About Parasitic Amp Draw (Battery Drain) in a passenger car.

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
39
Recently, I faced a difficult scenario that my car battery keep failing few occasions to start the engine, and I measured below 12V, mechanics did diagnose telling me, is not the battery faulty, instead is the adjacent components, need time to find out/narrow down the faulty part.

I am very interested to understand a passenger car, what is/are the component or parts or assemblies directly connected to the battery without an On/Off switch, I understand the alternator is one of them, any others? Best I could read from the electrical schematic drawing of a car.

Thank You.
 
Usually something like this is a damaged wire that hasn't fully shorted to ground. Maybe a corrosion problem in connectors, a wire that is partially rubbed through and not making good enough contact yet to blow the fuse, or something along those lines.

Another possibility is a glove box or trunk light that turns on when it shouldn't be.

Normally control modules have a battery connection to keep the memory alive. Remote locks and start functions need battery power. Radio uses it to keep your presets and clock active.

That is just a couple things off the top of my head. What year, make, model?
 
Another couple thoughts... Does it happen when the weather is colder? Do you smell rotten eggs or a sulfur smell? I have load tested a fair amount of batteries that have checked out fine only to find out they were really burning themselves up inside.
 

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
39
Another couple thoughts... Does it happen when the weather is colder? Do you smell rotten eggs or a sulfur smell? I have load tested a fair amount of batteries that have checked out fine only to find out they were really burning themselves up inside.
Thank you for your advice reply. I will let the mechanics take his time to rectify the problem.

However, mechanics told me likely is the faulty component rather than the wires issue, drain Amp measured consistency at 0.6A/600mA, somehow, as long as it happens I get my car battery lower than 12V without knowing, I could not start it while it is in Summer days. Already replaced a new battery, the same problem happened again.
 

Rich2

Joined Mar 3, 2014
241
My cars computers shut down after 10 minutes of inactivity. Leave the bonnet/hood up and retest after an hour, don't even open a door or the timer resets.
 
However, mechanics told me likely is the faulty component rather than the wires issue, drain Amp measured consistency at 0.6A/600mA, somehow, as long as it happens I get my car battery lower than 12V without knowing, I could not start it while it is in Summer days. Already replaced a new battery, the same problem happened again.
It ls nice to see your mechanics measured the draw. That is a huge relief by itself. Too many people want to load the parts cannon... I work with a few like that and it drives me insane!!
 

crutschow

Joined Mar 14, 2008
29,466
As a workaround, If there's a smaller wire from the battery that powers everything besides the starter, then you could put a switch or large relay in series with that to disconnect the parasitic load.
Of course some of your electronics may lose their memory when you do that and you'll have to manually open your door, but it should keep your battery from discharging.
 

sghioto

Joined Dec 31, 2017
3,058
My 2016 Subaru Outback has a similar problem, an intermittent parasitic current draw . As long as you drive the car every three days to keep the battery charged there's no problem. Subaru claims they have a software fix for this and a pending lawsuit about the issue. It appears the software fix has to do with the alternator and battery charging system.
 

DC_Kid

Joined Feb 25, 2008
924
If the car has an ECU, then it always has power.
Heck, my rav4 will do gas tank evap purge in the middle of the night, because the vehicle sensed it needed to. Hence, always using batt power, even when "off". ECU (or modules) firmware can be a culprit too. Some rav4's had "bad" code causing batt to drain too fast when car "off". Toyota fixed the code and issued firmware upgrade, batt issue fixed.

So, although batts can go bad by themselves w/o even being used, 99.99% of the time a drained batt is due to some bad load. Tracing the amps is key. OBD-II using diag app might show individual modules in the vehicle along with their V and A readings. The tricky part here though, to use ODB-II you usually need to turn "on" the ECU to power up OBD-II, but, excessive amp readings on any one module might show up, etc.

Other factors? Is the vehicle always wet? Environmental factors can cause gremlins.
 

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
39
As a workaround, If there's a smaller wire from the battery that powers everything besides the starter, then you could put a switch or large relay in series with that to disconnect the parasitic load.
Of course some of your electronics may lose their memory when you do that and you'll have to manually open your door, but it should keep your battery from discharging.
Good Advice ,
Thanks.
 

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
39
The simplest way to narrow down the search for the faulty component is to pull fuses, one at a time, while measuring the battery current. You will need to reset your radio and clock but it's a small price to pay.
Thank you for your advice, my mechanics informed me that, he sorted out the "Backup Fuse" by removing it, the drain Amp read at 0.18A from 0.6A, he thinks the culprit should be the Constant Power Source/line connected to the Radio, he needs to put back the fuse and again to dismantle the radio player to further confirm.
 

Attachments

Sounds like you are getting somewhere at least.

Many times I forget the vehicles I work on are usually a generation or two behind cars when it comes to electronics. Usually after 500,000 miles is when the strange electrical issues start to happen and for me it is normally something just gets worn through from vibration. The video made me realize I have some studying to do.
 

Reloadron

Joined Jan 15, 2015
6,507
Recently, I faced a difficult scenario that my car battery keep failing few occasions to start the engine, and I measured below 12V, mechanics did diagnose telling me, is not the battery faulty, instead is the adjacent components, need time to find out/narrow down the faulty part.

I am very interested to understand a passenger car, what is/are the component or parts or assemblies directly connected to the battery without an On/Off switch, I understand the alternator is one of them, any others? Best I could read from the electrical schematic drawing of a car.

Thank You.
Years ago my mom would park her car in the garage. Next day or two battery was fine and car starts. Waiting more days car would not start, weak battery. Drove my father nuts. Then suddenly the problem went away. The car had a switch activated trunk light. Switch failed in the On position. Looking at the finally burned out lamp the socket was pretty cooked. That bulb was always on, in the trunk, till it finally died of natural causes.

Hard to know what does and what does not have always on power of a significant enough load to run down a battery over time. If you have a DMM or something to measure current you may want to disconnect a battery terminal, place an ammeter in series and measure the load. Then begin looking at what would draw the load current you are seeing. You can always begin yanking fuses for always on circuits and see when the current on the measuring device stops.

Ron
 
Jonathan;
You have not yet told us the make/ year of your vehicle.
For the past 25 years I have worked for automotive electronics module manufacturers, and have seen technologies evolve.
As Alberthall mentioned, automobiles are loaded with electronic modules, literally dozens of them, and its number is increasing with each vehicle generation.

In the past 5 years or so, the trend has been to use smart CANBus transceivers which are always powered up from the battery. These transceivers, besides transferring the huge amounts of data from their assigned module, also turn on and off the module’s own power supply. A specific bus command sequence wakes the module, or puts it to sleep.

Thus finding the defective module which doesn’t want to go to sleep is quite difficult indeed.
 

Tonyr1084

Joined Sep 24, 2015
6,861
Really can't add anything to what's been said thus far except for to relate a couple experiences I've had.

Back in the late 70's I had a 72 Chevy Nova. One evening I stopped to buy a slurpee at my local convenience store. When I came out the car would not start. The battery was discharged. I got a jump start and drove home. The next morning the car would not start, so I put it on the fast charger. Later that afternoon I started the car and drove it for a while. Shut it off and started it several times. No problem.

The next day I started the car and drove it again, starting and stopping. No problem whatsoever. I thought the issue had resolved itself. But it didn't because I didn't find the cause. Whenever I drove at night with headlights on the car would not start. Keep that part in mind - the part about the lights. I went out and bought a new battery and installed it thinking it must somehow be a coincidence with the battery. But the same habit repeated itself over and over. One afternoon with the hood up I was staring at the alternator. I started the car and checked the battery voltage. 13.8 V right where it should be. So I shut it off and began poking around. I knew the battery was good and the alternator was doing its job. When running I had no apparent electrical troubles. So flummoxed, I gave the fan belt a tug. It was loose. In fact I was able to grab the alternator (engine NOT running) and give it a turn without making the engine turn. There it was - a loose fan belt allowing the alternator to slip when the electrical load was high. Such as at night when the radio was playing, the heater was running and the headlights were on - draining the battery because the alternator was slipping. I tightened up the belt and that was the end of the problems.

For years I've seen people with similar circumstances. It most often presents itself when the weather starts turning cold late fall, early winter. The bottom line - WITH THE ENGINE OFF - NOT RUNNING - if you can turn the alternator either by hand or with a wrench and the whole motor remains still then the fan belt is either loose or worn out.

Second issue to cover: A neighbor had a car that if he started it every day he had no problems. But when the car sat for two or more days the battery would be dead. So using the "Pull one fuse at a time" method we determined it was the automatic door lock circuit that was drawing power constantly. With that fuse pulled the battery could sit for weeks and still start the car.

So either the belt is slipping - or you have to find which circuit is drawing power when it shouldn't be. In the case of the locks - that circuit was supposed to be live all the time waiting for a key-fob to command it to unlock. For unknown reasons the system had begun drawing more current than it should have. So start testing. And we didn't test with a current meter, we tested with a voltage meter. Watching the battery voltage while pulling fuses. If the voltage went up when a fuse was pulled we made a note of it and how fast it was rising. When we found the door lock fuse and a much higher and faster voltage climb we knew we found the culprit. Not exactly the right way to go about it - but it worked for us.

Good luck with your automotive issue.
 
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