For the most part, it will almost always be a long process to track down the cause of a parasitic battery drain in a late-model vehicle, as there are so many modules, accessories and networks involved. In any case, there are a few things that I have found that will help more quickly narrow down the possibilities and also prevent overlooking some basics that cause severe headaches for some technicians.
Vehicle: 2008 Chevrolet equipped with 4.8 liter engine and automatic transmission
Odometer: 55, 291 miles
Concern: Vehicle will not start after sitting for a few days
The first step to diagnosing any parasitic draw starts with what’s being drawn down to begin with — the battery. I think more than any other component, the battery is the most overlooked item when it comes to testing for a parasitic drain. Where I live, most summer days are 100 degrees F or higher, which definitely takes its toll on the life of a battery. In fact, a large portion of batteries that I find worn out are only 2-3 years old. So before any testing can even start, we need to make sure the battery is sufficiently charged. Unless the open circuit voltage of the battery is 12.45V or higher, the battery needs to be charged before testing. While the battery is being recharged, this is also a perfect time to multitask and check for TSBs, hotline archives and groups like iATN for similar vehicles that have experienced the same problem. Many times techs will try to diagnose a system draw with a battery that will barely crank the vehicle’s engine over to start it; this can cause inaccurate test results. Some signs of battery problems are obvious, like visible leakage, especially across the top of the battery, which will cause case drain. The leaking acid creates a conductive path that can easily drain a fully charged battery overnight. Then there is a problem you can’t see, at least with the naked eye, which is a shorted battery. When measuring the open circuit voltage of the battery and the DMM reads in the area of 10.5V, you have a bad or shorted cell and it must be replaced.
One of the most important but overlooked steps in diagnosing a battery drain problem is verifying the vehicle has a good battery. If recharging is needed before testing can begin, it is a good idea to use the time to multitask and research the problem looking for TSB, hotline archives and technician sites like iATN.