Over time, corrosion forms on the terminals that connect the battery to the unit. This corrosion is common, but if it is not cleaned off, the battery will not charge properly. The Basement Watchdog backup and combination systems can detect this corrosion and will sound an alarm when cleaning of the battery terminals is needed. Use a stiff wire brush or sandpaper to clean the battery terminal posts, the rings on the end of the battery cable, and the wing nuts until they are shiny. See the product manual included your backup or combo system for detailed instructions and safety precautions.