Voltage drop data logger in automotive applications

wayneh

Joined Sep 9, 2010
16,509
I'd like to build a voltage drop data logger to help find what circuit is draining a cars battery intermittently.
I've been thinking about this the wrong way. I thought you meant something was drawing too much current while running, but you're describing something that causes a dead battery once in a while, for no apparent reason. Right?

In that case I think I'd look at it more globally and have a sort of alarm that triggers when the current draw while "off" is (maybe 2? times) more than whatever the typical value is. This wouldn't identify the circuit but if you could examine the vehicle when the fault is occurring then the culprit wouldn't be too hard to pin down.

Or were you thinking of something that could travel along with the car and log over time, and thereby catch the culprit in the act, in or out of the shop?
 

Reloadron

Joined Jan 15, 2015
5,715
In the years of working on cars, and trouble shooting over night or a few days discharging, what I've found most times were the lights under the hood (bonnet) or the trunk lid(boot). The flimsy brackets get bent and the light stays on when it should be off. This runs down a battery. First thing I told people with the discharge problem is to remove those bulbs and see if that fixes the problem. Many times it did.
Years ago my mom had a Buick and if left for several days the battery would go dead. Overnight it would start the next day. Then one day, like a miracle, it healed and worked fine. Then one day my dad went to use her car and opened the trunk. The area around the trunk light was all discolored and the lamp opened up. That bulb stayed on till it finally died. The trunk lights on that year used a tilt sensor which failed. Soon as the bulb was replaced the lamp lit and moving the sensor the lamp remained lit.

Ron
 

Reloadron

Joined Jan 15, 2015
5,715
I've been thinking about this the wrong way. I thought you meant something was drawing too much current while running, but you're describing something that causes a dead battery once in a while, for no apparent reason. Right?

In that case I think I'd look at it more globally and have a sort of alarm that triggers when the current draw while "off" is (maybe 2? times) more than whatever the typical value is. This wouldn't identify the circuit but if you could examine the vehicle when the fault is occurring then the culprit wouldn't be too hard to pin down.

Or were you thinking of something that could travel along with the car and log over time, and thereby catch the culprit in the act, in or out of the shop?
That is some good food for thought.

Ron
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
Then post some links to this multitude of tools. All I did was make a few suggestions and tossed out a few links. This is about as useful as teats on a bull. I believe your reference is to cartridge fuses as I am unfamiliar with a cartage fuse.

Thank You
Ron
Cartage fuses are over 50% of the fuses in the box in most new cars. On top of fuse taps for blade and glass fuses There are many back probes one can use I hope that helps clarify thing. Sorry to have ruffled your feathers Ron, just trying to concentrate on the circuit part of the project.
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
In the years of working on cars, and trouble shooting over night or a few days discharging, what I've found most times were the lights under the hood (bonnet) or the trunk lid(boot). The flimsy brackets get bent and the light stays on when it should be off. This runs down a battery. First thing I told people with the discharge problem is to remove those bulbs and see if that fixes the problem. Many times it did.
I agree but in the past few years I've seen motors, specially rear wiper motors drain the battery. I also found a short in a can bus pair that would keep one module from going to sleep, and it would ping other modules and wake them up. Those are not an overnight issue when the weather is warm, but at -30C or below, it is enough for the car not to start in the morning.
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
Until such time that you have some form of working monitoring logging system to identify which fuse circuit is causing the battery discharge – I offer this solution:-

Given that most vehicle batteries have a rating of 60Ah or more, for the fault to completely discharge a battery overnight, the current draw must be at least 3A for a considerable time (or a higher current for a shorter period). Therefore if you replace all the vehicle fuses with a 1A fuse – any circuit experiencing a current draw which would flatten the battery would cause the 1A fuse to operate.

A 1A fuse can pass 2A for some time before operating (equal to 24W at 12V), therefore having installed all 1A fuses, operating the remote vehicle locking system should not blow any fuse.

1A auto fuses are available, see the link below.

https://www.ebay.co.uk/itm/100x-Sta...=item19ef0cdd69:g:uu4AAOSwFnFWAYTX:rk:14:pf:0

Tip: before pulling all the fuses, take a photo of the fuse box so that you can ensure that the correctly rated fuses are replaced after the test.
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
Doing this means the car stays at the shop because as soon as you would turn the key, fuses would blow. When the car is in the shop, if the issue is present, I can find it by measuring the voltage drop on the fuses and finding which circuit to tare apart.

When the issue happens rarely, it usually happens when the car is in the customers care and it is driven. By monitoring voltage present on circuit that only powered off when the key is off, I know when to read my hot at all times fuses. Monitoring the battery voltage tells me when to save a data set and I can monitor the charging system to an extant.

I really liked the circuit you first drew. I think it is in the right direction. The micro controller would be powered independently with maybe a charger to top up it battery when the engine is running. Ideally, I'd like the circuit to be independent like a DVM is when you take a voltage drop measurement.

I saw a board that is sort of what I need. The ACS712 module read current and set the sig to .5 the VCC voltage as 0. so when the voltage on sig is between 0 and half VCC it is negative and from half VCC to VCC it is a positive reading. There 2 major issues with it that make it not adequate for what I need, the fist is the module need to be in the circuit and second the range is to high.
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
I've been thinking about this the wrong way. I thought you meant something was drawing too much current while running, but you're describing something that causes a dead battery once in a while, for no apparent reason. Right?
Exactly

In that case I think I'd look at it more globally and have a sort of alarm that triggers when the current draw while "off" is (maybe 2? times) more than whatever the typical value is. This wouldn't identify the circuit but if you could examine the vehicle when the fault is occurring then the culprit wouldn't be too hard to pin down.
Not quite. When the fault is occurring, measuring the voltage drop on the fuses points me the appropriate faulty circuit as it draws more current than it should. I can then look up the diagram for that circuit and make a diagnostic plan to pinpoint the issue.

Or were you thinking of something that could travel along with the car and log over time, and thereby catch the culprit in the act, in or out of the shop?
This is what I'm looking for. Back in the old days the where about a dozen circuits in a car. In today's cars you can have hundreds with 3 or more fuse panels. The circuits are usually leveled with a main fuse in the underhood panel that feeds a subgroup of smaller fuse panels or something to that effect. When the issue is not the typical light or wiper motor, and is intermittent, finding the circuit group the fault belongs to at the very least can greatly reduce time needed to find the culprit.

To accomplish this I need to be able to read the voltage drop across fuses when the car is not running and all the modules are a sleep. The total current draw should be lower than 50 mA.

Measuring a voltage drop with a micro controller safely is what need to find how to do safely with a precisiom of 0.1 mV
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
The inputs to the PSOC mux are differential. One is + side, or battery side
of fuse, the other - lead, load side of fuse. If leads reversed V measured
would be a - value, rather than a + value. If you never have a situation in
system where load side of fuse is at a higher V than battery you could
treat leads as non polarized and handle the sign changes in software.

One other comment about design, since CMOS is used the inputs to
mux have to use a series R and diode clamp on them to prevent damage
to inputs if connections are made to a blown fuse causing 12 V to be pre-
sented to inputs. There are R and Diode arrays available to make this
trivial to implement.

Lastly you can control the range the A/D digitizes, such that the resolution,
in 16 bit case, can range from .2 uV / lsb to 2 mV / lsb, ranges .064V and 6V
respectively, or several other ranges/resolutions in-between.

No need for an IA.....

View attachment 167704

Also shown is onchip LCD/Touchscreen support and SDCARD support. Again this is all on one chip.
The LCD can be SPI, I2C, Parallel, graphic, character, whatever.


Regards, Dana.
I don't quite understand how it would work but it sounds promising specially if the PSOC 5LP can do all of this because of it's small footprint. You might need to explain in more details or point to a good resource maybe.
 

Reloadron

Joined Jan 15, 2015
5,715
Cartage fuses are over 50% of the fuses in the box in most new cars. On top of fuse taps for blade and glass fuses There are many back probes one can use I hope that helps clarify thing. Sorry to have ruffled your feathers Ron, just trying to concentrate on the circuit part of the project.
You aren't going to ruffle my feathers, believe me. :)
It's just a matter of understanding what you want and getting some solutions. What was your view as to an off the shelf amplifier syatem, would that maybe work?

Ron
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
You aren't going to ruffle my feathers, believe me. :)
It's just a matter of understanding what you want and getting some solutions. What was your view as to an off the shelf amplifier syatem, would that maybe work?

Ron
I don't know honestly. DMM circuit's all seem to use amplifier but 'm not up to par with the knowledge to understand those circuits. It didn't seem to hard to do a simple voltage drop reading with a micro controller at first, but it looks like reading a voltage drop might be way more complicated that I imagined.
 

wayneh

Joined Sep 9, 2010
16,509
Measuring a voltage drop with a micro controller safely is what need to find how to do safely with a precisiom of 0.1 mV
Challenging. I’m picturing an octopus harness where you replace every fuse with one that is hooked up to a logger. Could be an interesting product.
 
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cork_ie

Joined Oct 8, 2011
428
I would like to add my tuppence worth to this if it may be of any help. I am working as an auto/marine electrician for 40 years and this is a very common problem and difficult enough to narrow down in modern vehicles.
With CAN & LIN networks; the drain current can be quite high, long after the ignition is switched off. It can often take up to half an hour for for the networks and electronics to finally shut down and reach quiescent ("sleep") mode. Before attempting to measure any drain , the car must appear, to all systems, that it is closed and locked so that the shutdown process can even begin. You will also need access to the interior fuse box(es) at the same time. Thus you will need to disable any ultrasonic intruder system and close the locks and any door ajar switches.
Initially it is a good idea to see what fuses are actually permanently powered, as the majority are likely to carry no power when the car is locked and in sleep mode. You can then try measuring voltage drop across the the live fuses , but I have found it a very unsatisfactory method as it is hard to equate the resistance of fuses to their rating, there are so many different types and ratings. The voltage drop is also likely to be in the microvolt rather than millivolt range and you will need to have a suitable DMM which are not that common and can be expensive.
It is easy enough to quickly determine that any current drain is less than 1-2A; anything above that is easy to check and I wont dwell on it here.

Beyond that, I have found far and away the best method to locate a parasitic drain is:
1)
Determine which fuses are live and make a list of their positions. Allow car enter quiescent mode for 30 mins
2) After everything has finally gone into quiescent mode, check if you can detect any voltage drop across any live fuses. If so check those circuits first.
3) Once you have determined which fuses control suspect circuits . Reopen the car and start all over again. Fit a 1 Ohm 25Watt resistor in series with the battery positive or negative terminal and further 1 Ohm resistors (fitted with in-line fuses for all circuits with a rating less than 10Amp.) in lieu of the suspect fuses.
4) Close car and again allow it to enter quiescent mode.
5) Check the voltage drop across the resistor fitted at the battery terminal , you can determine total drain current directly from it 1mA-1mV.
6) Check the individual resistors you have fitted in the fuse locations and again measure voltage drop and hence circuit current, as already described
Very quickly it will be apparent which circuit is the culprit, in some cases it may not be a fused circuit at fault i.e. wet starter solenoid or faulty alternator , wet connector etc., the same principal applies. If you wish to datalog the various systems then it is a simple matter to sample the voltage drops across each resistor as previously described in this thread.
I have found this system to be 100% accurate, every time & with very simple equipment. Even normal DC clamp on ammeters are not accurate at anything below 1Amp.
Overall the time is quite short as you can do other things while waiting for the networks to shut down, so it is not as time consuming as it may seem.
In a modern car anything up to 80mA quiescent drain is possible and acceptable, 50mA being more typical.
 

Thread Starter

sid1202

Joined Jul 25, 2016
25
I would like to add my tuppence worth to this if it may be of any help. I am working as an auto/marine electrician for 40 years and this is a very common problem and difficult enough to narrow down in modern vehicles.
With CAN & LIN networks; the drain current can be quite high, long after the ignition is switched off. It can often take up to half an hour for for the networks and electronics to finally shut down and reach quiescent ("sleep") mode. Before attempting to measure any drain , the car must appear, to all systems, that it is closed and locked so that the shutdown process can even begin. You will also need access to the interior fuse box(es) at the same time. Thus you will need to disable any ultrasonic intruder system and close the locks and any door ajar switches.
Initially it is a good idea to see what fuses are actually permanently powered, as the majority are likely to carry no power when the car is locked and in sleep mode. You can then try measuring voltage drop across the the live fuses , but I have found it a very unsatisfactory method as it is hard to equate the resistance of fuses to their rating, there are so many different types and ratings. The voltage drop is also likely to be in the microvolt rather than millivolt range and you will need to have a suitable DMM which are not that common and can be expensive.
It is easy enough to quickly determine that any current drain is less than 1-2A; anything above that is easy to check and I wont dwell on it here.

Beyond that, I have found far and away the best method to locate a parasitic drain is:
1)
Determine which fuses are live and make a list of their positions. Allow car enter quiescent mode for 30 mins
2) After everything has finally gone into quiescent mode, check if you can detect any voltage drop across any live fuses. If so check those circuits first.
3) Once you have determined which fuses control suspect circuits . Reopen the car and start all over again. Fit a 1 Ohm 25Watt resistor in series with the battery positive or negative terminal and further 1 Ohm resistors (fitted with in-line fuses for all circuits with a rating less than 10Amp.) in lieu of the suspect fuses.
4) Close car and again allow it to enter quiescent mode.
5) Check the voltage drop across the resistor fitted at the battery terminal , you can determine total drain current directly from it 1mA-1mV.
6) Check the individual resistors you have fitted in the fuse locations and again measure voltage drop and hence circuit current, as already described
Very quickly it will be apparent which circuit is the culprit, in some cases it may not be a fused circuit at fault i.e. wet starter solenoid or faulty alternator , wet connector etc., the same principal applies. If you wish to datalog the various systems then it is a simple matter to sample the voltage drops across each resistor as previously described in this thread.
I have found this system to be 100% accurate, every time & with very simple equipment. Even normal DC clamp on ammeters are not accurate at anything below 1Amp.
Overall the time is quite short as you can do other things while waiting for the networks to shut down, so it is not as time consuming as it may seem.
In a modern car anything up to 80mA quiescent drain is possible and acceptable, 50mA being more typical.
If you still do diagnostics, you can use this chart to help out blade fuse readings and convert mV to mA. As for cartridge fuses you have to go to a site like littlefuse and get the datasheet for the cartridge type the car uses.

All of this though does not explain the circuit needed to do the voltage drop reading with a micro controller.
 

cork_ie

Joined Oct 8, 2011
428
If you still do diagnostics, you can use this chart to help out blade fuse readings and convert mV to mA. As for cartridge fuses you have to go to a site like littlefuse and get the datasheet for the cartridge type the car uses.

All of this though does not explain the circuit needed to do the voltage drop reading with a micro controller.
Believe me that chart, in practice, is wildly incorrect. A lot depends on the resistance of the fuse , which varies widely from maker to maker. I have tried it several times and could never get consistent readings, good enough to give an accurate estimation of current draw. In addition there are many fuses that don't have probe points in front. I don't know of any vehicle manufacturer fitting "Littlefuse" brand as standard. They are only one brand and there are many others - all different. It depends very much on whether copper, aluminium or some other alloy is used as the fuse link.
I have merely explained the best way to accurately determine total parasitic drain and individual circuit drain, indirectly but with good accuracy using voltage drop across a fixed resistor in any circuit. Such drop being easily measured by readily available equipment.
After that it is a relatively easy matter to log the individual voltage drops on your arduino using multiplex or otherwise . I use a Hantek recording voltmeter to do the same thing.
 

djsfantasi

Joined Apr 11, 2010
7,213
Are there many different fuse blocks (1000s) or is the number of different fuse blocks a much smaller number? My thought being is to create a daughter board that the fuses are plugged into and the entire unit is then plugged into the vehicles original fuse block. This device could be used for the same type of vehicle or perhaps several vehicles using the same fuse block.

The device could also have onboard intelligence, to mux measurements to an offboard device. Or perform data logging as in the original post.

It’s the monolithic nature of this device that is its attraction. Removing and replacing dozens of fuses is simplified to one operation.
 

shortbus

Joined Sep 30, 2009
8,179
Are there many different fuse blocks (1000s) or is the number of different fuse blocks a much smaller number?
Don't know if it stretches into the 1000's but when I was still working, one of our plants molded over 30 part numbers just for GM cars. And that was back in the 1990's. And don't even know what was being done in other plants, in other states. Back then cars only had 1 or maybe 2 fuse blocks, now they have many.
 

danadak

Joined Mar 10, 2018
4,057
To do this right one has to use precision shunt in test fuse cartridge, and kelvin
sense the voltage. That way absolute accuracy would be assured due to variation
in fuses. And the higher current loads would be confined to flowing thru just shunt,
not instrument which would demand heavier leads, impractical . To handle the wide
range of currents one would want a wide dynamic range converter so that mA to
10's or amps could be handled. So that's 4 decades of current measurement or at
least 17 bit A/D. That way one shunt could be used across the design. Or design
for multiple valued shunts, more complicated.

At 12 bits, let alone 17 or higher, absolute accuracy over T and V very challenging.
Relative accuracy if allowed much easier to achieve.

Regards, Dana.
 
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Reloadron

Joined Jan 15, 2015
5,715
I don't know honestly. DMM circuit's all seem to use amplifier but 'm not up to par with the knowledge to understand those circuits. It didn't seem to hard to do a simple voltage drop reading with a micro controller at first, but it looks like reading a voltage drop might be way more complicated that I imagined.
OK, DMMs aside for a moment. Your objective here is simply to measure the voltage drop across a fuse, any of several automotive fuses. The voltage drop across the fuse will be a function of the current through the fuse. The fuse has some resistance to the flow of current so we get a subsequent voltage drop across the fuse which is proportional to the current flow through the fuse. You want to measure that small voltage drop across the fuse and you also want to data log that voltage drop over a period of time.

Starting with the fuse. We know the fuse has a resistance and we know that resistance is low, very low. Fuses are designed to have very low resistance since we want the voltage to be applied to a load and not dropped across a fuse. The actual fuse element resistance will be a function of its current rating and even the fusible link material and even the contact material used such as tin plated zinc spades or silver plated spades along with ambient temperature. All of these characteristics can vary based on manufacturer. However, they are likely close as to resistance based on rated current. We can also expect the voltage drop across the fuse to change base on the fuse element getting warmer as fuse current increases. The temperature increase will cause the element resistance to increase adding to the subsequent voltage drop across the fuse. I would think all of that should warrant some consideration in the measurement plane.

Data acquisition will present a new set of challenges. How often should the data be sampled? Once a second? Ten times a second? One-Hundred times a second? Where do we put the sampled data? Using a uC (micro-Controller) to acquire the data is fine but there are some considerations.

The following images reflect data taken at an ambient room temperature of 70 degrees F (21.1 C). The meter is using a 4 wire kelvin measurement on a standard One Ohm precision resistance.
One Ohm Standard.png

Meter reading
One Ohm Metered.png

I think we can say the meter is accurate enough to measure a few fuses.

Two Amp Fuse
Fuse 2 Amp.png

Meter reading of two amp fuse at ambient temperature
Meter 2 Amp.png

So at ambient less any current to speak of a two amp fuse is about 42 milli-ohm (o.042 Ohm).

Thirty-Five Amp Fuse
Fuse 35 Amp.png

Meter reading of thirty-five amp fuse
Meter 35 Amp.png

The ambient temperature reading of a 35 Amp fuse is just about 3.0 milli-ohms (0.003 Ohm).

Any voltage drop across these fuses, especially higher current rated fuses will literally be down in the dirt and noise. Using a 10 bit Analog to Digital converter as in most micro-Controllers with a 5.0 volt reference is going to give us 1024 quantization levels or the best resolution we can hope for is 5 volts / 1024 bits = 4.882 mV which is as good as it gets and does not compensate for any A/D errors which there will be. It becomes obvious any DC Voltage Drop across even a 2 Amp fuse will need to be amplified just to use any form of A/D conversion and even if we went with a better A/D which starts adding to cost we will need amplification and precision amplification. This does not even begin to consider the temperature coefficient curve of the fuse. That curve needs to be figured into the software routine for the data acquisition system.

Less any consideration for the temperature coefficient of the fuse, assuming there is none, here is what we would get. A two amp fuse with a fixed resistance of 0.042 Ohms and 2.0 Amps of current will have a voltage drop across it of about 8.4 mV. If we amplify that X100 we get about 0.84 Volts which is close to doable for measuring. But if we now look at the 35 Amp fuse we get .003 * 35 = 0.105 Volts and we amplify X100 we get 10.5 volts. That is a problem using an A/D with a 5 volt reference.

Earlier I mentioned using an amplifier with multiple gain settings and that is why. Now looking at what danadak mentions:
At 12 bits, let alone 17 or higher, absolute accuracy over T and V very challenging.
Relative accuracy if allowed much easier to achieve.
As we increase the ability of our A/D we increase the cost. There is quite a bit to all of this to accurately know how much current a fuse is passing and when we toss in the temperature coefficient of the fuse alloy it adds to the cost. Using voltage drop across a fuse is not going to be cheap. Very doable but far from inexpensive to do it accurately and correctly.

Ron
 

wayneh

Joined Sep 9, 2010
16,509
OK, DMMs aside for a moment. Your objective here is simply to measure the voltage drop across a fuse, any of several automotive fuses. The voltage drop across the fuse will be a function of the current through the fuse.
In my vision of the test rig, the normal fuses are completely replaced by test shunts. These would have to still provide a fuse-like function for safety, but would have a bit higher resistance by design so that they show a higher voltage drop, like maybe 100mV at 1A, or 0.1Ω. The shunt would be bypassed to a regular fuse when the car ignition is on, taking the test rig out of the circuit during normal operation and bringing it online only when the car is switched off.

I didn't say it was simple, but I don't see why it wouldn't be doable.
 
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