What the heck is going on with Tesla batteries?

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Disclaimer: I am not a car guy. Read the following with that in mind.

As long-time EG readers will know, I've complained in the past that my Prius has a feeble starter/electronics battery which is easy to run down even by leaving the interior lights on. This despite the fact that the Prius has a huge battery running the hybrid system to draw on. But I certainly didn't want this. Michael DeGusta reports that if you leave your Tesla parked for a long time (like months), then the car bleeds enough power off of the battery to run the auxilary vehicle systems [parasitic load] to drain it down into deep discharge (and hance damage to the battery) territory:

A Tesla Roadster that is simply parked without being plugged in will eventually become a "brick". The parasitic load from the car's always-on subsystems continually drains the battery and if the battery's charge is ever totally depleted, it is essentially destroyed. Complete discharge can happen even when the car is plugged in if it isn't receiving sufficient current to charge, which can be caused by something as simple as using an extension cord. After battery death, the car is completely inoperable. At least in the case of the Tesla Roadster, it's not even possible to enable tow mode, meaning the wheels will not turn and the vehicle cannot be pushed nor transported to a repair facility by traditional means.

The amount of time it takes an unplugged Tesla to die varies. Tesla's Roadster Owners Manual [Full Zipped PDF] states that the battery should take approximately 11 weeks of inactivity to completely discharge [Page 5-2, Column 3: PDF]. However, that is from a full 100% charge. If the car has been driven first, say to be parked at an airport for a long trip, that time can be substantially reduced. If the car is driven to nearly its maximum range and then left unplugged, it could potentially "brick" in about one week.1 Many other scenarios are possible: for example, the car becomes unplugged by accident, or is unwittingly plugged into an extension cord that is defective or too long.

When a Tesla battery does reach total discharge, it cannot be recovered and must be entirely replaced. Unlike a normal car battery, the best-case replacement cost of the Tesla battery is currently at least $32,000, not including labor and taxes that can add thousands more to the cost.

There's been a lot of controversy about this report (see, for instance, this defense), but Tesla's response seems to by consistent with DeGusta's basic argument, as does the letter that Jalopnik reproduces above:

All automobiles require some level of owner care. For example, combustion vehicles require regular oil changes or the engine will be destroyed. Electric vehicles should be plugged in and charging when not in use for maximum performance. All batteries are subject to damage if the charge is kept at zero for long periods of time. However, Tesla avoids this problem in virtually all instances with numerous counter-measures. Tesla batteries can remain unplugged for weeks (even months), without reaching zero state of charge. Owners of Roadster 2.0 and all subsequent Tesla products can request that their vehicle alert Tesla if SOC falls to a low level. All Tesla vehicles emit various visual and audible warnings if the battery pack falls below 5 percent SOC. Tesla provides extensive maintenance recommendations as part of the customer experience.

At present, then, the agreed upon facts seem to be that:

  1. If you leave the Tesla's batteries at zero charge, battery damage occurs.
  2. If you leave a Tesla unplugged for long enough, even with a charged battery, parasitic load from the vehicle systems will eventually consume the battery's charge, leaving you in state (1) above. [Note that this appears to exceed the Lithium-Ion self-discharge rate, so it likely is parasitic load.]

The controversy really seems to be about who's fault this is, namely whether the customer should have known better, whether Tesla notified them correctly, etc. I don't have a Tesla so I don't care about that. I'm much more interested in the engineering question of what's going on and what, if anything, can be done about it.

The parasitic load thing isn't totally unfamiliar territory, of course. Any modern vehicle has electronics and those need power, which they get from the battery. Some do a better job than others. My BMW R1200GS motorcycle, for instance, has this problem and the manual explicitly tells you to connect it to a trickle charger (an expensive BMW model, of course, though you can use a standard one if you're willing to do a tiny bit of work) if you're not going to drive it for a while, and I duly plug it into the wall whenever I get home. If you don't do that, however, the worst you're going to be out is new lead-acid battery, which depending on what vehicle you have, leaves you out something like $50-$200, not $40,000.

However, the level of load we're talking about here seems awful high. Remember that we're talking about a battery capable of powering your car for 200 miles or so on a single charge (53 kWh). In order to deplete the battery in 11 weeks (~2000 hrs) you would need continuous battery consumption of around 30 W. For comparison, a Macbook Air has a 50Wh battery and gets something like 5 hours on a charge, so it's like the Tesla is running 5 Airs at once 24x7. It's natural to ask where all that power is going, since you don't need anywhere near that much to keep a vehicle on standby. One likely source seems to be the battery cooling system, of which Wikipedia says "Coolant is pumped continuously through the ESS both when the car is running and when the car is turned off if the pack retains more than a 90% charge. The coolant pump draws 146 watts." [Original reference and long discussion here. Note that this post is due to Martin Eberhard, one of the Tesla Founders but apparently no longer with the company at the time he wrote it. Thanks Wayback Machine for preserving this!].

Obviously, if you have a load this high, then you're going to deplete the battery. The question then becomes whether there is some way of avoiding permanent battery damage as the depletion gets to dangerous levels. The natural thing to do is install some sort of cutoff that turns off all power drain once you get close to that level. This may end up blowing away a bunch of the car's configuration (though really, it's not that hard to store that stuff in flash memory, even though historically manufacturers have tended not to), but surely it's cheaper to reboot your car than replace the entire battery pack. However, if the power is going to the cooling system and the cooling system is doing something important, like keeping the battery from being damaged by excessive heat, then this may not help.

Oh, one more thing. DeGusta claims that Tesla has the capability to remotely monitor the battery and locate the car, and has sent people out to fix it:

In at least one case, Tesla went even further. The Tesla service manager admitted that, unable to contact an owner by phone, Tesla remotely activated a dying vehicle's GPS to determine its location and then dispatched Tesla staff to go there. It is not clear if Tesla had obtained this owner's consent to allow this tracking5, or if the owner is even aware that his vehicle had been tracked. Further, the service manager acknowledged that this use of tracking was not something they generally tell customers about.

If true, that would be... interesting.

2 Comments

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"Coolant is pumped continuously through the ESS both when the car is running and when the car is turned off if the pack retains more than a 90% charge"

To me, this indicates the cooling pump turns off when the vehicle is off at battery levels below 90%, exactly the situation here. I agree there should be a parasitic load kill switch at, say, 5% charge level to prevent complete discharge. Not having this seems like a design error. Consider that the 30W load may come in part from the computer system that allowed them to remotely turn on the GPS in an attempt to warn the user of excessive parasitic-induced discharge.

Also, the fact that the car apparently can't be trickle charged through a long extension cord seems like a design flaw, in order to keep it topped up, the system requires a full capacity power connection.


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