A very useful homemade construction. We can safely connect all kinds of rechargeable batteries. We need a car battery between 35-44 Ah (Here the battery is 100 Ah but we don't need so much current) , a strong relay from a scooter or a motorbike or from a car ( the ignition relay) . The electrodes must be from bronze or from copper. We also need a small switch to give in an instant the order to the relay to close the circuit. We must push the electrodes on the battery but not very hard to not destroy it. The batteries don' t become so hot because the time we need to weld is less than 1 sec. This method is much better than to weld with soldering iron, witch makes all the area very hot. Take care of your hands to not burn them. Rings , bracelets and other metal accessories are forbitten to wear during the welding.