Auto Electronics: electrcal short, electrical short, parasitic drain


Question
how do i locate a dead short?
I suspect the short is associated with the install of a car radio a few months back. I have ruined 4 batteries now in 41/2 months. I've tried unplugging the fuse to no avail. Any direction will be appreciated.

Answer
John, finding a short can be a time-consuming exercise in futility, unless you are willing to dig deep to resolve it. From your posting I’m not convinced you have one. A dead electrical short would blow the protective fuse, fuse link or fusible element,  bringing down everything that is connected to the circuit. The only way this could kill a battery would be if the fuse was bypassed, even then there would be extreme heat, smoke, and possibly fire caused by burning of the conductor’s insulation. Batteries should be properly charged and load tested before replacement. It sounds more like you have what’s called a parasitic drain on the battery. The only way to know for sure is to test for an excessive drain with an ammeter. I don’t know what kinds of tools you have or even if you are planning on doing the repair yourself, but the old method of checking for a drain with a test light will not work on late model vehicles. Diagnosis of this problem can get very involved, the cause of an excessive drain can range from a bad diode in the alternator to a glove box light that stays on, or even a bad module that won’t go into ‘sleep mode’. All vehicles have some residual parasitic drain even after all the system’s modules have gone into ‘sleep mode’. The ballpark figure for this is less than 50 milliamperes. Rather than give you details on how to proceed with testing I will tell you to try and figure out what might have caused the drain problem, with the hope that it will lead to the fix. Try to think about what changes have been made to the vehicle that might have caused this to begin with. Have there been any repairs done to the vehicle lately, including body repairs? Are there any aftermarket devices installed, new sound system, remote starter or anti-theft system? These are always primary suspects when chasing down a battery drain problem. Your new sound system could very well be the cause of the excessive drain and can only be resolved by replacing the defective component.

Hope this helps, Don.