diff --git a/libraries/AP_AccelCal/AP_AccelCal.cpp b/libraries/AP_AccelCal/AP_AccelCal.cpp
index ebab633157..f75d6e94c9 100644
--- a/libraries/AP_AccelCal/AP_AccelCal.cpp
+++ b/libraries/AP_AccelCal/AP_AccelCal.cpp
@@ -22,6 +22,9 @@ const extern AP_HAL::HAL& hal;
 static bool _start_collect_sample;
 static void _snoop(const mavlink_message_t* msg);
 
+uint8_t AP_AccelCal::_num_clients = 0;
+AP_AccelCal_Client* AP_AccelCal::_clients[AP_ACCELCAL_MAX_NUM_CLIENTS] {};
+
 void AP_AccelCal::update()
 {
     if (!get_calibrator(0)) {
@@ -245,13 +248,10 @@ void AP_AccelCal::collect_sample()
 }
 
 void AP_AccelCal::register_client(AP_AccelCal_Client* client) {
-    if (client == NULL || _num_clients == AP_ACCELCAL_MAX_NUM_CLIENTS) {
+    if (client == NULL || _num_clients >= AP_ACCELCAL_MAX_NUM_CLIENTS) {
         return;
     }
 
-    if (_started) {
-        fail();
-    }
 
     for(uint8_t i=0; i<_num_clients; i++) {
         if(_clients[i] == client) {
diff --git a/libraries/AP_AccelCal/AP_AccelCal.h b/libraries/AP_AccelCal/AP_AccelCal.h
index 6ace5761e1..192ec52441 100644
--- a/libraries/AP_AccelCal/AP_AccelCal.h
+++ b/libraries/AP_AccelCal/AP_AccelCal.h
@@ -11,7 +11,6 @@ class AP_AccelCal_Client;
 class AP_AccelCal {
 public:
     AP_AccelCal():
-    _num_clients(0),
     _started(false),
     _saving(false)
     { update_status(); }
@@ -25,18 +24,19 @@ public:
     // Run an iteration of all registered calibrations
     void update();
 
-    // interface to the clients for registration
-    void register_client(AP_AccelCal_Client* client);
-
     // get the status of the calibrator server as a whole
     accel_cal_status_t get_status() { return _status; }
 
+    // interface to the clients for registration
+    static void register_client(AP_AccelCal_Client* client);
+
 private:
     GCS_MAVLINK *_gcs;
     uint8_t _step;
     accel_cal_status_t _status;
-    uint8_t _num_clients;
-    AP_AccelCal_Client* _clients[AP_ACCELCAL_MAX_NUM_CLIENTS];
+
+    static uint8_t _num_clients;
+    static AP_AccelCal_Client* _clients[AP_ACCELCAL_MAX_NUM_CLIENTS];
 
     // called on calibration success
     void success();