Old over age batteries can be dangerous. It depends on the format and chemistry of the battery. Since batteries contain stored energy, there is a risk of uncontrolled release of the energy. It is very important to make sure these batteries are stored in the right manner. Always store batteries as per manufacturer’s instructions mentioned. They need to be stored away from any conductive material that has a tendency to pass electrons through the poles of the battery. Keep them away from metals. It could lead to a dangerous accident if the chemical content of it flows out. If alkaline primary cells are left in the device for a very long period of time, they might start leaking out. Lower the cell voltage, lower will be the risks. If exposed to fire, the batteries start to destruct as soon as their casing is destroyed.