Remember, the cables to the aux battery are hot all the time unless you disconnect the main battery. I just wrap the positive aux battery cable in a heavy kitchen towel when replacing the aux battery to prevent shorting the cable to ground and taking out the aux battery relay ($$$) or worse. As JC said, the aux battery needs to be filled with the included electrolyte, permitted to stand per the instructions, sealed and then charged before installation. Really not very difficult to replace, you can get the battery from McParts for about $100 and MB gets about $300 for the job. Mark