AA batteries are a negligible risk when you touch metal with skin. That resistance is very high. Resistance inside your body is much lower -- 100 ohms is typica

AA batteries are a negligible risk when you touch metal with skin. That resistance is very high. Resistance inside your body is much lower -- 100 ohms is typical. If you were to get 0.5v between you arms, you could get unsafe currents. With electrodes, the current can be /much/ greater than touching metal wires with a finger. Single-digit kilo ohms is typical. The big ECG paddles are tens of ohms, probably. Another way to think about it -- have you ever felt even a 24v battery? No. Now You know that 9v tingling on your tongue? That's saliva and a tiny contact area. Now think of a large patch of engineered electrolyte can do.