Battery draw test setup procedure
Performing a draw test without the proper setup can be a big waste of time. If not setup properly you can get erroneous results. In the past my go to method was to just put a meter on the battery and measure the correct draw, wait for 30-45 minutes and recheck. There is a much better process that may yield more accurate results. This of course is performed after a complete vehicle scan and thorough visual inspection
Charge/Test/Replace battery as needed.
Perform a road test over 30 mph. During and after the road test activate all the functions in the vehicle. This is crucial for intermittent draws cause my modules that stay awake. This would include, but not be limited to, open and close all doors, use the climate control, audio systems, heated seats, all the lights, all gear positions, activate windows, and all other functions of the vehicle.
After the road test, roll down the windows and leave the doors open with latches closed (use a screwdriver to latch them) if there are fuse panels in the engine compartment or trunk, leave those open and bypass ajar switches as well.
If the vehicle has Smart keys (the kind that work when in your pocket) relocate them far enough away from the vehicle so they do not read. 50 ft should be enough.
Use a jump box to bypass the battery by connecting positive cable to battery and negative cable to chassis or engine ground. This is important to maintain power to the modules that have been activated on the road test.
Disconnect negative battery cable and install Amp meter, disconnect jump box. Take the initial current draw reading.
You can at this point either leave the meter in place or reconnect jump box to ground and remove meter while you wait for the vehicle to sleep.
Following this process will maintain the modules state after road-testing. I have had several vehicles with intermittent draws that could not be duplicated due to depowering the modules from not maintaining the power supply to them.
Once the draw is measured beyond the manufacturer's specified sleep time (typically 30-45 minutes) you can reference the draw specification in the service information. Most manufacturers require less than 50 mA or 0.050 A draw. Most vehicles are in the 10-20 mA range.
If there is an excessive draw, there is access to under hood, trunk mounted and interior fuse boxes without reawakening the vehicle.
If continuity is broken, the process should be started over.
Hey Chris, nice write up. Any reason you prefer this method over an inductive amp clamp? Accuracy? The test drive and operation of all function is great advice and key for intermittent conditions.
I've honestly never had great luck with an amp clamp finding small draws.
I like the accuracy of the meter. I don't have an amp clamp that measures small enough. I should have also noted that my meter leads have an inline 7.5 amp mini fuse that keeps my meter safe during current testing. I have another meter that is good for 15 amps and for that one I use a 10 amp fuse.
Good Morning Chris,
Thank you for an informative write-up.
I have found the Kaise SK-7830 Low current clamp meter excellent for these scenarios; in the lower settings, it has a resolution of +/- 1mA.
This also allows continued use of scanning equipment, without the associated effect of interrupted vehicle power faults.
The one step of the process that I don't like is using the jump box and unhooking the battery. This can be tricky sometimes, and if the jump box clamp pops off, then you have to start over. So what I did was make a tool to help. You can make a similar tool. It's made from basic stuff you can find. In the attached picture, you can see that it is just a battery disconnect switch with two long battery cables and then terminals on the ends. You hook this up to the negative side of the battery BEFORE starting your process.
You'll notice that there are two small blue vacuum hose that are covering two bolt studs. When you want to make your measurement with your meter inline, then pull the hoses off, hook your meter up, set on amps, and flip the big battery switch so that the current will pass through the meter only. The picture of the underside of the tool shows how I accomplished this. Just two bolts coming up from the bottom, and two wires attached. Really easy to build this, and makes the measurement step so much easier. Plus, you can make the cables longer for those hard to access batteries.
I like the setup. The process outlined above is with equipment that should be readily available to every tech. Specialized equipment may be better depending on application. The most important part of the process it to maintain a constant power to the modules and the pretesting road test.
Great write up Chris. That's a sure fire way to simplify the process. Another method I use as well, is to perform a voltage drop test across fuses to locate parasitic draws. As long as I follow the steps you listed above and let all the modules sleep, it works really well. It all comes down to a quicker, more streamlined diagnosis. Followed up by happy customers.
Beware of the fuse voltage drop method. In theory it will work fine, but different fuse manufacturers may have a variance in resistance and may cause inaccurate results. I believe Mercedes has a Voltage drop calculator editable pdf that will convert the voltage drop to current based on the fuse size. I attached a screen shot calculating the current draw based on a 3 mA drop on various size Mercedes fuses.
Very true Chris. I've run across multiple vehicles that had cheap chinese/.99 cent store/Harbor Freight type fuses. It can definitely bite you in the ass. I wasn't aware of the Mercedes calculator, thanks for the heads up.
When I use the fuse VD method I don't usually don't worry about how much of a drop I see. Anything other than zero means there is current flow.
Agreed, I'm looking for ANY fuse with a voltage drop. I haven't needed to calculate the approx draw from the mV. I will make a quick sweep with the volt meter to see which fuses still have power so I don't waste time checking the dead ones. I always repeat the measurement several times, sticking and re-sticking the probes to get a reliable reading.
I made a similar tool with a high amperage switch. It's especially useful if you need to start the car and move it out of the way. Or if you even just cycle the key on it can be enough to blow a low amp fuse.
Yes, there are situations that come up where you need to switch between using the meter to measure the draw and then where you need to go back to a higher amperage connection. Fiddling with a jump box was not for me. Having this tool connected before you even start your process means you are good to go, and no worrying about accidental disconnects.
I used to have a basic disconnect switch that mounts to the battery post, but that is such a pain because you can't use that in tight spots. Having cables attached allows for positioning the switch to a more convenient location and hooking up to the battery is way easier. I strongly recommend it to anyone who has not made one yet.
Don't forget about voltage drop across the fuses.
I like to start with the Pico TA189 amp clamp, then voltage drop the fuses to narrow it down. Then I will put my graphing ammeter inline, either in the single circuit or the whole car if needed.