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Measuring car battery drain without interrupting power?

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klugesmith

Mon Jan 19 2015, 07:11AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Many references suggest that before removing and replacing a car starter battery, one should connect a backup power source "to preserve settings in engine controller, audio systems, etc.". How bad can it be, to have no voltage on the battery bus for 5 minutes? How about 5 seconds, or 1/2 second? I haven't noticed any immediately bad consequences from accidental complete discharges.

The question came up when I wanted to put an ammeter in series with a car battery, in order to measure and track down a spurious load that runs the battery down in just a few days.

Perhaps unnecessarily, I inserted the ammeter without ever disconnecting the battery. First made a small threaded hole in the negative battery post. Used a screw to attach the negative wire from meter. The surrounding lead-metal lug is contacted by a big alligator clip, which goes to the positive meter wire.

Then when the lug is slipped off of battery post, the meter takes all the current.

Keep away from the starter switch whenever a meter like this is in the circuit.

It would be safer and much faster to use a clip-on current probe that does DC. We have one at work, but its opening might be too small for a thick car battery cable.

Kolas

Mon Jan 19 2015, 07:24AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

It occurs to me that you could simply leave the engine running while you preform the switcharoo. The alternator will provide plenty for power for the short time you need it to take up the slack. Cars with dead batteries fool people all the time because the alternator keeps it going while the battery continues to deteriorate through deep cycling as the lead mesh dissolves over time.

Bjørn

Mon Jan 19 2015, 09:54AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Just a warning, those multimeters can be unreliable in places with infrared light. I have had one show 60V with no input because IR light made its way to the die of the controller.

Better check if it is a chip on board design without a light shield.

Wolfram

Mon Jan 19 2015, 10:42AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Kolas wrote ...

It occurs to me that you could simply leave the engine running while you preform the switcharoo. The alternator will provide plenty for power for the short time you need it to take up the slack. Cars with dead batteries fool people all the time because the alternator keeps it going while the battery continues to deteriorate through deep cycling as the lead mesh dissolves over time.

This can be a very very bad idea. Without the battery in circuit, the 12 V bus can rise uncontrollably, killing electronics. Modern cars are supposed to be protected from this sort of damage, but I still wouldn't risk it. From a document by Vishay

The worst instances of surge voltage are generated when the battery is disconnected when the engine is in operation, and the alternator is supplying current to the power line of the vehicle. This condition is known as â€œload dumpâ€, ...

Steve Conner

Mon Jan 19 2015, 11:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

In my opinion (based on not very much concrete at all :) ) it's good to disconnect the battery of a modern car now and again, so its dozens of microprocessors get a reset.

When I disconnect the battery on mine, the radio forgets its preset stations, the clock needs reset, and I have to drive a few hundred yards in a straight line to recalibrate the power steering. I think the ECU also forgets any mixture and timing corrections it might have learned, but those could probably do with an occasional reset too.

The only thing I ever had a problem with was a second-hand vehicle with a security coded radio. When I reconnected the battery, the radio asked for a PIN code that the previous owner forgot to give me. This required a trip to the local car audio installer to have the code hacked out.

Also it is a really bad idea to disconnect the battery while the engine is running! The inductive energy stored in the alternator windings will cause a load dump transient as mentioned above. Having said this, I'd expect the average 4hv member to have replaced the battery with a hybrid Li-poly and ultracapacitor module anyway. :)

Kolas

Mon Jan 19 2015, 01:05PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Well, I stand corrected. I suppose my pontification could have been the source of a very very expensive mistake. For that I apologize.

Conundrum

Tue Jan 20 2015, 06:00AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Running the engine with a flat battery which has been left (ie sulphated) is also unwise, less so than disconnecting it entirely but the same thing can happen.

I once owned a car where the battery had an (unknown to me) internal fault that caused the engine to die while driving due to a 12V bus undervolt.
This caused the ECU to go completely Strange Loop and the engine slowly lost power and cut out usually on a hill or in front of a queue of traffic.
Let it recover for 5 minutes and it started up and ran fine for a while until it happened again.
I found that for some reason it happened less when the battery was kept charged with a solar panel which was just fine in the summer but as autumn and winter approached it became more and more annoying.

Only found out when the battery got changed because it finally died completely and I found a huge lesion on the back of the battery right in the middle where it had evidently suffered an internal short between plates.

For those unfortunate to own a G1 Prius this can also happen if the 12V "system" battery dies, it stops everything else working even if the traction pack is fully charged.
I had endless amusement from jump starting an electric car with my old boat anchor, despite the owner being concerned about the dangerous HV from the 12V battery.

Ash Small

Tue Jan 20 2015, 12:50PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Personally, I'd remove fuses one at a time and connect the Ammeter accross the fuse terminals to identify which circuit is live when it shouldn't be. (assuming ammeter is fused).

The first Jaguar XJ6 I bought (an old Sseries 2 with carburettors) had an 'elusive' electrical problem which caused the same effect (battery drain while switched off) and the previous owner had spent quite a lot trying to have it traced.

The asking price reflected this. I paid him the money, opened the bonnet and disconnected the 'anti-run on valve solenoid'. This circuit, I'd checked, was the only circuit that 'could' be live when the ignition was 'off', as it opens a valve in the inlet manifold to prevent 'engine run-on' after the ignition is turned off, not a circuit that is really needed anyway, and only used on some cars with carburettors.

The vendor's jaw hit the floor.

This reminds me also of one of the V12 Jags I bought. I went to see it and it was parked outside the owner's hotel. He started it, but it wouldn't move when 'drive' was engaged. He was most upset as he'd recently paid to have the transmission overhauled. I looked under the car and saw it had an oil leak, and offered him half the asking price, which he accepted.

I then left and returned ten minutes later with a jug of transmission oil, which I topped up the transmission with, and then drove the car away. It was a fairly simple job to fix the leaking transmission later.

klugesmith

Tue Jan 20 2015, 06:36PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Bjorn, thank you for the tip about IR sensitivity. I do know that those ubiquitous DMM's don't last as long as ones with two- or three-digit price tags.

My first measurement of battery drain in the offending Volkswagen, with car all the way off, was 0.57 amperes.

At the fuse panel we can probe both ends of each fuse. Found most fuses at 0 volts w/r/t ground. The few at 12 volts mostly had end-to-end voltage drop of 0 mV. Two fuses were dropping about 0.5 and 4.5 mV, which I translated into amps after bench measurement of spare fuses. Later found fuse-mV-to-ampere lookup tables in a VW technical service bulletin on a car forum.

Was pleased to find glove compartment light staying on when door was shut. Verified that with a cell phone video, then used same trick to answer the refrigerator light question. (later found 100s of those on YouTube).

In the following week or two, battery drainage continued to strike. Newly measured 0.33 A can't be normal. Who knows what is normal?

Ash Small

Tue Jan 20 2015, 08:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Alternator rectifier diodes have been known to be problematic.

Otherwise, I suggest there may be more fuses elsewhere. More modern cars tend to have safety features like 'door unlocking on impact', etc which don't require ign to be 'on'.

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