The Most Coddled Automotive Battery Ever?

We get some great questions and comments from the readers of our blogs, and this post takes its title from Brent, who on May 30 wrote:

The Roadster’s battery is arguably the most coddled automotive battery in history. It has its own climate control system, several monitoring computers, and perhaps some other mojo 'they' are not telling us.

The battery pack gets a lift

The Tesla Roadster’s battery is extraordinary and in some respects is our company's “secret sauce.” We call it the Energy Storage System (ESS). It is comprised of 6,831 individual lithium ion cells, each similar to the 6 to 12 cells (made by top-tier 18650 cell providers) found in many standard laptop computers. The cells, in turn, are housed in 11 modules. The ESS weighs about 900 pounds and contains about 53 kilowatt-hours of energy.

Our ESS contains multiple overlapping safety systems – some active and some passive. Each module has its own microprocessor circuitry that monitors a wide range of conditions as well as actively balancing the cell voltage to help mitigate the danger of overcharging (and potential damage from over-discharge). There are sensors inside the ESS that sniff for smoke, check for the existence of water immersions (if the car were driven into a lake, for example), and continually monitor the pack to ensure that it is thermally stable. Each cell is independently fused, not once, but twice. And the pack has been designed to passively contain any single cell entering thermal runaway (e.g. start burning) without propagating further. (For more information on the Tesla Roadster's ESS, see our blog and white paper on the subject.)

Collectively all these features (plus some others) give us confidence that the ESS is among the safest Li-ion battery packs made. In this post, I’ll address some of the regulatory work and support testing that we have done to validate the safety of the ESS. Given the size of our Li-ion pack, our volume projections, and the automotive future that Tesla Motors represents, we are getting attention from regulators (and not just in the United States).

To begin with, let’s talk about some of the process steps in bringing a new large Li-ion battery to market. Almost anybody can start a company and assemble batteries. However, depending on the volumes and chemistry involved, you can’t sell and ship them to the general public or untrained users without going through a whole battery (get the pun? :)) of tests to demonstrate safety.

The exact regulations may vary by country, but there has been a major effort via the United Nations (UN) to harmonize (e.g. propose and standardize) the regulations that govern safety and commerce for a variety of “dangerous goods” that may present a risk in transportation. These efforts include harmonization of transportation regulations for such things as Li-ion batteries, pesticides, radioactive materials, explosives, compressed gases, and flammable materials like paints. The governing regulations are based on the UN Recommendations on the Safe Transport of Dangerous Goods Model Regulations, and related UN Manual Tests and Criteria. The regulations vary by mode of transportation when these items are shipped (e.g. ocean vessel, rail, truck, aircraft).


In the regulatory world of Li-ion batteries, it turns out that there are two mutually exclusive categories that are treated very differently:

  1. Prototypes: Packs made with electrically interconnected cells that have not completed a rigorous set of UN-defined safety tests.
  2. Production: Packs made with electrically interconnected cells that have passed the UN-defined safety tests.

Basically, when you have to ship a “prototype” Li-ion battery there is a very strict set of regulations and conditions that govern its packaging, shipping and testing. For Tesla Motors, this involved the creation of a super robust crate that required its own testing and certification program. The adjacent picture (right) shows the candidate crate mid-fall during its certification testing.

Approved crate for prototype ESS

In addition, the Tesla Motors ESS is so large and heavy that we had to secure approval from Thailand’s transport authority to ship the “prototype” ESS from Thailand, where the battery packs are made. We also secured similar approvals from government transportation regulators in the U.K. (where the Tesla Roadster is assembled) and the U.S. (where we do R&D and most of the testing work). A picture is adjacent (left) of the fully tested and approved crate for our "prototype" ESS.

At this point in our story, Tesla Motors was able to ship prototype packs all over for our vehicle development and battery testing purposes. But we still had the small problem of needing to pass the required UN tests so that we could actually deliver cars to our customers. These tests weren’t originally drafted for packs as large as our ESS. As a result they are stricter than one would normally expect. Yet these are the rules and for the standard test protocol numerous test articles are required to sequentially pass the following tests:

Vibe testing
  1. Altitude simulation: simulating air transport.
  2. Thermal cycling: assessing cell and battery seal integrity.
  3. Vibration: simulating vibration during transport.
  4. Shock: simulating possible impacts during transport.
  5. External short circuit: simulating an external short circuit.
  6. Overcharge: evaluating ability of a rechargeable battery to withstand overcharging.

We have spent more than three years perfecting our design with innumerable trips to testing labs along the way to test design iterations. We only use cells that have passed both UN and UL 1642 testing criteria as well as our own specifications. Some of our tests are done at the cell level and some at the module level. And, of course, we test all sorts of other things using the full ESS pack. The picture at right shows an ESS on the vibe table – 38 hours of a particular vibration profile simulates 100,000 miles of driving.

Along the way we have also consulted with a wide range of experts, and we’ve also worked very closely with the U.S. authorities who regulate Dangerous Goods at the U.S. Department of Transportation (DOT). And now, a drum roll please...

It is with great satisfaction that we can now state that we meet the requirements for a “production” Li-ion battery! This is the largest Li-ion battery that we are aware of that has met this milestone. And much more importantly, this represents a significant milestone toward our being able to ship Tesla Roadsters to customers.

Additionally, Tesla Motors has recently received permission to fly the production ESS and cars containing it on cargo aircraft. This has been a huge effort by Tesla Motors and has involved an additional level of scrutiny by the DOT. Shall we say that the Tesla ESS has passed this safety review with “flying” colors? :)

So what do you think? Is this the most coddled battery in automotive history?