Battery / product failure involved in fire 1

[rjgoebel]

Hello,

This is my first post, but it definitely seems like this board is a great resource. I just graduated from a good engineering school with a degree in EE, and am now getting into forensics. I'm pretty green but am already on a big case involving a certain company and a product of theirs that has caused a large-loss fire.

So I am taking apart the product, learning about how it works, taking out the PCB, and looking at the remains of some of the products that have failed. Basically what happened was that the battery literally blew out- they were 9V duracells, standard stuff, and you can literally tell on a few of these products that somehow something caused these things to explode (on the side opposite the cathode and anode).

I'm not really allowed to get into any more detail about the comapny that makes the product, but I did call Duracell and spoke to a low-level consumer questions guy. He casually mentioned that there is a known compatibility issue between the Duracells and this product, and he had no idea what it was caused by. However, he was not able to directly put me in contact with a design person or engineer at Duracell, or someone that could tell me a little more about what might cause a battery to do that.

I would love to talk to some engineers at the unnamed company, but because of the nature of our business, there are a lot of legal issues at stake and a product recall is potentially in the works, so I guess it's not good to tip our hand at this point. So my task as an aspiring green forensics EE is to determine the cause of this failure without talking to any engineers or design people at the unnamed company. This could prove to be tough, and I need suggestions or resources for getting in touch with people that might be able to give me specifics about battery failure.

I also need information on how to get in contact with important people in large companies. Duracell, I am sure, would love to learn more about this issue, so I think talking to a battery engineer there wouldn't be a problem, but I am running into the same problem that people have when they need to speak to the President of the United States. And as of right now, (graduated a month ago), I do not have an enourmous list of contacts in the EE field that I can turn to for expert advice on certain subjects.

Jimmy Goebel
Mid-West Forensic Services, Inc.

 

[zeusfaber]

Contacts, when you're starting out, is hard work - especially if you're working for a small company and are in the sort of business where you can't tell in advance which firms you're going to need to develop a relationship with.

I've always found walking the floor at tradeshows to be quite productive - but then I work for a large, quite well known organisation and have a job title that hints at buying influence.

If you can't talk to the specialists, your next resort is to get hold of the battery spec sheet, (also any national/trade/military specs it complies with) and see if there's anything about the application under normal or single fault conditions which might violate the spec. Suspect parameters for a popping battery could be forward and reverse current, reverse charge, environmental temperature and explosive decompression.

You could then test whether that particular spec violation delivers the right symptoms.

After all that, nine times out of ten it'll still turn out to be a shorted diode, a whisker, or stupidity causing the device to attempt to charge a primary battery against its will.

A.

 

[rjgoebel]

You hit the nail on the head when you said small company and that I do not know what companies I need to develop a relationship with. I have only been here for about 3 weeks now, but the thing I love most so far about this job is that its small size allows me to feel like I can really make an impact.

How do you learn about tradeshows? And do you have any more information on those parameters you mentioned that could cause that sort of battery failure? Isn't reverse charge and reverse current virtually the same thing?

How should I go about testing a diode to see if it is shorted? (while on a PCB, or do I have to remove it from the PCB?) I have isolated the diode on the PCB that should make the battery only be able to act as a source and not be able to have current flow in the reverse direction.

The issue I am running into now as a forensics person is that I can't just go into a lab setting and test and play with my circuits like I could in school. There is a lot of red tape and what evidence we have, we cannot conduct any destructive testing on it unless all parties are present (meaning parties representing the potentially liable). So that makes it tough because I have products that have failed, (batteries popped out), I have some that were consumed by the fire, and I have a few more that are in good working condition. My testing methods need to focus on forming hypothesis for what has caused this, and trying out failure methods on working products. That way I won't have to worry about the red tape of destructively testing evidence.

Jimmy Goebel
Mid-West Forensic Services, Inc.

 

[zeusfaber]

Starting off as a singleton in a small organisation sounds like good fun, but it's certainly the hard way when it comes to building a personal list of professional contacts.

I guess the tradeshows start with looking through likely adverts in trade magazines, and some inspired google searches. Once you have a few good contacts, you start to hear about these things by word of mouth.

Reverse Current is about charging a cell that isn't designed to accept (a/that much) charging current. Reverse charge is about forcing forward (discharge) current through a cell after it's gone flat - it sometimes happens if you've got lots of cells in series, and one has a lower capacity than the others, or if a battery is connected to a system the wrong way round

Whether you can test the diode in-situ depends on the design of the circuit, and on whether you're allowed to remove the battery.

If lots of the devices are failing, you could try analysis of the design (including component stressing - for which Mil Hdbk 217 might finally be of some value to somebody and pcb track spacing) to see if a failure is inevitable; and build quality inspection (is anything assembled wrong in the failed devices, and the general standard of cleanliness).

.... and of course you will have to prove that the fire didn't make the batteries pop, rather than the other way around.

A.

 

[rjgoebel]

The diode adjacent to the battery is designed to allow current to only flow in one direction in relation to the battery. If I use a jumper to short around the diode, and see if the result is the same as for our products that failed, does this sound like a valid way to do fault testing?

My concerns are:

1) We have no way of determining how long it might have taken these products to fail like this, (So how long do we leave this thing on and wait for batteries to pop) and
2) If this diode did become shorted, why did it do this on multiple units? ( We have at least 2 of the 5 units that this battery problem is evident on)

 

[zeusfaber]

Taking (2) first.

Reasons why charge current might get past the diode:

This could be down to a duff batch of diodes, or diodes that are underspecced for some element of the (electrical or physical) environment (the reference to Mil Hdbk 217 is because this has a rule of thumb way of working out how long things like diodes might last under various levels of abuse), or they might have been put in back to front in some units, or a mounting screw might be shorting a couple of tracks together, bypassing the diode altogether.

Or it might not be down to charging the battery at all (it's awfully easy to get carried away with the first promising theory you come up with).

If you haven't got stacks of product to mess around with, I'd be disinclined to try wrecking another one. All this really allows you to do is to see if the battery looks the same after it's popped - significant if it doesn't, but otherwise inconclusive.

You don't say whether any of the failed units are in good enough nick to measure the electrical conditions at the battery terminals if you put a new battery in. If so, comparing with "unit that hasn't failed yet" and "educated expectation" could tell you a lot.

You might get away with shorting the blocking diode on a good unit out just briefly to measure the resulting current (not familiar enough with forensic conditions to know whether this would be OK). This would allow you to stew as many batteries as you like under representative conditions without jeopardising any more units.

All ways round, unless Duracell have got it badly wrong, or the unit deliberately abuses the battery, what you're seeing is a secondary failure: your job (by speculation backed up by analysis, examination and measurement) is to find the corresponding primary failure, and the reason this could have cropped up so often. I wouldn't try too hard to demonstrate the primary-secondary link until I was pretty confident I had found a credible and demonstrable primary failure.

A.

 

[rjgoebel]

That was a very helpful post.

One thing I am now thinking about, is the fact that the units that did fail, the PCB board does look like it is still in good condition. The two recovered failed units were not damaged by the fire; the only visible damage to them is to the battery.

So if I put in another working battery, and establish that the unit has current going into the battery backwards, or that there is maybe slightly more current enterring the battery than exiting the battery, this might tell me a little bit about what it is that could be going on with that battery or elsewhere. I think that is what I will try to do, as long as I get cleared to do it to the units that are considered faulty (which I am not sure I can because of legal issues).

I'm also new to failure analysis jargon - but I am guessing that primary failure refers to the individual component that failed while secondary failure refers to the overall system failure?

A way that I just thought of to test those diodes is to hook up a different battery in reverse direction, with the intention of sourcing current through the diode backwards, and measuring the current. If I=0 then the diode did not fail (short) but if there is noticeable current then there must be a problem there. I think I'm going to be needing a resistor in there, too, to limit the current to something just in case it is shorted.

What do you think?


Jim Goebel,
Electrical Engineer
Mid-West Forensics, Inc.

 

[rjgoebel]

Something I just thought about was: how do you measure the charging of a capacitor by measuring a difference in current in vs. current out. Is that possible? Is it possible to do something similiar with the battery in a unit that failed? I just realized that I've never actually done anything like that before, and as a testing method I'm not sure that will work.

In addition, if there is a difference in the amount of current going in vs. out, (in the correct direction) does this point to a battery failure rather than a problem with the circuitry?

Jim Goebel,
Electrical Engineer
Mid-West Forensics, Inc.

 

[zeusfaber]

Primary failure = The whole lot was working properly, and then this component failed (either randomly, or for some good reason) (for instance, some twit put a nail through a cable)

Secondary failure = Something had already failed, causing this other component to fail also (for instance, the short caused by the compromised cable burnt the distribution board out).

What you can do to test the diode depends a lot on the layout of the circuit (which you will need to understand quite thoroughly anyway). If you're lucky, once you've taken the battery out, a quick prod around with a multimeter will tell you a lot about the state of the diode. Not sure how much more you'd get by abusing the system by putting a battery in the wrong way round.

Don't really understand your current in vs current out question.

A.

 

[rjgoebel]

It is possible that I am way off-base on this but current = charge/time, so I am wondering if there is a way to determine if the charge entering the battery is greater than the charge leaving the battery, albeit on a very small scale. I've never heard of this being done before but it seems as though any component capable of storing charge (having capacitence), would HAVE to have a different amount of current in vs. current out as it was storing charge. Maybe this relationship is hard to quantify, or more of a theoretical/acedemic matter, but it might still be worth looking into?

And I do plan to use a multimeter and probing around the diode to determine if it is shorted or open, which hopefully should not be all that difficult to do. If it is, more than likely that is the primary failure of the circuit, although I guess in fairness that also could have been caused by the failure of some other component. What causes a diode to short out? Too much current, voltage?

Jim Goebel,
Electrical Engineer
Mid-West Forensics, Inc.

 

[zeusfaber]

I think you need to have a think about the difference between batteries and capacitors.

Capcitors let you peel charge out of one side of the device, and pump it in the other without building up too large a voltage in the process. Provided you don't leak too much across the dielectric, you're going to be able to ship all that charge back. Where the system is inefficient, you will tend to lose voltage, rather than ability to return the charge.

In a battery, you're moving charge through the chemistry of the cell, and storing energy in that chemistry in the hope that you'll be able to use some of this energy to pump charge in the opposite direction later. Losses here read right back into how long the pumping process can be sustained, and from there into how much charge you can extract. In all practical circumstances, you will get a lot less charge through a battery on discharge than you put through when you were winding it up. Especially if the battery isn't designed for recharging.

Could also interpret the question to mean you're planning to measure currents in both battery leads simultaneously, expecting them to be different. This isn't how current works (go back to Kirchoff's first law) - you move charge about, rather than creating it. It's one of things that's conserved in nature.

Lots of things will make a diode go short - often, when you find one that's open-circuit, that's because it's gone first short, then bang. Excessive reverse voltage (even short-lived, low-energy spikes) is quite a reliable way of getting there. Try googling something like diode failure cause, and see what you can find.

A.