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Why Tesla Cars Aren't Bricked By Failing Batteries
itwbennett writes "Don't believe recent claims made by a blogger that non-functioning batteries in the Tesla Roadster cause the electric cars to be bricked, says IDC analyst Sam Jaffe. 'Here's the primary fact that the blogger in question doesn't understand: the Tesla battery pack is not a battery,' says Jaffe. 'It's a collection of more than 8,000 individual batteries. Each of those cells is independently managed. So there's only two ways for the entire battery pack to fail. The first is if all 8,000 cells individually fail (highly unlikely except in the case of something catastrophic like a fire). The second failure mechanism is if the battery management system tells the pack to shut down because it has detected a dangerous situation, such as an extremely low depth of discharge. If that's the case, all that needs to be done is to tow the vehicle to a charger, recharge the batteries and then reboot the battery management system. This is the most likely explanation for the five 'bricks' that the blogger claims to have heard about.'"
I would have to assume that a Tesla wouldn't be "bricked" by a failed battery either, as the batteries are presumably replaceable by the manufacturer.
Remember: Bricked = Failed and unrepairable.
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When you hear Lithium Ion Battery, you need to understand there are many different types of cell.
A battery consists of an Anode, Cathode and Electrolyte.
In LiIon based batteries, the electrolyte is a Lithium Salt, and the Anode is generally Carbon.
In LiPolymer batteries the electrolyte is held in a polymer of Lithium Cobalt or Lithium Maganese (this is the most common format of battery in consumer electronics)
In a recent project a for a hand held RF device, we chose LiFePO4. Mainly because it is so robust. Although it does not have the same capacity as LiPoly, you can grossly overcharge it and even drive a nail through it and it wont catch on fire. It also has much longer life over LiPoly.
LiPoly are very sensitive to overcharge, overdischarge, and mechanical damage, thus have a circuit to disconnect the battery when over discharged, thus the 'bricking' effect.
Tesla orginally used 18650 LiIon batteries with I believe had a LiCoO2 cathode, although I now think they are changing to pupose built cells. They would have a more sophisticated battery management that would prevent 'bricking'...... well at least one would hope...
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However... the Tesla vehicles already take advantage of "becoming a generator" as that is part of the "regenerative braking system" used in the vehicle. That ability to "generate electricity" not only doesn't damage the battery, but it helps to recharge the system as well and is an intended behavior... at least if you are going downhill with a tailwind.
I've seen several electric vehicles that have a gasoline-powered "pusher" trailer that provides "emergency power" for long haul trips instead of looking for an outlet for the car. It isn't even that new of an idea for that matter.
Regardless, because of the simplicity of the drive train and that the engine is not an internal combustion engine, calling a dead Roadster "a brick" is going over the top even if you can't disengage the engine from the transmission. Yes, there is a transmission in a Roadster, and there was even going to be a "clutch", but that feature was removed due to the torque issues and other problems from the supplier that was originally going to provide the transmission (something that nearly killed the Roadster when it went into production).
The Roadster is a rear wheel drive vehicle, so I don't think the front wheels are connected to anything other than the steering mechanism. In that regard, it is more like a conventional automobile too. In other words, towing the car is just like towing any other vehicle when you don't have the keys to unlock the transmission from the drive train.
It seems Jaffe only speculated the five bricked cars only needed servicing. Jalopnik did the research and also got an admission from Tesla. http://jalopnik.com/5887265/tesla-motors-devastating-design-problem
No, not semantics, but rather proper use of terminology. The AC is correct - a battery is a collection of cells. A battery pack is a collection of batteries. (The battery being the smallest individually replaceable part in the pack.)
That's why a "D" cell battery is called a "single cell battery" - because, duh, there's only a single cell in the battery. That's why automotive type batteries (of the type you add water to) have multiple vent caps - because each individual cell must be separately vented and/or topped off with water.
The quoted IDC analyst adds to the confusion (at least to those of us versed in standard terminology) by using the terms battery and cell interchangeably, which is the same mistake often made by the general public - you for example.
Regenerative braking requires some pretty sophisticated power electronics, controls, and software. The Tesla's motor is an AC induction motor. (The AC induction motor was invented by Nikola Tesla.) An AC induction motor has copper coils for both the rotor and the stator. This is different from a DC motor (brushed or brushless) where (usually) the rotor has permanent magnets on it.
Backdriving an induction motor will result in no power generation unless the stator is energized. Even then, the associated power electronics have to commutate which phase of the stator is energized in sync with the spinning rotor. In other words, you need at least some external (i.e., battery) power in order to regenerate - this is true of all induction generators. Without the stator being energized, you're just spinning one set of copper coils past another set (this is different from a DC motor, where the rotor has permanent magnets, which will induce current in the copper coils).
So the Tesla cannot be "jumpstarted" by towing it or rolling down a hill if the battery has discharged so deeply that it has disabled itself.
that depends entirely on the architecture of the motor, motor drive, and battery management circuits. The Tesla roadster, for instance, uses an AC induction motor, which has no permanent magnets in it. Unless the stator is energized and properly commutated, backdriving the wheels will not generate any power. Even in the case of a DC motor, backdriving the wheels will generate power, but if the motor drive is disabled, that power won't backfeed onto the power bus. Even then, if the battery has discharged so deeply that it has disconnected itself internally, it won't accept power unless it first communicates with a compatible charger.
Just what we need--more "analysts" fighting bad bloggers' bad information with more bad information.
Let's start with the "more than 8000 individual batteries". These are 18650 cells (a standard form factor, a bit larger than an AA cell), and a Roaster has only 6831 of them. They are not "individually managed". Rather, they are grouped into a 69-parallel module, with 99 modules in series. (69x99==6831)
It is asinine and a distraction that Tesla (and everyone else) constantly obsesses about the 6831 cells. For all practical purposes it is a 99-cell Li battery, but rather than using monolithic cell modules, Tesla (like ACP before them) builds modules from smaller component cells, because they yield better cost ($/Wh) and specific energy (Wh/kg), with more-favorable cooling and safety characteristics.
Other than a built-in per-cell PTC device (which Tesla is likely no longer using), any "management" is done at the module level, and the battery is treated as a 99-cell series pack. The PTC is a passive cell protection device, designed to save a cell from a failed-short condition, but they also cause as many problems as they prevent.
Secondly, the "solution" is not nearly as simple as "shutting the pack down" when it reaches "an extremely low depth of discharge".
The Li cells themselves do not discharge themselves quickly when idle--perhaps 5 or 10 percent per year. However, small parasitic (e.g. maintenance) loads will slowly deplete the cells' energy. Herein lies the "grain of truth" that is probably at the center of this greatly dramatized "journalism".
Li traction batteries typically have on the order of 100 cells (or more for 600V systems), and each cell must be monitored to keep its voltage and temperature within a safe range. Typically the monitors are powered from the cell modules directly, and the competing design constraints are many: Small packaging, low cost, low power, electrical isolation, and so on. It is possible, but not trivial (nor cheap) to make a cell monitor draw zero current when its host module is at low voltage.
The original rant (er, blog) claimed that the parasitics would deplete a battery in 11 weeks, which is bordering on implausible, and if true, it would represent a staggeringly high rate of self discharge. Per Chelsea Sexton (who knows what she's talking about), there has not been "a single 'brick' story that didn't involve some extraordinary circumstances".
Lastly is the notion that the traction pack is necessarily destroyed by a deep discharge event. While it is true that deep discharge (and particularly cell reversal) will cause some permanent damage, the damage is in the form of higher impedance, and this is far from rendering the module useless. The battery can be brought back via trickle charging and a per-module impedance test will reveal if any are too far gone.