Battery draws… I’ll jump in if you don’t mind, I’ve been down this road. I hope this helps you out and sheds some light on the subject. What follows is my experience working on battery draws for BMW, so I cannot help you with your car, but maybe this will help to answer your questions in general and show you how Pico can help.
The question begs asking as to why one should look at some sort of graph, and of course, how.
Todays vehicles use more and more electronics. And it’s only going to get worse. Electric vehicles, hydrogen vehicles, hybrids of all sorts. Control units that control other control units. The BMW 7 series has over 50 modules alone. Each one is programmed and coded. There is a central ‘Power Module’ that controls virtually all power to the vehicle. On the newer vehicles there is a microprocessor on the negative battery cable that monitors every electron. It sends the data it collects to the DME and other modules for processing. There is now an ‘Energy Diagnosis’ test plan built into the factory test equipment we can use to help determine the cause of a flat battery. The amount of data processed and presented can be overwhelming sometimes. There has been concern in the shop lately as to when too much information gets in the way.
The power module has the ability to shut off different circuits if it is determined that there is over 80ma of current leakage.
It can take up to 70 minutes for the vehicle to go to sleep. Different vehicle systems and creature comforts require this. The telephone control module for example: if a handset is plugged into the system, it can charge the handset battery for up to one hour. Furthermore, some TCU’s can cause 500ma spikes every 15min, lasting for 14hrs. This is normal behavior. The instrument cluster: you will see tiny spikes of about 200ma lasting about 2 seconds. The cluster is interrogating the outside temp sensor. But in order to calculate a correct outside temperature, you must also take into consideration the waste heat from the engine. In order to read the coolant temp sensor, you must wake the DME, which means waking up the PT-CAN bus, that’s 8amps for just under 5 minutes. It’ll do this at pre-determined time intervals. There is a basic draw of 200ma for the first 30-40 minutes while the system keeps the body relay active.
That brings us to the 8 minute rule. The power module is set to ignore draws that last under 8 minutes as they may be just normal activity. After 70 minutes, the circuits that are capable of being monitored begin being monitored. If the draw lasts only a few minutes, and then goes away, the circuit is never cut. This intermittent action can kill the battery even though the circuit is monitored. You need the graph. Because looking at the graph allows you to determine right away normal vs. not normal. You can tell even before the vehicle goes to sleep. What if the draw is on a monitored circuit, and you let the vehicle go to sleep, when you check for a draw… there is none.
You must also keep in mind that the software that runs all these modules is constantly being updated and improved, so what might be normal now, no longer is after a module change or vehicle squirt.
Sooo… Looking at the graph, you will be able to see what’s normal and what’s not. You will see if the PM cut the power, and you will see exactly how much leakage you have. 200ma – dome light, relay coil, etc. 8 amps – which control module is waking your bus?
My personal experience is that you need to see the graph. To do so, I use my old 212/3. It supports using Picolog. As far as I know, my 3423 is still incompatible w/Picolog (haven’t tested it in quite a while). I suggest using the logger because it’s tailor made for the job. There’s a software setting that you can use to scale your clamp properly. As far as readings and duration, set it up to take a reading once every second and set the total duration in seconds to whatever you want. This is more than enough detail and Picolog allows zooming etc.
I use the Prova 15 amp clamp to get the signal into the scope. It’s jaws are big enough to get around the negative cable and it scales from 0 - 400ma, 0 - 4 amps, and 0 - 30 amps. I can use it intermittently during the day and it’ll run overnight 3 to 4 nights before I have to replace the batteries. It also serves as my current ramper. For those quick checks, I hook it up to my Fluke 77. I created a reference chart that correlates the readings on the meter to the chosen amp clamp scale.
The Prova 15, like most other amp clamps is prone to saturation. The higher the current and the longer it lasts, the faster it will saturate. In all cases you will easily recognize it on the graph. Not to worry.
I’ve seen quite a few draws in my day. No two seem to be alike. They are all challenging, especially the intermittent ones. The trick in finding a draw isn’t in diagnosing where it’s leaking from, but getting it to leak. It leaves no stains.
The usual disclaimers apply and of course ymmv.
Jim / jperras
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