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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.
TECHNICAL RECOMMENDATIONS FOR EXISTING EAA-PHEV 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.
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.
REMINDER ABOUT DISCLAIMER LANGUAGE
I recommend we all review and heed the EAA-PHEV:General disclaimer at http://www.eaa-phev.org/wiki/EAA-PHEV:General_disclaimer
and the PriusPlus disclaimer below and at http://www.eaa-phev.org/wiki/PriusPlus :
HV (High Voltage) DC (Direct Current) Warning: Traction Battery Packs, Motors, Chargers, and other HV sources could cause serious injury or death if proper precautions are not taken while working on or around such High Voltage Direct Current sources.
Use this information at your own risk: There is no warranty expressed nor implied and we are not liable for any of your past, present, nor future actions. Even should you perform these modifications to the letter you could still damage any number of components in your vehicle causing it to no longer function. Even if it appears to function properly your actions may cause it to self destruct with collateral damage to surrounding properties other than your vehicle. By utilizing these ideas and instructions in an attempting to enhance national security, reduce gas consumption, vehicle "emissions", your carbon footprint, or smog, you do so at your own risk & peril.
Warranty: In performing some of these modifications you may void your warranty with the vehicles manufacturer.
Sincerely,
/ron gremban
+++++++++++++++++++++++++++++++++++++++++ Ron Gremban, rgremban@... California Cars Initiative, a nonprofit organization: http://www.CalCars.org Moderator & Technical Lead http://www.priusplus.org PRIUS+ PHEV Conversion Group: http://groups.yahoo.com/group/priusplus Newsletter: http://groups.yahoo.com/group/calcars-news Do-it-yourself PHEVs: http://www.PriusPlus.org +++++++++++++++++++++++++++++++++++++++++
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