Battery Fire

Most informative. Loose connections cause arcing, causing smoldering and eventually fire. Obviously a ordinary mechanic was not electrically astute. Or just forgot to tighten?

 

So the cause was the Gen II batteries being used?

 

The cause was loose battery module connections to the bus bars which resulted in electrical resistance in the circuit. That resistance resulted in heat being generated, sufficient to cause a fire which melted the sheet steel battery case.

The peak current flow to and from the traction battery is ~80A. According to Ohm's Law, I^2 x R = P where I is current, R is resistance and P is power.

Suppose you have 1 ohm of resistance in the circuit, which is a pretty small amount. 80 x 80 x 1 = 6,400 watts. That is like the heat produced by 4+ hair dryers.

 

Too bad we don't know the whole story. I'd love to know whos handy work that was.

Avi

 

Surprised there aren't more fires esp since so many people seem to cheap out and go for a module swap w/o knowing the torque specs or use the services of a questionable craigslist advertiser.

No implication implied whatsoever to Strawbrad; he obviously knows what he is doing and discovered the cause: crappy workmanship and incorrect parts.

 

Brad. Is there any discoloration of any of the bus bars or terminal nuts due to heat? From your pictures it is difficult to tell.
Most of the fire damage is or appears to be on the top of the battery, and not at the bus bar area. I am in no way trying to discredit your findings, but trying to make sure 100% the problem was cased by poor bus bar connections although I agree that any resistance due to poor contact is going to produce localized heating. As such I would expect to see melted plastic around the offending terminals.
Could this fire be caused for instance by a faulty or corroded internal connection between cells close to the area where the most fire damage is.
The gen2 modules have only one internal connection to the next cell. In the gen3 modules this was altered to give two internal connections to reduce both internal heating and battery resistance.

John

 

How do we know that the serrated nuts did not have full contact to the electrical surface so therefore heated up and cause them to become loose?

 

...looks like plastic burned?

To be clear these are NiMH batteries, so the electrolyte inside the battery is water-based. So the battery chemistry itself is not sustaining the fire (of course the spark it's giving). Whereas a Lithium battery has flammable electrolyte so the whole battery would have gone up in flames. Of course, greater design safety pre-cautions are taken with Li-batts for this reason.

 

Okay if the load is not going thru the nuts then I will agree with that summation you gave. So what do the small nuts achieve? Just hold downs? My experience is that unless the threads are interfereance type the tightness of the nut onto the surface does make an electrical difference insofar as to achieve the best contact as possible. Thats why in most instances brass is used so that the washer plate deforms to fill in the small gaps to achieve the best electrical contact. Also the amount of tightness make the threads achieve the best contact thread to thread within the fastener.

 

Thanks I'll amend my post accordingly.

 

An important thing to note in this failure is it was contained (just barely) by the design of the battery pack and though the failure is the most extreme case I've seen in a Prius, it did not set the rest of the car on fire, which explains alot about why Prius was such a popular vehicle in its first decade of production.

 

Somewhere there are photos of a Gen 1 battery fire that did much more extensive damage to the car. They are still vividly in my head from when I saw them maybe a dozen years ago, but I've tried a few times to re-find the link without success so far.

 

How much of the upholstery did you see burned? Maybe in terms of % of the interior damaged?

 

Definitely the headlining, C pillar trim, package shelf, rear seats, center stop light ... pretty much everything you'd see if you were in the front of the car looking back.

 

It's good to know the two worst examples of battery failure were in Gen1... I wonder if that had any influence on downsizing from 38 modules to 28?

What I do know is my buddy who's been teaching hybrid repair since Gen 1 days has said more than once that the Gen1 Panasonic modules were horribly designed and he won't work on them anymore.

 

I don't know if there's any basis for saying those are the two worst; they're two I've seen photos of.

 

I've seen Pri on auction sites with extensive burn damage in the rear only. Since I have no way of determining the cause (HV, 12v, gas, a cigarette, etc.) I've never considered posting it. Just because one has never seen something doesn't mean it never happens.

 

You left out arson.

 

I have a heck of a time convincing people to actually torque the nuts properly. "It can't possibly be that important!". Yes. Yes it is.