
+class ChildAssertHelper : public std::stringstream {
+  public:
+    ChildAssertHelper(bool res, const char* expr, const char* file, int line) : res_(res) {
+        (*this) << file << ":" << line << ": `" << expr << "` evaluates to false\n";
+    }
+    ~ChildAssertHelper() {
+        EXPECT_TRUE(res_) << str();
+        if (!res_) exit(EX_SOFTWARE);
+    }
+
+  private:
+    bool res_;
+    DISALLOW_COPY_AND_ASSIGN(ChildAssertHelper);
+};
+#define CHILD_ASSERT_TRUE(expr) ChildAssertHelper(expr, #expr, __FILE__, __LINE__)
+
+// Run |test_body| in a chroot jail in a forked process. |subdir| is a sub-directory in testdata.
+// Within |test_body|,
+// - non-fatal errors may be reported using EXPECT_* macro as usual.
+// - fatal errors must be reported using CHILD_ASSERT_TRUE macro. ASSERT_* must not be used.
+void ForkTest(const std::string& subdir, const std::function<void(void)>& test_body) {
+    pid_t pid = fork();
+    ASSERT_GE(pid, 0) << "Fork fails: " << strerror(errno);
+    if (pid == 0) {
+        // child
+        CHILD_ASSERT_TRUE(
+                chroot((android::base::GetExecutableDirectory() + "/" + subdir).c_str()) != -1)
+                << "Failed to chroot to " << subdir << ": " << strerror(errno);
+        test_body();
+        // EXPECT_* macros may set the HasFailure bit without calling exit(). Set exit status
+        // accordingly.
+        exit(::testing::Test::HasFailure() ? EX_SOFTWARE : EX_OK);
+    }
+    // parent
+    int status;
+    ASSERT_NE(-1, waitpid(pid, &status, 0)) << "waitpid() fails: " << strerror(errno);
+    ASSERT_TRUE(WIFEXITED(status)) << "Test fails, waitpid() returns " << status;
+    ASSERT_EQ(EX_OK, WEXITSTATUS(status)) << "Test fails, child process returns " << status;
+}
+
+class MockHealth : public android::hardware::health::V2_1::IHealth {
+    MOCK_METHOD(Return<::android::hardware::health::V2_0::Result>, registerCallback,
+                (const sp<::android::hardware::health::V2_0::IHealthInfoCallback>& callback));
+    MOCK_METHOD(Return<::android::hardware::health::V2_0::Result>, unregisterCallback,
+                (const sp<::android::hardware::health::V2_0::IHealthInfoCallback>& callback));
+    MOCK_METHOD(Return<::android::hardware::health::V2_0::Result>, update, ());
+    MOCK_METHOD(Return<void>, getChargeCounter, (getChargeCounter_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getCurrentNow, (getCurrentNow_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getCurrentAverage, (getCurrentAverage_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getCapacity, (getCapacity_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getEnergyCounter, (getEnergyCounter_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getChargeStatus, (getChargeStatus_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getStorageInfo, (getStorageInfo_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getDiskStats, (getDiskStats_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getHealthInfo, (getHealthInfo_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getHealthConfig, (getHealthConfig_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, getHealthInfo_2_1, (getHealthInfo_2_1_cb _hidl_cb));
+    MOCK_METHOD(Return<void>, shouldKeepScreenOn, (shouldKeepScreenOn_cb _hidl_cb));
+};
+
+class TestCharger : public Charger {
+  public:
+    // Inherit constructor.
+    using Charger::Charger;
+    // Expose protected functions to be used in tests.
+    void Init(struct healthd_config* config) override { Charger::Init(config); }
+    MOCK_METHOD(int, CreateDisplaySurface, (const std::string& name, GRSurface** surface));
+    MOCK_METHOD(int, CreateMultiDisplaySurface,
+                (const std::string& name, int* frames, int* fps, GRSurface*** surface));
+};
+
+// Intentionally leak TestCharger instance to avoid calling ~HealthLoop() because ~HealthLoop()
+// should never be called. But still verify expected calls upon destruction.
+class VerifiedTestCharger {
+  public:
+    VerifiedTestCharger(TestCharger* charger) : charger_(charger) {
+        testing::Mock::AllowLeak(charger_);
+    }
+    TestCharger& operator*() { return *charger_; }
+    TestCharger* operator->() { return charger_; }
+    ~VerifiedTestCharger() { testing::Mock::VerifyAndClearExpectations(charger_); }
+
+  private:
+    TestCharger* charger_;
+};
+
+// Do not use SetUp and TearDown of a test suite, as they will be invoked in the parent process, not
+// the child process. In particular, if the test suite contains mocks, they will not be verified in
+// the child process. Instead, create mocks within closures in each tests.
+void ExpectChargerResAt(const std::string& root) {
+    sp<NiceMock<MockHealth>> health(new NiceMock<MockHealth>());
+    VerifiedTestCharger charger(new NiceMock<TestCharger>(health));
+
+    // Only one frame in all testdata/**/animation.txt
+    GRSurface* multi[] = {nullptr};
+
+    EXPECT_CALL(*charger, CreateDisplaySurface(StrEq(root + "charger/battery_fail.png"), _))
+            .WillRepeatedly(Invoke([](const auto&, GRSurface** surface) {
+                *surface = nullptr;
+                return 0;
+            }));
+    EXPECT_CALL(*charger,
+                CreateMultiDisplaySurface(StrEq(root + "charger/battery_scale.png"), _, _, _))
+            .WillRepeatedly(Invoke([&](const auto&, int* frames, int* fps, GRSurface*** surface) {
+                *frames = arraysize(multi);
+                *fps = 60;  // Unused fps value
+                *surface = multi;
+                return 0;
+            }));
+    struct healthd_config healthd_config;
+    InitHealthdConfig(&healthd_config);
+    charger->Init(&healthd_config);
+};
+
+// Test that if resources does not exist in /res or in /product/etc/res, load from /system.
+TEST(ChargerLoadAnimationRes, Empty) {
+    ForkTest("empty", std::bind(&ExpectChargerResAt, "/system/etc/res/images/"));
+}
+
+// Test loading everything from /res
+TEST(ChargerLoadAnimationRes, Legacy) {
+    ForkTest("legacy", std::bind(&ExpectChargerResAt, "/res/images/"));
+}
+
+// Test loading animation text from /res but images from /system if images does not exist under
+// /res.
+TEST(ChargerLoadAnimationRes, LegacyTextSystemImages) {
+    ForkTest("legacy_text_system_images",
+             std::bind(&ExpectChargerResAt, "/system/etc/res/images/"));
+}
+
+// Test loading everything from /product
+TEST(ChargerLoadAnimationRes, Product) {
+    ForkTest("product", std::bind(&ExpectChargerResAt, "/product/etc/res/images/"));
+}
+
+}  // namespace android
diff --git a/healthd/testdata/Android.bp b/healthd/testdata/Android.bp
new file mode 100644
index 0000000..110c79a
--- /dev/null
+++ b/healthd/testdata/Android.bp
@@ -0,0 +1,20 @@
+//
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+filegroup {
+    name: "libhealthd_charger_test_data",
+    srcs: ["**/*.*"],
+}
diff --git a/healthd/testdata/empty/ensure_directory_creation.txt b/healthd/testdata/empty/ensure_directory_creation.txt
new file mode 100644
index 0000000..36ceff4
--- /dev/null
+++ b/healthd/testdata/empty/ensure_directory_creation.txt
@@ -0,0 +1 @@
+File is placed to ensure directory is created on the device.
diff --git a/healthd/testdata/legacy/res/images/charger/battery_fail.png b/healthd/testdata/legacy/res/images/charger/battery_fail.png
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/healthd/testdata/legacy/res/images/charger/battery_fail.png
diff --git a/healthd/testdata/legacy/res/images/charger/battery_scale.png b/healthd/testdata/legacy/res/images/charger/battery_scale.png
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/healthd/testdata/legacy/res/images/charger/battery_scale.png
diff --git a/healthd/testdata/legacy/res/values/charger/animation.txt b/healthd/testdata/legacy/res/values/charger/animation.txt
new file mode 100644
index 0000000..0753336
--- /dev/null
+++ b/healthd/testdata/legacy/res/values/charger/animation.txt
@@ -0,0 +1,9 @@
+# Sample Animation file for testing.
+
+# animation: num_cycles, first_frame_repeats, animation_file
+animation: 2 1 charger/battery_scale
+
+fail: charger/battery_fail
+
+# frame: disp_time min_level max_level
+frame: 15 0 100
diff --git a/healthd/testdata/legacy_text_system_images/res/values/charger/animation.txt b/healthd/testdata/legacy_text_system_images/res/values/charger/animation.txt
new file mode 100644
index 0000000..0753336
--- /dev/null
+++ b/healthd/testdata/legacy_text_system_images/res/values/charger/animation.txt
@@ -0,0 +1,9 @@
+# Sample Animation file for testing.
+
+# animation: num_cycles, first_frame_repeats, animation_file
+animation: 2 1 charger/battery_scale
+
+fail: charger/battery_fail
+
+# frame: disp_time min_level max_level
+frame: 15 0 100
diff --git a/healthd/testdata/product/product/etc/res/images/charger/battery_fail.png b/healthd/testdata/product/product/etc/res/images/charger/battery_fail.png
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/healthd/testdata/product/product/etc/res/images/charger/battery_fail.png
diff --git a/healthd/testdata/product/product/etc/res/images/charger/battery_scale.png b/healthd/testdata/product/product/etc/res/images/charger/battery_scale.png
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/healthd/testdata/product/product/etc/res/images/charger/battery_scale.png
diff --git a/healthd/testdata/product/product/etc/res/values/charger/animation.txt b/healthd/testdata/product/product/etc/res/values/charger/animation.txt
new file mode 100644
index 0000000..0753336
--- /dev/null
+++ b/healthd/testdata/product/product/etc/res/values/charger/animation.txt
@@ -0,0 +1,9 @@
+# Sample Animation file for testing.
+
+# animation: num_cycles, first_frame_repeats, animation_file
+animation: 2 1 charger/battery_scale
+
+fail: charger/battery_fail
+
+# frame: disp_time min_level max_level
+frame: 15 0 100