Short Answer: 2 possible reasons are a dead battery and/or a ground loop.
Long Answer: The 2 most common reasons the SuperSting™ will give you weird values are a depleted battery or if a ground loop is occurring. Here is how to fix both of these issues:
1. Double check your battery
Is the battery voltage above 12.1 Volt? If not, your battery is probably depleted. A newly charged 12 Volt battery will usually be over 12.6 Volt when fully charged.
2. Isolate your equipment
Ground loops can sometimes occur. Isolate the SuperSting™ by putting it on something electrically insulated. Do the same for your battery/power source.