An ECG monitoring system for ambulatory patients includes a small multi-electrode patch that adhesively attaches to the chest of a patient. A reusable battery-powered ECG monitor clips onto the patch and receives patient electrical signals from the electrodes of the patch. A processor continuously processes received ECG signals and stores the signals in memory in the monitor. Processed ECG signals and cardiac event information are sent wirelessly to a cellphone handset for relay to a monitoring center. The ECG monitor is contained in a watertight sealed case with only electrical contacts on the outside of the case. The electrical contacts electrically couple the ECG monitor to the electrodes of the patch during patient monitoring and to a charger during recharge of the battery.