# What is normal parasitic current drain?

I have a 2004 Chevy TrailBlazer that recently refused to start for my wife. I’ve got it going again and with a new battery. but I checked the key-off battery current drain with an ammeter (negative lead disconnected, meter between the cable and the battery) and the drain is 92 milliamperes. That seems high to me.

Does anyone know what a “normal” key-off electrical current from the battery should be? I’ve pulled every fuse, every circuit breaker, and have disconnected the alternator, and no matter what I do, the meter still shows right around 92 milliamps.

The only reference I’ve found to tell me what’s “normal” is the document at this site: {http://flashoffroad.com/electrical/CurrentDrain/currentdrain.pdf) – which says on the second page that “A full option GM vehicle can experience a parasitic load of 25-35 milliamps.” But does anyone know it that is correct and if 92 is too much?

Secondly, supposing ~30 milliamperes is about right, does anyone have an idea where the extra 60 milliamperes is going? Since it is not going through the fuses or the circuit breakers or the alternator, What’s left??

Thanks.

This is Dave again. If that link mentioned above does not work, try going to Google and typing in: “Parasitic Current Drain”. That should bring it up.

To me, 92 doesn’t sound that high—that’s less than 1/10 of an amp, or about 1.1 Watts of power. If the car has keyless entry, a stereo, Onstar, an alarm, in addition to the vehicle’s own computers, it’s possible that just maintaining the memory of these devices is using this much power. Of course I don’t know what the draw should be, so I can’t answer your original question. What kind of meter did you use to measure the current draw? The cheapie analog meters (the kind with the needle that swings) have pretty lousy accuracy at measuring low voltages and currents.

92 milliamps sounds high. I think it should be around 50 milliamps.

Did you wait about 30 minutes before checking the amp draw? When the battery is disconnected then reconnected it takes about 30 minutes for everything to power up then shut down. So if you didn’t wait long enough you could be getting a false reading.

Just opening the door can wake up some microprocessors in some vehicles. That puts you in the awkward position of having to crawl in the window to pull fuses after you wait for everything to go asleep. 90-100 mA seems a bit high, but look at it this way. I presume that your TB has a pretty big battery. The reserve capacity might be 100 Ah. If you do the calculation it would take 500 hours to discharge the battery 50%. That is less than a month so it seems kind of high.

Are you sure that your ammeter is correct?

Thanks for the reply. The meter I used was a pretty nice one – a digital multimeter with three current ranges available. I have a DC Clamp ammeter ordered so I can check things with the cables connected normally. That will be about 5 days from now, though.

Dear Willey, Yes, I checked the current over a period of several hours and then again after sitting all night. Thanks.

Dear Beadsandbeads - Your reserve calulation is interesting. I expect the battery gradually lost ground after many weeks because my wife usually makes short trips with the car. On her last trip, as I mentioned, it just couldn’t crank the engine again. That fits with your reserve calculation at least in a general way.
Ammeter Calibration – Good idea – I’ll go to Radio Shack, buy a resistor or two, and double check the calibration of the meter at near this current level and on the same scale.
By the way, I did a side-by-side test with my two cars the other night. First I topped off both batteries with a trickle charger until there was almost zero charging current draw for both batteries. Then I let both cars sit overnight (about 10 hours) with the key off and no charger. Then I put the trickle charger on each car, one at a time. The Buick needed only a brief surge (1/4 scale) of current in about TEN SECONDS the charger’s ammeter was back to virtually zero . Then I did the same with the TrailBlazer. The TrailBlazer pulled the charger’s ammeter fully to 1/2 scale and it took over a half hour to get it gradually back close to the “zero” level that it had showed when topped off the night before. This is an indirect indication, but pretty strong evidence that there’s a whole lot more parasitic draw on the TB than the Buick.

`` Thanks, Dave``

Regarding accuracy of ammeter – I just checked it with known resistive load and it checks out just about perfectly. Three 470 ohm resistors in parallel make 156.7 ohms. Battery voltage of 13.55 should give 86.5 milliamps. And that’s what it reads. - Dave

Here is a TSB that might apply:

Bulletin No.: 02-01-39-007B

Date: November 05, 2007

INFORMATION

Subject:
Automatic Dual Zone HVAC Battery Draw

Models:
2004-2007 Buick Rainier
2002-2008 Chevrolet TrailBlazer
2003-2008 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2002-2008 GMC Envoy Models
2003-2008 GMC Sierra, Yukon Models
2003-2008 HUMMER H2, H3
2005-2008 Saab 9-7X

Supercede:

This bulletin is being revised to add models and model years. Please discard Corporate Bulletin Number 02-01-39-007A (Section 01 - HVAC).

When diagnosing battery draws on trucks equipped with the automatic dual zone HVAC controls (RPO CJ2), technicians should keep in mind that the control head does not completely “go to sleep” until after 150-250 minutes, or up to 4-1/4 hours, from when the ignition key is turned OFF. This is a normal condition. In these cases, DO NOT replace the control head.

Before anyone can tell you if 92ma is normal, you need to provide more information about your particular Trailblazer. Is it 4x4? Is it equipped with OnStar? Do you have any aftermarket accessories?

I own an '04 LS 4x4. I can check my normal draw as a comparison but I want to know if we’ll be comparing apples-apples beforehand.

Your TB is equipped with a power saving feature that is supposed to prevent the battery from being discharged enough that it cannot be started. However, this system only works if the battery is in good shape and the charging system is working and allowed time to replenish the lost charge from starting. Infrequent, short trips are killers for batteries and as it sulfates, it will spiral downward rapidly. 4-5 years is not outside the unexpected time for a battery replacement in those situations.

Here is the test procedure from ALLDATA…

TOOLS REQUIRED
J 38758 Parasitic Draw Test Switch

DIAGNOSTIC AIDS

Be sure to rule out any possible obvious influences, such as customer error or aftermarket equipment.
Customer driving habits, such as regular short trips. This does not allow enough time to properly charge the battery. Refer to Battery Description and Operation.
Verify that the battery and charging system are in proper working order. Refer to Battery Charging and Charging System Test. See: Battery Charging Procedure See: Charging System Test
A battery discharging for no apparent reason while the vehicle is parked can be caused by an intermittent draw, such as a module waking up, or a continuous draw, such as a dome light or stuck relay.
Some systems and modules such as OnStar®, and regulated voltage control (RVC), if equipped, are designed to wake-up, perform a task, and go back asleep at regular intervals. Refer to Body Control System Description and Operation in Body Control System for the system or modules description and operation.
Remote keyless entry (RKE) will wake up due to an outside input. Refer to Keyless Entry System Description and Operation in Keyless Entry.
IMPORTANT: The battery specification listed below is a generic specification. Refer to Battery Usage when testing the battery.

The battery run down time will vary depending on cold cranking amperage (CCA) and reserve capacity (RC). If the CCA and RC are higher, then the battery run down time would be longer. If the CCA and RC are lower, then the battery run down time would be shorter. The graph below indicates roughly how many days a 690 CCA battery with at 110 mm . RC (60.5 AH) starting at 80 percent state of charge will last with a constant current draw until it reaches 50 percent state of charge. Differences in battery rating and temperature will affect the results.
CAUTION: Refer to Battery Disconnect Caution in Service Precautions .

NOTE:

Do not turn the parasitic draw test switch to the OFF position with the engine running. Damage will occur to the vehicle’s electrical system.
The test switch must be in the ON position when removing the fuses in order to maintain continuity in the electrical system. This avoids damaging the digital multimeter due to accidental overloading, such as a door being opened to change a fuse.
IMPORTANT: The switch knob (1) on the J 38758 is marked ON and OFF. When the switch knob is in the ON position, the circuit is closed and electrical current will pass through the switch. When the switch knob is in the OFF position, the circuit is open and electrical current will not pass through the switch.

Disconnect the battery negative cable from the battery negative terminal.
Install the male end of the J 38758 to the battery ground terminal.
Turn the J 38758 knob to the OFF position.
Install the battery negative cable to the female end of the J 38758.
Turn the J 38758 knob to the ON position.
Road test the vehicle and activate ALL of the accessories, including the radio and air conditioning. This may take up to 30 minutes .
Park the vehicle. Turn the ignition switch to the OFF position and remove the ignition switch key.
Connect a 10A fused jumper wire to the test switch tool terminals.
Turn the J 38758 knob to the OFF position. The current now flows through the jumper wire.
Wait 1 minute . If the fuse blows, install an inductive ammeter and go to step 20.
Remove the fused jumper wire.
Set a digital multimeter to the 10A scale.
Connect the digital multimeter to the test switch tool terminals.
Turn the J 38758 knob to the OFF position. The current flows now through the digital multimeter.
Wait 1 minute . Check and record the current reading.
15.1. When there is a current reading on 2A or less, turn the J 38758 knob to the ON position. The electrical current will now pass through the switch.
15.2. Then switch the digital multimeter down to the 2A scale for a more accurate reading when the J 38758 knob is turned OFF.

Turn the J 38758 knob to the OFF position. Wait 15 minutes for most vehicles.
Check and record the current reading.
Note the battery reserve capacity, amp hour rating. Refer to Battery Usage.
18.1. Divide the reserve capacity by 4, amp hour rating by 2.4.
18.2. Compare this to the multimeter milliampere reading taken in the previous step. The parasitic current drain should not exceed this number. Example: If a battery has a reserve capacity of 100 minutes, (60 A/H) the current drain should not exceed 25 mA .

If excessive current drain is not found at this time and there are no other apparent causes, complete the following:
Using the MIN/MAX function of the digital multimeter, monitor the parasitic drain overnight or during the day. This will determine if something has been activated during that time frame. NOTE: The test switch must be in the ON position when removing the fuses in order to maintain continuity in the electrical system. This avoids damaging the digital multimeter due to accidental overloading, such as a door being opened to change a fuse. IMPORTANT: Removing fuses, relays, and connectors to determine the failure area may wake up modules. You must wait for these modules to go to sleep or use the sleep function on the scan tool.
When the vehicle has an unacceptable amount of parasitic current drain, remove each fuse one at a time until the current drain falls to an acceptable level. This will indicate which circuit is causing the drain. Refer to Power Distribution Schematics in Diagnostic Aids to diagnose exactly which pan of the suspect circuit is causing the parasitic drain. In some cases a non-fused circuit or component, such as a relay, is the cause of excessive parasitic current drain.
Repeat the parasitic current drain test procedure after any repair has been completed to make sure that the parasitic current drain is at an acceptable level.
When the cause of the excessive current drain has been located and repaired, remove the J 38758.
Connect the battery negative cable to the battery negative terminal.

I would suspect the normal draw should be around 30 milliamps but I don’t know for sure what it should be. Other cars use about that range of current.

Since pulling fuses dosen’t seem to help locate the current path to the draw you may have to follow the hot lead from the battery and see what is connected to it and then remove whatever branches are connected to it in order to find the path.

This is Dave again. The question is now answered and the problem has been solved, so I’m posting this for those who might benefit in the future. I’ll tell you (1)what I believe the correct answer to the question to be, (2) how the problem was resolved, and (3) what I learned about current-measuring test instruments along the way. Since some asked, my Trailblazer is a 4x4 with no aftermarket options.
Item (1). Acceptable Current Draw: After a little research, the 25 - 35 milliamp figure mentioned above still looks like a good rule of thumb to me. AllDatadiy.com provides a list of GM service bulletins for this car, and one bulletin shows parasitic loads for six GM vehicles (none of them TrailBlazers) that range from 11 to 28 milliamperes (.011 to .028 amps). That matches pretty well with the figure I mentioned in my original question – about 25 - 35 milliamps. The source of that figure was an article written by someone named Larry Hammer, and can be found at this website: http://flashoffroad.com/electrical/CurrentDrain/currentdrain.pdf. If that does not work, Google “Parasitic Current Drain”. Now that the problem is fixed, my TrailBlazer draws less than 10 milliamps – after the computers on board shut things down (this takes about fifteen seconds or so).
Item (2). How the problem was resolved: I found that my (digital) ammeter had nearly the full battery voltage drop across it while reading 92 milliamps. I had expected the ammeter to have nearly zero volts across it. (The meter was just protecting itself.) This meant that the vehicle was not operating with normal voltages and the 92 milliamp measurement should be ignored. Time to start over.
I had already ordered a clamp-type ammeter but when it arrived I found that it could not be used as a “clamp meter” except in the very high current ranges (like 40 amps or more). Some research led me to the one I now have on order – an ExTech Model MA220 from Calright Instruments at calright.com. Cost is a around \$100. However, I wanted to fix this car immediately if I could, so I went to Radio Shack and bought sixteen 1-ohm Ten Watt Resistors, and connected them together in a bundle to make a 1-ohm 160-Watt resistor. I then connected this 1-ohm resistor between the negative terminal of the battery and the black battery lead – with the resistor temporarily SHORTED OUT with a test lead with alligator clips. I DID NOT ATTEMPT TO START THE VEHICLE OR OPERATE POWER WINDOWS ETC. WITH THIS TEST SETUP IN PLACE. After giving the car a minute or two for the computers to go through their paces and settle down, I removed the SHORTING test lead and connected my digital voltmeter across the 1-ohm resistor. With this setup, 1 volt means 1 amp, 1 millivolt means 1 milliamp, etc. The computers need 10 volts or so to operate normally, so any reading above 2 or 3 volts means the car may not be operating “normally”. That is why a clamp meter would be so nice to have. (As I write this, mine is still in the mail.)
What I found was that the parasitic load was not 92 milliamps but more than an amp! It was the dome light. But the dome lights were not lit up. Why? Because the car has a dome light override switch in the lower left corner of the dash. Push it once and the dome lights do not come on when the car doors open. Push it again and they do. It was switched to the position where they do not come on. Meanwhile, the instrument panel lights “DIMMER” control was turned all the way clockise – which turns the dome lights on. Here’s the key to the whole matter: even with the override switch keeping the dome lights turned off, there is still a big current draw with the switches positioned as described. Voila! Big current draw through the dimmer/dome light path with the dome lights NOT LIT UP. Chevrolet are you listening? How about a change in the logic so this cannot happen? On future cars of course.
Item (3). Well, I answered most of this at the beginning of the section above. I plan to use the ExTech MA220 in the future if this ever comes up again. The 1-ohm resistor described is clumsy and one has to be very careful not to let anything short out by accident.
One other thought might be helpful. I can now tell if I have a parasitic current draw just by using my tricke charger (which has its own ammeter). First, I charge the battery for an hour or two or until the the trickle charger’s ammeter indicates that “almost zero” charging is going on. Remember where the meter’s pointer is – even if it has no numbers on the scale. Then let the car sit overnight or for 5 or 6 hours with KEY OFF and NO LIGHTS ON. If there is a light that illuminates the engine compartment when the hood is up, lower the hood until the light goes out or remove the bulb if necessry. Disconnect the trickle charger. Then the next morning, reconnect the trickle charger and watch the pointer closely when the charger is plugged in. The charger current should move up to maybe half scale or so but you should immediately see the pointer start falling back toward zero. In 10 or 15 SECONDS, mine is almost back to where it was the previous night when the battery was fully charged. That’s when there is no parasitic load problem. When my TrailBlazer had its problem, the trickle charger would need 15 to 30 MINUTES to get itself back to “almost zero” charging current. I think this is a good poor-man’s way to assure oneself that such a problem is fixed.
Let me say thanks to those of you who wrote with helpful information. This is my first use of the Car Talk chat line, and I was amazed by the response.
I have found via Google searches that people sometimes have a frustrating time getting these parasitic current drain problems fixed. I hope that these notes will be helpful to someone in the future.

I could have told you the clamp was not going to work for the range you needed. I’m confused, doesn’t your DVM have a DC current input? Every meter I have ever owned will read current. The shunt works too, don’t get me wrong but my approach would be to connect the meter in line with the negative battery lead on the 10A input. Turn on key. If reading settles to <2A, then reconnect meter to 2A input and short across meter leads. Turn key on, wait observed time until settled and remove jumper. Same result you got without messing with a high wattage shunt resistor.

I noticed the other night when I messed with the interior lights as you described that the headlight switch lamp appears to be slightly lit at all times, even when the interior lights have gone out. I think I’ll check it out now to see if I have a similar situation. Thanks for the heads up!

Bravo to you for writing a follow up and enlightening all of us with your new found knowledge. I’ve been troubleshooting electrical and electronic systems for 35 years and it’s still a frequent learning experience.

You are welcome for the help Dave and glad to hear that you solved the trouble.

I used my Fluke 87 (put in series, on negative side for current measurements) and never had the meter influence my parasitic draw tests (worked for GM,always used the figure 30 milliamps for max)

I think GM approved of the Fluke 87 for diagnostic use.

You would think if the problem was initated by opening the doors (whether the dome came on or no) that a connection would have been made by seeing a meter spike when opening the door.

I have seen “dome overide” complicate other electrical diag.

Good Job