Why do Tesla batteries not overheat? Tesla’s battery cooling system.

As you should know by now from examples like the Samsung Note 7, lithium ion batteries get hot. If they’re not fitted with an appropriate cooling system and are not regulated in an effective manner, they can overheat, potentially causing lots of damage to both the car and all of the humans inside.

When Tesla was developing the original roadster, they had issues keeping the batteries cool. Consequently, they developed an ingenious cooling system. Since very few other companies were making electric cars, Tesla was one of the first to pioneer these systems. Here’s how they did it:

Why do batteries need thermal regulation?

As stated above, all lithium ion batteries need some form of thermal regulation. Predictably, the larger the battery the more heat it gives off, thus needing a more powerful cooling system.

Phone Batteries

On a small scale, let’s start with smartphones. When playing a game or doing an internally intensive task, it’s likely you’ve felt your phone warm up. That’s the internals giving off heat, not necessarily the battery.

However, if you charge the phone at the same time, you’ll now notice that your phone gets even hotter. That’s battery temperature because the battery is experiencing both charging and discharging at the same time in large relative quantities.

Instead of placing in powerful cooling systems, phone manufacturers usually place sensors inside their phones. These monitor temperatures at several points inside and cause the CPU to throttle (reduce clock speed) if temperatures get too high. Consequently, your phone never overheats or catches fire.

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iPhone X Lithium-Ion Battery. Source – iFixit

Generally, phones use a passive cooling system, i.e. a large surface area which acts as a heatsink to dissipate the heat. At most, phones may use small copper pipes to take heat away from important internals.

In the case of the Samsung Galaxy Note 7, one of the thermal regulation systems at the battery failed, resulting in the battery overheating when charging. A very small number of phones ended up exploding due to this design flaw.

As you can imagine, if something like that happened in an electric car, the scale of disaster would be much larger and more dangerous.

Tesla Batteries

Electric car batteries work in a similar way but are cooled slightly differently. In order to supply and receive peak power, the battery must be within a suitable temperature range of about 40-50 degrees Celcius. This range depends on the battery chemistry; Tesla hasn’t published official numbers.

Tesla Model S 85 kWh Battery
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Tesla Battery Pack

This temperature is perfect for short, peak performance like rapid accelerations and top speed runs. Sometimes, the battery can get warmer than that, though more likely in the northern parts of the world its colder.

Subsequently, the battery pack must be both heated and cooled in order to create the perfect temperature range. This is done with the BMS (battery management system).

Furthermore, when the V3 superchargers were released, a new Model 3 setting was engaged which would allow batteries to be ‘preconditioned’ before receiving maximum charging speeds. This preconditioning will include heating or cooling to get the battery to about 30 degrees Celsius or a little more for superfast charging.

Additionally, batteries with no thermal management systems have considerably reduced lifespans due to the increased thermal pressure placed on the batteries. This would massively change an electric cars lifespan even if it didn’t catch fire in the meantime.

How does the BMS work?

Originally, when Tesla was developing the first Roadster in 2006, there were some thermal issues when accelerating fast and pushing the car to its limits. Clearly, a risk of fire or overheating is not something which Tesla was looking for when developing an electric car which would prove the technology to the world.

Consequently, Tesla developed the BMS, a system which is still used as a similar design in the Model S, 3 and X today.

Whatever type of BMS Tesla uses, it must be pretty powerful in order to cool all of the 7000 cells. Tesla patented a battery cooling system for the Model S which allowed each of the cells to make contact with the coolant pipe. Here’s what it looks like on the scale of a few cells:

Cell Level

Tesla gm cooling gb slide 2
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BMS 1

As you can see, a ribbon like pipe snakes through the battery pack, making contact with each and every cell. This is far superior to a straight pipe due to the increased energy density and better thermal management it facilitates.

Each circular cell above-numbered ‘203’ are the battery cells themselves. The patent diagram shows a small section of one of the 16 battery modules in the Model S. As you can see, multiple cells can be packed closely together, following the curve of the ribbon.

Cooling Tube

Tesla gm cooling gb slide 1
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BMS 2

In BMS 2 above, you can see the radiator design of the BMS, as well as some of the cooling tube. Tesla uses fluid Glycol to cool the cells. Glycol has fairly high thermal conductivity, though there are other fluids which are better at removing heat, giving them a larger heat transfer coefficient.

In order to prevent Glycol from leaking from the tube and into the battery array, Tesla used a metallic inner tube. Presumably, this tube is highly resistant to impacts.

Battery Module Cooling

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BMS 3

Finally, here’s one full Model S battery module. Although this diagram doesn’t outline the ribbon shape of the cooling pipe, it shows the general flow of heat. As you can see, coolant goes in, snakes round the cells whilst making contact with each and every one of them, before being pumped out to a radiator and the heat is dissipated.

Each battery module has its own cooling system with separate radiators attached. Since the car monitors the temperature of each module, presumable the cooling system can specify to cool a particular module more than others if it gets too hot. Since power draw and input is evenly distributed, I doubt certain modules ever get noticeably hotter than others.

What About Charging

The act of charging the battery actually warms it up. If the temperature outside is reasonable, the BMS may not even kick in when charging. However, when supercharging it may.

For storage, it’s best not to take Lithium-ion batteries over 30 degrees Celsius for extended periods of time. Leaving your Tesla plugged in is recommended in order to keep the BMS working and the batteries cool.

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A conventional wall charger is fine when storing a Tesla.

Since your Tesla will not overcharge due to safety measures implemented into software, there’s no risk leaving it plugged in. This is due to a control system which prevents the battery from overheating, similar to the one that failed in the Note 7.

So, when you’re storing your car for many days, weeks or months, make sure you leave it plugged in. It doesn’t have to be plugged into a fast charger, a standard wall outlet should be fine, especially considering the relatively tame temperature difference on earth.

Can I heat the battery by driving?

Although driving has no effect on the cooling side of the BMS, it does warm the battery. This is simply because power is being drawn from the battery, thus making it heat up due to energy transfers not being 100% efficient.

Tesla owners may notice that when it is extremely cold or hot outside, peak power from the battery is limited. During extreme acceleration tests, the batteries heat up significantly, causing the car to limit peak performance until the batteries cool down.

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If it’s this cold outside, you may want to consider heating the battery.

In places like Norway where it can get really cold outside, peak power may be limited due to a number of factors. You may be required to drive down the road just to heat up the batteries before you can drive at any sensible speed on a major road.

Sometimes this is due to the tires being too cold. Extreme cold makes rubber brittle, causing it to crack and break under stress. Consequently, Tesla limits the stress on the rubber in the cold by forcing you to drive slowly at first in order to heat the tires up.

Ludicrous Mode

In the P100D variants of the Model S and X, you may have a model which supports ludicrous mode. This is an option when buying the car and is a few thousands pounds extra.

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Put simply, ludicrous mode unleashes the absolute maximum peak performance of the batteries, allowing the car to accelerate at ludicrous speeds. Before enabling, a notice pops up saying that the battery is just preconditioning to around 50 degrees Celsius. This is the temperature needed for maximum peak performance from Tesla batteries.

Next, the BMS activates, heating or cooling the battery modules depending on their temperatures at the time. It usually takes a few minutes for ludicrous mode to become fully enabled.

Conclusion

We’ve established that batteries must be cooled or heated in order to facilitate both performance and storage metrics. Here’s a rundown of how to store your Tesla:

  • When storing your Tesla for multiple days, try to keep the charger connected.
  • If your battery is too cool, you can drive to warm it up. Remember to not place too much stress on freezing tire rubber.
  • The ludicrous mode will precondition the battery to about 50 degrees Celsius. This uses battery power and may be damaging to battery health when done repeatedly.

Do you have any recommendations on storing your Tesla?

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