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diff --git a/src/tangara/test/battery/test_battery.cpp b/src/tangara/test/battery/test_battery.cpp
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+++ b/src/tangara/test/battery/test_battery.cpp
@@ -0,0 +1,67 @@
+/*
+ * Copyright 2024 jacqueline <me@jacqueline.id.au>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#include "battery/battery.hpp"
+
+#include <cstdint>
+#include <memory>
+
+#include "catch2/catch.hpp"
+#include "drivers/adc.hpp"
+#include "i2c_fixture.hpp"
+
+namespace battery {
+
+class FakeAdc : public drivers::AdcBattery {
+ private:
+  uint32_t mv_;
+
+ public:
+  virtual auto Millivolts() -> uint32_t override { return mv_; }
+  auto Millivolts(uint32_t mv) -> void { mv_ = mv; }
+};
+
+TEST_CASE("battery charge state", "[unit]") {
+  I2CFixture i2c;
+
+  // FIXME: mock the SAMD21 as well.
+  std::unique_ptr<drivers::Samd> samd{drivers::Samd::Create()};
+  FakeAdc* adc = new FakeAdc{};  // Freed by Battery.
+  Battery battery{*samd, std::unique_ptr<drivers::AdcBattery>{adc}};
+
+  SECTION("full charge is 100%") {
+    // NOTE: in practice, our curve-fitting slightly undershoots
+    adc->Millivolts(4210);
+
+    battery.Update();
+
+    auto state = battery.State();
+    REQUIRE(state.has_value());
+    REQUIRE(state->percent == 100);
+  }
+
+  SECTION("empty charge is 0%") {
+    adc->Millivolts(3000);
+
+    battery.Update();
+
+    auto state = battery.State();
+    REQUIRE(state.has_value());
+    REQUIRE(state->percent == 0);
+  }
+
+  SECTION("overcharge is clamped to 100%") {
+    adc->Millivolts(5000);
+
+    battery.Update();
+
+    auto state = battery.State();
+    REQUIRE(state.has_value());
+    REQUIRE(state->percent == 100);
+  }
+}
+
+}  // namespace battery