My Tacoma's battery would die after sitting for a couple of weeks so being an Electronics Engineer I did some research and discovered the newer vehicles have many computers constantly drawing power from the battery. This continual draw is called Parasitic current, and this is what I found for the amount of current draw. “Typically, the normal amount of parasitic draw is between 50 and 85 milliamps in newer cars and less than 50 milliamps for older cars.”
What I did to determine what my truck was drawing is to remove one of the leads off of the battery and hook up a Volt/ohm/current meter at the 10 amp setting. Then I closed all the doors, shut down the engine, removed the key and I was able to watch the current draw go down and finally stabilize at 62 milliamps. Car batteries are designed to have lots of starting or cranking current but very little current for being continuously drawn upon to keep electronics powered up. I looked at my batteries' Amp hour rating and divided it by the 62 milliamps and the result was about 325 hours or around 2 weeks.
What you could do is measure the current like I did but I would suggest you have a battery charger or jump start battery connected to both cables or battery clamps when you disconnect one of them so your truck will always have power to keep all the computer memory intact. Once you have your meter connected between one of the battery terminals and the cable which will power up all the computers you can disconnect the charger so all the power is coming from the battery and not the charger. You will need to reconnect the charger before removing the meter to keep power to the electronics while you reconnect the terminal. The goal is to ensure you have power to the truck's electronics at all time to avoid memory loss to the computers.
If you like to troubleshoot your issue yourself you could connect up the meter then pull out one fuse at a time from each fuse box and see which fuse is drawing all the power. You might just find one circuit or fuse doing all the power drain or several fuses draining the power. At least you can now go to the dealer and tell them which fuses are doing all the power draw. Just make you don't have any electronics powered up or truck doors open which will activate some computers or lights.
You would think the dealer's service department could do this but sometimes the Manufacturer tells the dealers what they can do to troubleshoot trucks and what they are not allowed to do. Perhaps this type of troubleshooting which is very detailed and will take several hours to complete is not allowed. This may be why they tell you they can't find the problem, perhaps the service department doesn't have techs that are trained to troubleshoot or repair the complex electronics.
I have had this experience with my Tacoma, dealer's service department and Toyota's manufacture's imposed limitation to what service techs can do. It is limited to plug and play type of service, diagnosis issue to a system then replace the whole system or major components within that system.
Next time you go in for service for a dead battery ask them if they have done a parasitic draw test and what are the results that were noted in the service log. Make sure you get the results, that will show they did the test. If it is above the limits ask them what troubleshooting they did to find the source of the draw. The steps above are the easiest way to single out the cause, if they didn't do it then ask what steps they did do to find the source of the current draw.
Good luck and hope novel this didn't put you to sleep.....
