diff --git a/libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Generic.cpp b/libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Generic.cpp
index 169dab3a35..337cd9f9ee 100644
--- a/libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Generic.cpp
+++ b/libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Generic.cpp
@@ -81,6 +81,12 @@ void AP_BattMonitor_SMBus_Generic::timer()
         }
     }
 
+    // we loop over something limted by
+    // BATTMONITOR_SMBUS_NUM_CELLS_MAX but assign into something
+    // limited by AP_BATT_MONITOR_CELLS_MAX - so make sure we won't
+    // over-write:
+    static_assert(BATTMONITOR_SMBUS_NUM_CELLS_MAX <= ARRAY_SIZE(_state.cell_voltages.cells), "BATTMONITOR_SMBUS_NUM_CELLS_MAX must be <= number of cells in state voltages");
+
     // read cell voltages
     for (uint8_t i = 0; i < (_cell_count_fixed ? _cell_count : BATTMONITOR_SMBUS_NUM_CELLS_MAX); i++) {
         if (read_word(smbus_cell_ids[i], data) && (data > 0) && (data < UINT16_MAX)) {