I like to fix stuff. I am into fixing my milwaukee batteries as they go bad. I have a 12ah battery. I have it all taken apart. There are 15 cells inside, 21700. Four cells are completely dead. I have a charger for that type of battery. They will not charge. I have 4 new ones to put into the pack. Question is explosions. I have only a little knowledge on this issue. I will spot weld the batteries into the pack. For you guys and girls that know your stuff, what can you tell me about these batteries? I know to wear a face mask and not let the spot weld get too hot. Other than that?????????