Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack

March 8, 2007

I'm familiar with the Tesla – a high-performance (0-60 in 4 secs) sportscar that just happens to be an electric vehicle — from an article in Wired magazine. So I knew that the car's designers relied on existing lithium ion technology rather than spending all their R&D cash trying for the Holy Grail of auto batteries.

What hadn't registered was that the car's battery pack relies on the self-same 18650 cells – all 6,800 of them — that power consumer cell phones and laptops. And sure enough, just like cell phone battery packs, each lithium ion cell in the Tesla has its own positive temperature coefficient (PTC) current limiting device acting as a fuse.

In addition, (and this is all from a white paper, "The Tesla Roadster Battery System") each cell has an internal current interrupt device (CID) that senses excessive internal pressure within the cell – if the pressure gets too high, the CID breaks and electrically disconnects the cell. High internal pressure within a cell is generally caused by over-temperature or other failures that then result in over-temperature, so over-pressure sensing can result in a sort of early-warning system for over-heating and consequent thermal runaway.

What is the makeup of this CID? I have no idea, but I'd sure like to find out.

Posted by Margery Conner on March 8, 2007 | Comments (5)

March 15, 2007

In response to: Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack
Battery Tester commented:

Sony has corrected their manufacturing process that released microscopic metal particles inside the cell. Also other notebooks suppliers used the same Sony cells as Dell without catastrophic failures. Many experts believe that the aggressive charge regime that Dell uses (ExpressCharge) caused the docile "soft" shorts to become "hard" shorts, resulting in fires. Tesla uses a very conservative charge regime--charging in several hours vs. around 1 hour for some notebook computers.

March 14, 2007

In response to: Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack
FyreStorm commented:

I'm not sure I see how this would protect the battery if there is an internal short like the Sony problem last summer. Once the cell goes into thermal runaway, breaking the electrical connection doesn't have any effect. The temperature and pressure builds until the case ruptures emitting the fumes and flames. Are the individual cells insulated or thermally isolated?

March 14, 2007

In response to: Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack
Battery Tester commented:

Each series strings consists of 69 cells in parallel so it would require a massive failure before any PTC was subjected to more than about 4V. Also, the fusible links, two in series with each cell, will pop very quickly should a PTC fail due to overvoltage.

March 12, 2007

In response to: Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack
Pual Rako commented:

Streetcar Eddie brings up a good point. If the Tesla cells are in a 375 volt string the voltage limits of the PTC in the cells may well be exceeded when they trip. I doubt the LiIon cell makers intended the cells to function in such high-voltage applications. As to the overpressure gizmo in the cells, I assume it is the same type of interrupter that is used on lead-acid gel cells.

March 10, 2007

In response to: Electric car: Tesla's use of PTC and CID protection in its lithium ion battery pack
Streetcar Eddie commented:

I am not familair with the particular implementation, but the PTC has voltage limits across (up to 60v)when it opens. They are like fuses that are resettable. (CLOSES when they cool)can be used again. The CID is a one shot device that mechanically seperates and disconnects the battery after the overheated cell swells. after that the cell is junk. Both appear to be overcurrent protective devices but do not protect from internal cell failure modes (such as denting the battery cell itself)

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