Measuring pressure keeps Li-ion batteries cool

Measuring pressure keeps Li-ion batteries cool

We’ve all seen the video clips of laptops inexplicably bursting into flame, or read about the Chevy Volt that erupted in a blaze weeks after completing a crash-test: Known as ‘thermal runaway’ events, these occurrences in Lithium Ion batteries are not only spectacular but also extremely dangerous.

Thermal runaway is usually caused by excessive current or high ambient temperature and involves several phases: 

  • Starting at 80ºC, the solid electrolyte interphase layer (SEI) begins to breakdown; after which the electrolyte reacts with the anode. This reaction is exo-thermal which rapidly drives the temperature higher.
  • Secondly, the elevated temperature causes the organic solvents to break down resulting in the release of gases – normally this starts at around 110ºC. During this phase the pressure inside the cells builds up and the temperature rises beyond the flashpoint. However, the gas does not ignite due to a lack of oxygen.
  • Finally at 135ºC the separator melts causing a short-circuit between the anode and cathode, leading to the metal-oxide cathode breaking down at 200ºC and releasing oxygen which allows the electrolyte and hydrogen gas to burn. This reaction is also exo-thermal and rapidly drives temperature and pressure even higher.

Liquid-cooled batteries; the answer to thermal runaway.

To regulate the temperature of the cells in high energy Li-ion electric vehicle battery packs manufacturers employ sophisticated Battery Thermal Management Systems, often incorporating fluid cooled heat sinks, to control both high and low temperatures.

But in order to implement an efficient fluid cooled heat sink design for an electric or hybrid vehicle battery it’s important to determine the battery’s temperature and heat flux profile through testing and recording values at multiple locations. This is done using thermocouples during the charge and discharge cycles of the battery. 

Once this data is collected and analyzed, trendlines are extrapolated to fit the heat flux data, then used to create equations for the heat flux profile during the charge and discharging phases. 

As soon as this profile has been recorded, a“half-heat sink” model is created using modeling software such as PTC Creo Parametric 3D.In so doing the proposed paths of the fluid flow channels can be laid out to create the desired cooling channel cross-sections along the critical paths. 

However, effective heat transfer requires a fine balance between the velocity, pressure, and temperature of the fluid flowing through the heat sink channels. Therefore it’s critical to optimize the inlet and outlet pressures to control the flow rate of the coolant through the heat sink. 

Accurately measuring pressures optimizes heat transfer 

And with a pressure differential of about 0.008273709 Bar deemed to be optimum, the pressure sensors used to measure the fluid pressures across the heat sink have to be incredibly accurate and stable across a wide range of temperatures and pressures. 

There are only a handful of pressure sensor manufacturers in the world producing instruments capable of reliably performing the task. Manufacturers are chosen to supply pressure sensors to development teams across the globe because of their accurate and consistent performance. 

The test results these quality sensors record are used to plot the maximum and minimum pressures at different volumetric flow rates, allowing different flow channel designs to be compared. 

As defined in Bernoulli’s Equation, where velocity squared varies inversely with pressure, the pressure drop increases quadratically as the volumetric flow rate increases. 

For this reason, engineers opt for wider channels with allow flow rate and more passes over the battery, thereby optimizing the heat transfer from the cells to the heat sink. 

So, largely thanks to accurate pressure measurements during the development phase the heat dissipated through forced convection has significantly improved the safety, reliability and cycleability of Li-ion batteries. 

Going electric steps up the pressure

Going electric steps up the pressure

As the world inches closer to ‘zero emissions,’ transportation engineers are under pressure to come up with creative ways to retain drivers’ confidence in the changing technologies.

Take the hydraulic braking system for example: The current hydraulic system is nothing short of an engineering masterpiece. What drivers take for granted when they push the brake pedal has taken decades to develop and perfect. While the system that slows the vehicle down is in itself a complex engineering feat, the servo-assisted pedal input from the driver is no less impressive.

If we examine the pedal force the driver exerts vs. the retardation of the vehicle we see that it’s not linear. With the assistance of the ‘brake booster’ the first part of the curve is steeper, so that the driver has a direct correlation of pedal effort to retardation. However at a certain point, known as the ‘knee-point,’ assistance is reduced to prevent the driver inadvertently locking the wheels, thereby reducing the braking efficiency.

Although brake manufacturers have got the science of optimizing these systems down to a fine art, there’s a very fine line between a great braking system and one that under extreme conditions can be dangerous. The experienced driver often picks this up under emergency braking, when the vehicle initially slows down as expected, only to ‘run out of brakes’ the moment before the accident. This is usually attributed to a severe drop off of servo assist, leaving the driver to exert excessive and unexpected pedal pressure at a critical stage of the operation.

While this may be a worst case scenario, even under day to day driving conditions a borderline braking system can deliver an unsatisfactory user experience: Consumers complaining about a lack of feel, commonly known in the industry as a ‘wooden pedal’ is usually as a result of the pedal effort appliednot matching the expected retardation. In this case the driver feels disconnected from the vehicle.

Nonetheless, after refining the system over several decades, the industry is being forced to rethink everything it has learnt: Electric vehicles are redefining vehicle control systems.

Brake by wire system of a Formula One race car
Image Source: https://www.formula1-dictionary.net

Revolutionizing the braking system for electric vehicles

As electrification takes hold and traditional internal combustion engines are phased out mechanical components, such as the vacuum servo, no longer have a ready power-source, which means electrically driven pumps and motors have to be developed to drive the control systems.

Furthermore to integrate automated driving systems, controls are rapidly moving to electric/ electronic (E/ E) architecture, often loosely referred to as ‘X by wire’ controls.

But, for a brake by wire system to function safely and effectively the integrity of the human machine interface (HMI) needs to be retained as is. And in order to achieve this engineers need to map out the two sets of forces (In this case measured in force/ area, or pressure): The pedal effort applied by the driver, and the resultant pressure on the caliper pistons/ wheel cylinders in the ‘traditional’ hydro mechanical system.

 Only high quality pressure sensors will do

The integrity of this data is crucial to the effective development of the E/ E system, so only high quality pressure sensors, that are capable of accurate and repeatable recordings, can be used.

Not only must these sensors be capable of capturing highly accurate data, but they need to do this in an environment where harsh chemicals, heat, vibration and space constraints do not always favor carefully calibrated measuring equipment.

For this reason development teams rely on a handful of quality pressure sensor suppliers to provide measuring equipment they can rely on.

It’s all about the feel

Armed with the input vs. output pressures engineers now need to try and replicate, not so much the outright stopping performance, but the feel of the traditional system. Using wheel speed sensors it is quite easy to maximize retardation, but it’s not that easy to replicate the drivers’ feel when carrying out very light ‘check braking’ at low speeds.

This is where the real world data is worth gold: The pedal effort vs. system pressure needs to be replicated by an electronic control unit which manages the rate at which the brakes are applied. This in itself is a mammoth task, as drivers apply the brakes at different rates depending on road and traffic conditions and personal preference: A driver in a hurry may leave braking to the last minute and have to brake harshly, whereas elderly people may expect a far more leisurely event.

The degree of difficulty in achieving this driver feedback can be gauged by the performance of the system when fitted to Formula One race cars: After three years there are still teams that are unable to provide the driver with a brake by wire system that provides enough feel for them to commit to heavy braking maneuvers.

So while brake by wire systems may still be a few years away from series production in mass volume, cost sensitive vehicles, brake system specialists have been able to accurately quantify, with the aid of pressure sensors, exactly what is required.

48 Volt electrification puts pressure on the engine’s cooling system

48 Volt electrification puts pressure on the engine’s cooling system

Set against the backdrop of the 2015 Paris Accord, which calls for an 80% reduction in Greenhouse Gasses by 2050, the automotive industry is fervently working towards ‘Zero Emissions’ vehicles. However, after more than a century, the internal combustion engine isn’t going to disappear overnight, leaving the industry no choice but to explore new technologies to clean up ICE emissions.

While full electrification is the end-goal, the technology is currently prohibitively expensive, with a lack of infrastructure to support a mass roll out. Hybridization on the other hand, in particular the 48-volt mild hybrid electric vehicle, offers a cost effective, easy-to-implement solution. 

Even though the technical implementation of a 48V powernet is relatively straightforward, it does require the fitment of a Li-ion battery designed to support cranking and restarting of the ICE, brake energy recovery and torque assist during acceleration. Although this is a very efficient source of electrical energy it has several drawbacks, including overheating which could lead to ‘thermal runaway’ and even cause spontaneous ignition of the battery cells. 

Thermal runaway is usually caused by excessive current or high ambient temperature andprogresses throughseveral phases: 

  • Starting at 80ºC, the solid electrolyte interphase layer (SEI) begins to breakdown, after which the electrolyte reacts with the anode. The heat generated by this reaction causes the organic solvents to break down releasing gases which increases the pressure inside the cells.
  • However, the gas does not ignite due to a lack of oxygen. But if the temperature continues to rise above 135ºC, the separator will liquefy causing a short-circuit between the anode and cathode, leading to the metal-oxide cathode breaking down at 200ºC and releasing oxygen which allows the electrolyte and hydrogen gas to burn.

This is a characteristic of the Li-ion battery and, as with full electric vehicles, requires efficient battery management and cooling to ensure safe, efficient operation.

Image 1: Porsche battery cooling system (Image source: Charged EVs)

Determining the 48V Li-ion battery’s cooling demands

To regulate the temperature of the cells in higher-power 48V Li-ion battery packs (above about 1,000Wh), manufacturers have developed sophisticated Battery Thermal Management Systems which often incorporate fluid cooled heat sinks to ensure thermal stability.

But, before implementing a fluid-cooled heat sink it’s important to determine and record the battery’s temperature and heat flux profile at multiple locations in the system. This is done using thermocouples during the charge and discharge cycles of the battery. 

Once this data is collected and analyzed, trendlines are extrapolated to fit the heat flux data, then used to create equations for the heat flux profile during the charge and discharging phases. 

As soon as this profile has been recorded, a“half-heat sink” model is created using modeling software such as PTC Creo Parametric 3D. In so doing the proposed paths of the fluid flow channels can belaid out to create the desired cooling channel cross-sections along the critical paths. 

Effective heat transfer requires a fine balance between the flow-rate, pressure, and temperature of the fluid flowing through the heat sink channels. Therefore it’s critical to optimize the inlet and outlet pressures to control the flow rate of the coolant through the heat sink. 

Pressure measurement key to the thermal balancing act 

And with a pressure differential of about 0.008273709 Bar deemed to be optimum, the pressure sensors used to measure the fluid pressures across the heat sink have to be incredibly accurate and stable across a wide range of temperatures and pressures. 

During this crucial stage of development manufactures rely on high quality pressure transmitters only available from a select group of manufacturers because of their accurate and consistent performance. 

Focusing on the heat sink, the measurements recorded during these tests are used to plot the maximum and minimum pressures at different volumetric flow rates within the heat sink, allowing different flow channel designs to be compared. 

As defined in Bernoulli’s Equation, where velocity squared varies inversely with pressure,the pressure drop increases quadratically as the volumetric flow rate increases. 

For this reason, engineers opt for wider channels with alow flow rate and more passes over the battery, thereby optimizing the heat transfer from the cells to the heat sink. 

Up to this point development is similar to that normally carried out during EV battery testing, but in the case of the 48V MHEV, battery cooling is commonly integrated into the ICE cooling system to reduce cost and complexity, thereby increasing the thermal load on the ICE’s cooling system. Thus, once the battery cooling has been optimized engineers need to integrate the heat sink cooling into the ICE’s cooling system. 

During this phase of the design, the development team not only monitor any change in the pressure differential across the heat sink, but also measure the pressure drop across the complete cooling system to ensure that engine cooling is not negatively affected by the inclusion of the battery cooling. 

Once again, with engineers looking for incremental differences in pressure readings, it’s important that the pressure sensors used to record these values are stable, and capable of repeatable results. 

So, although 48V battery liquid cooling integrated into the ICE’s system increases the thermal loading, thoughtful design and development, particularly regarding system pressures, significantly improve the safety, reliability and cycleability of the Li-ion battery without affecting engine cooling.