Data logger to measure car battery drain overnight 1

[adamb1]

Hi all, I'm new to the forum.

My car has a battery drain problem that is proving difficult to diagnose.

I want to measure the current being drawn over a long period of time (e.g. 24 hours). I know it decreases to about 80mA after 20 minutes, but I suspect the car wakes up periodically and consumes much more.
None of the cheap current data loggers cover the necessary Amp range or resolution.

So my plan is to connect a 1 ohm, 10 watt resistor between the battery negative lead and the terminal on the car body.
I will then connect a Fourtec Microlite II 32K USB voltmeter (0-10V range; resolution 1mV; max load 12V; sampling rate once per second), measuring the voltage drop across the resistor.

I should then be able to view the data on a PC and verify whether the car is waking up periodically.

Can anyone find any errors and omissions in this plan? Or risks? Could the resistor overheat and cause a dangerous situation?
The ignition will remain switched off (the max current draw I have seen in this state is 6 amps).

Really grateful for help and suggestions.
Adam

 

[MikeHalloran]

A very high drain lasting less than a second might go undetected.
... Go ahead and measure as you propose, but if you don't find any spikes, it may be just that you happened to miss them.

... in which case, the next step would involve sampling at a much higher rate, and discarding adjacent identical samples if you are limited on memory. If every sample is time-stamped when stored, or time stamps are inserted between retained samples, this scheme would not be throwing away anything useful.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Is the battery actually getting completely drained? Even at 1 amp, it would take about 60 hr to drain a typical car battery.

TTFN
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[LiteYear]

Logger is battery powered and not connected to mains equipment, so no referencing issues.

If you really do have 6A current events then obviously the wattage rating of the resistor will be exceeded. As long as the events don't last more than about half a second you might get away with it, but it might be safer to put two of your resistors in parallel to be sure.

As mentioned, once-per-second recording might miss some events, but then again events of less than a second are unlikely to represent a great deal of battery drain.

Putting the resistor "between the battery negative lead and the terminal on the car body" might leave an alternate path if there's a cable running from the negative battery post directly to the starter motor for example. To ensure you get them all you might want to use hose clamps to clamp the resistor between all negative leads and the negative post.

Otherwise sounds okay on a first pass.

 

[bradrs]

If you start drawing much current across that resistor, the big voltage drop could result in a problem with some of the electronics. Particularly aftermarket stuff.

6A would result in a 6V drop across that resistor, bring total system voltage into the <6V range, which would start putting some electronics into brownout range.

From personal experience, I have seen relays start buzzing when their controller started seeing low voltage.

I'd consider a much lower resistance for that reason. To try to minimize your effect on the system, as you measure it. 0.1 Ohms would be better.

Good Luck

 

[adamb1]

Thanks for your responses.
Very useful comments so far.
I am now thinking of getting a 0.1R resistor. But given the risk of higher current I think I should up it to 100w rather than 10w.
For the same reason I'm going to need some better rated leads to connect the resistor to the battery terminal.

The car is a Range Rover L322. It tends to run at 3A to 6A for the first 16 minutes after shut down, dropping to 85mA when it's asleep.
There will be other earth connections other than the main one that might be causing issues, but I'd first like to investigate need whether the car wakes up periodically, drawing what current, and why. The battery doesn't get completely drained - it used to but I have since replaced the battery and fitted a 'Battery Brain' which isolates the battery if the voltage drops below 11.7V. Handy.

Thanks

 

[IRstuff]

Sounds good, so long as you don't have one of those annoying car radios that require a password after it loses power. It's a POS anyway, but its list price is high, I guess.

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[IRstuff]

oh, nonetheless, if your circuit gets to the point of cutting out the battery, it's a got a horrifically severe load, i.e., something on the order of leaving the lights on. That's not just random waking up. One possibility might be a cooling fan; some cars have fans that run after the engine stops to cool the catalytic converter, or something. If the temp sensor is malfunctioning, it could presumably come on in the middle of the night and stay on.

TTFN
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[DanEE]

I got a bit of a surprise and education when I left the vehicle in the garage with the doors open for the purpose of drying out the carpet (and all dome lights switched off) with a fan blowing through the open doors after the wife has spilled some water inside the vehicle...

Dead battery the next morning.. and prompted me to make similar measurements by lifting the negative lead off the battery and inserting a current meter in series.. Many vehicles these days have so-called "body computers" that perform the security function and in many, operate the power windows and other devices, e.g. the power window switches are merely signal inputs to the body computer (and wiring can be downsized accordingly). The computer does things such as window reversal (child entrapment/suffocation issues with head in window). The body computer is located on the back of a fuse panel that also contains the relays the body computer operates.

Anyway, the body computer, ECU computer normally go into a sleep mode to save battery draw when the vehicle is not in use. The engine ECU on many vehicles comes up to full power mode when the driver door is opened in prep for starting the vehicle. Perhaps this little bit of info about these systems will help you find the culprit. You may have a problem with some event keeping the computers from going into sleep mode.

If you have a vehicle with the newer keyless entry system (the key fob is sensed by radio, no button pressing required, (e.g. the fob unlocks the car and enables ignition start with the fob in your pocket), you have to be careful with storing the key fob within radio range of the vehicle as it can keep the security computer from going into sleep mode. I only have documentation on Toyotas with this feature, but to avoid running down the battery, their design will time out after 15 minutes of key fob in range of the vehicle and no vehicle entry or start.

Toyota's troubleshooting procedure for finding excessive parasitic loads.. Their maximum spec'd parasitic draw is 50ma

http://home.4x4wire.com/deddleman/photos/battery_parasitic_load.jpg

Measurements I made on my 4x4 vehicle.. Note various doors being opened and subsequent effect on body computer and ECU electrical draw (all dome lights switched off for these measurements). Ignore measurements on 2nd battery.. that's a custom installation with accessories operating off the 2nd battery with voltage drop threshold automatic shutdown.

http://home.4x4wire.com/deddleman/photos/battery_current_draw.jpg

 

[waross]

Check the duty cycle specs on your instrument. I have seen multimeters that were only rated for short time use. I forget the time but less than an hour. Hopefully this is not an issue with your instrument but worth checking.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tmoose]

Is the root problem a battery that dies?

A "bad" battery will do that. A friend recently had a 2 or 3 year old battery that after extended charging tested "good" with a tester like this

http://www.restockit.com/images/Product/medium/OTC3180.jpg

But it was too weak to start the car after sitting a few days.
A fancier tester (done free at a chain) detected some internal cell issues.

 

[MRSSPOCK]

I didn't read all the above posts because I haven't time, but it reminds me of a story a friend told me.

Maybe completely irrelevant, but here it is.

He tried for ages to find out why the battery was going flat.

Eventually he gave up and took it to the local autospark.

The spark told him too open the boot (trunk if your in the usa), he then said to my friend, "get in".

He got in and the spark closed the boot lid.

He shouted in, "is the light on".

My friend was happy to say, "yes".

That was the problem sorted in about 2 seconds.

Nice story.

Pity they all weren't all so easily diagnosed.

 

[MacGyverS2000]

An engineer would have just pressed the trunk lid's button and looked at the light ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

Nah, he would have wired up an Arduino with a light sensor and coded up a routine to send him the light sensor data through Bluetooth to his phone.

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[LionelHutz]

Ya, what button? I don't recall ever seening a button on a trunk or hood light. They seemed to have mercury switches in them on older cars and nowadays are tied to the BCM which is monitoring the status of the latch.