You raise a very good point about the Volt fire, ChriSharek. However, I'm going to take a slight issue with what you've said. The truth is that the battery cells had nothing to do with the Volt fire, but the battery pack did. You're correct that the fire occurred after coolant leaked onto a printed circuit board at the top of the battery pack. Let's remember, though, that the coolant is there precisely because this is an energetic chemistry. And when GM fixed the Volt, all of the fixes were done to the battery pack. They beefed up the battery safety cage to resist the kinds of forces seen in the NHTSA tests; they added a sensor to monitor battery coolant levels; they provided a tamper-resistant bracket to prevent overfilling of the battery's coolant reservoir. Perhaps we're both putting too fine a point on it, but all these components are part of the battery, and they wouldn't have been there in the first place if lithium-ion cells didn't have such an edgy chemistry. To say that the fire had "nothing" to do with the lithium-ion battery is incorrect.
That's a very timely question, Greg. And the answer is, yes, there is a safer chemistry on the horizon. We haven't written about it yet because it's still in the early stages (as far as I know), but there are soild state lithium batteries that eliminate the liquid electrolyte, and are therefore regarded as being safer. Toyota is working on a solid state lithium "super-ionic" battery. As with all battery research, however, the schedule is unclear. I've heard projections of 10 years for solid state lithium. And, typically, when battery researchers say ten years, it means more than ten years. Perhaps one of our readers is familiar with the solid state lithium technology and can provide a little more info here.
Great post Charles--OK, I'm probably going to show my ignorance here but I'm not close to being a battery expert.
QUESTION: When overheating begins--is this a cascading event? Does it always have to end with a catastrophic failure? I know we have cause/effect situations but is there any way to monitor temperature rise and dissipate that rise by appropriate cooling? I'm sure all companies using this technology have evaluated any contribution from surrounding sources. Are there any trends regarding installation and mounting of the battery? Again--great post and thank you for keeping us up to date.
@ChasChas: the reason that the battery electrodes are so very close is that they need to be close to avoid losses in the electrolyt, and also to keep the batteries compact.
On the other hand, it might be possible to build the batteries in a manner similar to those "self-healing" capacitors that appeared during the early 1960s era. The concept was that a short circuit would result in burning away a small segment of the electrodes in the area of the fault, removing the shorted circuit connection. Of course, there may be some very good reasons why that would not work in a battery like this but it certainly seems that it could be worth cnsidering. An added advantage is that it is certainly not a new concept.
It has been my experience that battery engineers over-charge the batteries. It doesn't surprise me that most of these battery failures occur when the vehicles are sitting and being charged. As the battery approaches full state of charge, the energy going into the battery to charge it winds up as heat. You need to measure heat dissipation at the battery assembly level. Battery heat dissipation varies as a function of state-of-charge, voltage, battery temperature, and charge current. Batteries are typically endothermic when charged at low states of charge, but they rapidly become endothermic as you approach full state of charge. We typically size the battery cooling for 3.5 watts/amp of discharge.
I'm going to take a pass on your thermal runaway questions, bobjengr, other than to say the obvious: Overheating can lead to a cascading event. As to when overheating turns into thermal runaway...we would need a material scientist or better, a lithium-ion battery expert, to go into greater detail. Perhaps a reader who is familiar with thermal runaway can chime in. As for battery monitoring: Yes, all of today's lithium-ion EV batteries use battery management systems that check for over-temperature, over-voltage and over-current conditions. Usually the battery managment system consists of a little IC that contains a microcontroller, memory and an A/D converter. Temperature measurements typically require a multitude of sensors around the battery pack. Toyota uses about 40 sensors around the pack in its Prius PHV plug-in hybrid. And, yes, the cooling systems can respond to those conditions. Toyota uses an active cooling systems with three or more fans. Chevy uses 144 metal plates with channels machined into them to allow for liquid coolant to run between the battery's 288 cells and draw the heat away, in a manner similar to that of an engine block and radiator. As you suggest, all of the automakers have put tremendous development effort into the design of these systems. But as recent events suggest, they're still learning.
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