diff --git a/board/hammer/board.c b/board/hammer/board.c
index cb6d7d836b..e17cacd141 100644
--- a/board/hammer/board.c
+++ b/board/hammer/board.c
@@ -270,11 +270,3 @@ int board_write_serial(const char *serialno)
 {
 	return 0;
 }
-
-#ifdef BOARD_WAND
-/* TODO(b:66575472): This assumes external power is always present. */
-int extpower_is_present(void)
-{
-	return 1;
-}
-#endif
diff --git a/common/charge_state_v2.c b/common/charge_state_v2.c
index d956fdcb5a..9b1573c59a 100644
--- a/common/charge_state_v2.c
+++ b/common/charge_state_v2.c
@@ -1071,6 +1071,14 @@ wait_for_it:
 			curr.requested_voltage = charger_closest_voltage(
 				curr.batt.voltage + info->voltage_step);
 			curr.requested_current = -1;
+#endif
+#ifdef CONFIG_EC_EC_COMM_BATTERY_SLAVE
+			/*
+			 * On EC-EC slave, do not charge if curr.ac is 0: there
+			 * might still be some external power available but we
+			 * do not want to use it for charging.
+			 */
+			curr.requested_current = 0;
 #endif
 		}
 		charge_request(curr.requested_voltage, curr.requested_current);
diff --git a/common/ec_ec_comm_slave.c b/common/ec_ec_comm_slave.c
index 40bfef3492..608e1edc4f 100644
--- a/common/ec_ec_comm_slave.c
+++ b/common/ec_ec_comm_slave.c
@@ -12,6 +12,7 @@
 #include "crc8.h"
 #include "ec_commands.h"
 #include "ec_ec_comm_slave.h"
+#include "extpower.h"
 #include "hwtimer.h"
 #include "queue.h"
 #include "queue_policies.h"
@@ -31,6 +32,9 @@
 struct ec_response_battery_static_info base_battery_static;
 struct ec_response_battery_dynamic_info base_battery_dynamic;
 
+/* Set if the master allows the slave to charge the battery. */
+static int charging_allowed;
+
 /*
  * Our command parameter buffer must be big enough to fit any command
  * parameter, and crc byte.
@@ -140,6 +144,7 @@ static void handle_cmd_charger_control(
 		charger_enable_otg_power(0);
 		charge_set_input_current_limit(
 			MIN(MAX_CURRENT_MA, params->max_current), 0);
+		charging_allowed = params->allow_charging;
 	} else {
 		if (-params->max_current > MAX_OTG_CURRENT_MA ||
 				params->otg_voltage > MAX_OTG_VOLTAGE_MV) {
@@ -153,11 +158,26 @@ static void handle_cmd_charger_control(
 		charger_set_otg_current_voltage(-params->max_current,
 						params->otg_voltage);
 		charger_enable_otg_power(1);
+		charging_allowed = 0;
 	}
 
 out:
 	write_response(ret, seq, NULL, 0);
 }
+
+/*
+ * On dual-battery slave, we use the charging allowed signal from master to
+ * indicate whether external power is present.
+ *
+ * In most cases, this actually matches the external power status of the master
+ * (slave battery charging when AC is connected, or discharging when slave
+ * battery still has enough capacity), with one exception: when we do master to
+ * slave battery charging (in this case the "external" power is the master).
+ */
+int extpower_is_present(void)
+{
+	return charging_allowed;
+}
 #endif
 
 void ec_ec_comm_slave_task(void *u)