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- 7 PriusPlus - Interim Control Board - May 3, 2008
- 8 Blue Thermal Runaway - May 3, 2006 post
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PriusPlus - Interim Control Board - May 3, 2008
- Subject: [eaa-phev] WARNING: Fail-safe systems needed to protect Prius battery in EAA-PHEV conversions
- From: Ron Gremban at CalCars
- Organization: California Cars Initiative
- To: EV-PHEV Email List
Last night I had an undesirable surprise event. Due to a combination of unexpected factors from the installation of new equipment, the original Prius' nickel-metal hydride battery got seriously overcharged and experienced a meltdown. Fortunately, Toyota's battery box was quite good, though not perfect, at keeping this extreme event contained. No one was injured, and my car suffered minimal damage beyond the readily available OEM (original equipment manufacturer) battery that was destroyed. We have been presented with a wake-up call, and this is an opportune time to map out additional safety systems, some of which I will propose below.
Attached to the next message (with it this message was too long) is a picture of the aftermath of last night's event [the attachment may not make it into archives]. I will publish more as we disassemble and replace the OEM battery pack before we replace it. Below, I have some specific safety recommendations for those who are using the Interim Control Board -- plus the added recommendation that we get our heads together to design redundant levels of assurance against OEM battery pack overcharge.
In retrospect, what happened was an unfortunate interaction between three devices, each of which works fine on its own or in combination with any one of the others.
As the world's first Prius PHEV conversion, since 2004, my car has repeatedly been a guinea pig for new technology (it's roughly on a version 3 battery box, control system, and set of batteries). Wednesday we added some data collection equipment that I will explain in detail in a future message. However, what's important feature relative to my event is that it writes messages onto the CAN bus not only when driving but also when the car is on charge. Meanwhile, my CAN-View V4 monitors the CAN bus and turns itself ON when it sees activity. Due to the new equipment, it began turning ON when the car is off but charging.
I noticed this upon installation of the new equipment. My workaround was to unplug the CAN-View when charging. Thursday night, however, I forgot. Also, the car was left with a low OEM SOC, causing the CAN-View, once turned ON during charging, to close a reed relay that the Interim Control Board in my car interprets as a requirement to parallel the battery packs. The Interim Control Board, being a super-basic design without all the niceties of a full control board, doesn't have a circuit to specifically inhibit PHEV operation when the Prius' hybrid system is inactive (causing our READY line, taken from SMRL3's coil, to be low).
Because of all these interactions, the PHEV contactors both closed, putting the battery packs in parallel while the PHEV pack was being charged. And because the OEM BMS was not powered, CAN-View received no messages indicating that the OEM SOC was increasing -- so it kept the contactors closed until the OEM pack got so overcharged that it began seriously overheating.
My girlfriend Lynne and I were at the dining room table not far from the garage when we heard loud but muffled pops. I ran into the garage, where I heard more such pops, now louder but still muffled. My car was full of smoke. I unplugged it and raised the hatchback. The plastic top was still removed from the OEM battery pack due to the previous day's installation work. Every few seconds, I heard an individual OEM battery cell or module blow, and simultaneously saw the top of the OEM battery box temporarily bow further upward. Two small fires had started in the rear seats' carpeting at either end of the OEM pack, the larger one on the driver's side where I had not replaced the heavy battery pack cover member that fits above the thinner cover over the contactors. I extinguished both quickly with a fire extinguisher. The experience, of course, left me thinking about what could have happened if I had not been home, alerted, and prepared with an extinguisher. (I have one powder-based extinguisher that works on electrical fires in my garage and another in my car at all times -- highly recommended for experimenters.)
Note: I need to eat some of my words about the added-battery conversion system (that piggybacks the original battery with a new pack) being safer than systems like Hybrids Plus' that replace both the OEM battery and BMS. The particular failure mode I experienced is unique to two-battery systems, though a BMS failure could possibly produce similar results, depending on the PHEV battery's failure characteristics. Also, this failure mode has little relevance to any systems an auto manufacturer might install in a newly designed PHEV.
Recommendations for Existing Interim Control Board Installations
As a first-stage fail-safe mechanism for systems using the Interim Control Board, I recommend -- on the Interim Control Board itself -- cutting the trace from chassis ground to J1F, the CAN-View connector. Add an NPN transistor (a 2N3904 will do) as follows (better directions and pictures will follow once I've done this once):
- Collector to chassis ground
- Emitter to J1F, where it used to be connected to chassis ground
- Base through a 10K, 1/4W resistor to the READY line. The READY line comes from the OEM battery box. After removing the cover to the compartment with the OEM contactors, you will see a 4-wire connector at the top rear. One of its wires is green on the outside but yellow on the inside. This is the power line to SMRL3's coil. The READY line is a tap off this line.
Since the OEM contactors are de-energized after a crash or in case of a really serious hybrid problem, it is a significant safety feature to inhibit PHEV system operation except when SMRL3 is energized. This was done on my earlier control boards, and, I believe, is built into Chris' CAN Control Board. But, for simplicity, it was left off the Interim Control Board. After this event, I highly recommend retrofitting all Interim Control Boards in the field, even though their use is intended to be temporary, only until the CAN Control Board is in production.
Note that after a crash or serious hybrid system problem, as well as during charging, we want to make sure the PHEV paralleling is not happening. Therefore, I doubly recommend dependency on the READY line, which will ensure just that.
Recommendations for Design and Safety Reviews
Since seriously overcharging the OEM battery turns out to have such potentially serious bad effects, I also recommend that we take a serious second look at our designs, their failure modes, and how we can build multiple layers of fail-safe systems into our conversions. For example:
- We can develop a fatal error mode that disallows system operation (the car is again a plain hybrid) until someone has checked the system and cleared the error.
- On contactor-based systems, we should continue to have two (not just one) contactors, and we can have a start-up test sequence like that of Toyota's to test and make sure neither contactor is shorted closed. A shorted contactor would cause a fatal error.
- Another fatal error could be set if the OEM battery's SOC, as read by the OEM BMS, exceeds e.g. 80%.
- We could have an independent temperature sensor on the OEM battery and set a fatal error if it exceeds a set excessive temperature.
I recommend that, at minimum, we open a thread on the topic of "Conversion safety", possibly culminating in a series of conference calls and/or a physical meeting.
Blue Thermal Runaway - May 3, 2006 post
- May 3, 2006 - Ron Gremban at CalCars wrote:
- > Last night I had an undesirable surprise event...
I'm so sorry to hear about Rons incident. Furthermore after the reply from Garry praising your courage and honesty for presenting this grim reminder of the dangers, I'm feeling ashamed for not publicly mentioning the similar incident that I myself experiences shortly after developing the new PiPrius conversion method with Rich.
Similar to Ron's case mine involved operating a PHEV system in an early development stage without all of the proper safety mechanisms in place. Further exacerbated by our forgetting to exercise the manual safeties we knew needed to be performed. Those being manually unplugging the CAN interface in Rons case or shutting down the changing system when CAN-View commands were no longer being sent to it in my case.
I should point out that the vast majority of the PHEV conversions to date do in fact have safeties in place to prevent incidents such as this. The exception being the Interim Control Board which Ron and others are currently developing and perhaps some other DIY conversions as mine was during it's early development.
In my particular incident I would like to point out that not only did I experience the initial Thermal Runaway event caused by the overcharging of the OEM battery. But 12 hours later I experienced a second thermal event with everything disconnected. So it should be made clear that if the OEM NiMH battery has been compromised that it should be promptly removed from the vehicle, it's interconnects should all be removed, and it should be stored in a safe location until it can be properly dealt with.
PriusBlue Thermal Runaway Event - June 1, 2006
June 1, 2006 was great at the Cascadia Transportation Conference. But later that night I encountered an event to rival that of: John Wayland "The Electrifying Birth of 'Plasma Boy'"
After a dozen or so PHEV runs in my car named PriusBlue I thought it was about time to give my wife a ride so she could see what all the trouble had been for the previous month or so. So we took a drive to the grocery store where she drove on the way and I drove back, she won with a score of 99.9 mpg and I somehow managed to bring it down to 98.9 or so when the defroster kicked on the ICE. Then once back home...
Having ignored a number of concerns which had been building regarding the automatic shut down of the charging system as well as higher than what would be considered prudent voltage settings of 242v. I finished off with a dash of bad basic operating procedure for a test vehicle under development by absentmindedly walking away without fully shutting down the system and then getting distracted from the intention to return immediately to finish buttoning things down for the evening.
Having at this point left the PFC floating the OEM NiMH pack at a dangerous level for nearly an hour a curious sound rang out. Thinking nothing of it at first and dismissing a second pop as another car door slamming. It instantly dawned on my with the third shot what had just occurred and been escalating for the previous hour. Dashing out to the Prius with a fire extinguisher I became all to apparent that my worst fear was realized, the OEM battery had gone into thermal runaway and was now engulfed in flames with cells now rapidly rupturing with eerie firecracker like explosions. Pumping the extinguishers contents into the seams of the battery case the beast was brought back under control if you can call it that while the gloom of what had just transpired began to settle in.
The next morning after little sleep the task of cleaning up the mess began. After having spent some time removing the rear seats and body panels while vacuuming up extinguisher guts it began to rain. So it was decided that it would be a good time for some lunch. Having nearly finished I received a frantic call from my wife that the car was on fire again, she had left work to check on things having had a "funny feeling". Somehow 12 hours after the initial event, perhaps having allowed oxygen to once again gain access to the cells, the pack had reignited. This time, with the interior already removed we had the hot OEM pack unbolted and out of the car in about 2 minutes flat. It turns out that this time the smoke had come from a roll of paper towels which was resting on top of the battery, which was very hot but not burning. Just to be sure now with it out of the car I proceeded to remove all of the module interconnects and awe at the charred and twisted metal case.
Even now, some years later the very thought of the event still turns my stomach, the 4th of July just isn't as much fun as it once was.