Lithium battery technology

Jan. 8, 2016
The Lithium-Ion family of battery technologies has quickly become the center stage product in the advanced technology vehicle market.

The Lithium-Ion family of battery technologies has quickly become the center stage product in the advanced technology vehicle market. Although Nickel Metal Hydride (NiMH) continues to be a staple product in the Hybrid Electric Vehicle (HEV) market, when it comes to applications that need additional energy storage for extending vehicle range, NiMH is not competitive with the Lithium family of battery technologies.  Plug-In Hybrid (PHEV), Battery Electric Vehicle (BEV) and Fuel Cell Electric Vehicle (FCEV) applications need the additional capacity storage capability, coupled with the smaller weight and packaging of Lithium to provide increased vehicle fuel economy and/or range.

Why use Lithium?
As the advanced technology (electric propulsion) systems continue to populate the vehicle market, the Lithium family of battery products becomes the choice of manufacturers that have entered the market in the past 4-8 years, due to its superior energy storage capability. For the foreseeable future, Toyota/Lexus continue to enjoy the majority of the advanced technology market and the battery technology utilized in their vehicles is predominately NiMH. One of the primary reasons for utilizing NiMH is that it is a very stable chemistry (not prone to thermal events or problems with overcharging), there is a long record of use in industrial applications and it is a known quantity that is predictable in its operating and failure modes.

Lithium products are manufactured with two basic formats – cylindrical 18650 and pouch.  The 18650 cell is slightly larger than a AA battery and the pouch style cell can be manufactured in many different size configurations, dependent upon application. 

Lithium 18650 cylindrical battery cell
Lithium pouch battery cell

When compared to NiMH, the Lithium family of products has significant advantages when considering capacity, mass (weight), resistance and size. However, since Lithium products must be controlled within a much narrower charging/discharging voltage range, this becomes a disadvantage for systems due to the cost of adding sophisticated control systems. When considering the cost of the hardware and software systems that are required to monitor and control cell performance, Lithium battery systems cost can be significantly higher than NiMH. 

Lithium battery families
Unlike NiMH, Lithium technology has numerous family categories. Each of these categories offer varied energy (capacity) and power characteristics. The primary families utilized in the automotive or medium/heavy duty market as of this printing are:

·      Lithium Cobalt Oxide

·      Lithium Manganese Oxide

·      Lithium Manganese Cobalt Oxide

·      Lithium Iron Phosphate

·      Lithium Nickel Cobalt Aluminum Oxide

·      Lithium Titanate

Each of these chemistries has a different characteristic (power, energy and discharge performance). The electrolytes can be significantly different, although each uses Lithium Salt as a basic element. Each may have different additives in the electrolyte that mitigate aging, permit enhanced performance, fire retardants, etc. Each of these additives will affect how the cell performs and its longevity. Unlike Lead Acid and NiMH batteries, which enjoyed stable and predictable performance metrics, the growing number of Lithium products with widely varying performance metrics will require technicians to more thoroughly understand failure modes and specialized diagnostic techniques. A quick internet search on the Battery University website can provide more detail on each chemistry.     

The advantages of Lithium, when compared to NiMH are in the areas of mass and energy (how much capacity can be stored for a given size and mass). To assist with clarity, a comparison of the Toyota Prius (NiMH) and Hyundai Sonata Lithium battery packs applications has been provided. Listed below are some of the key metrics of Lithium as compared to NiMH. To help understand the comparisons (written in engineering “speak”) here are some definitions:

·      W-hr./kg = Watt Hours per kilogram:  How much energy can be stored for a the given mass (weight) of a battery cell

·      W-hr./L = Watt Hours per Liter:  How much energy can be stored for the given physical cubic size of the battery cell

NiMH vs.Lithium cell comparison
NiHM vs. Lithium density comparison

The comparisons of the NIMH and Lithium are very clear. In the examples, the Lithium cell voltage is 3.8 volts when compared to NiMH with 1.2 volts. Therefore, Lithium can achieve voltage levels with fewer cells. When comparing W-hr. per kilogram and Liter, the comparison is just as stellar. Lithium has approximately 2.60 – 3.50 times the W-hr. per kilogram (1 kilogram = pounds) so, it stores more energy for a given mass. And, Lithium has approximately 1.50 – 1.80 times the W-hr. per Liter so, it stores more energy for the physical cubic space it occupies. There are incredible differences and advantages for Lithium when compared to NiMH. Therefore, more battery cells can be placed in series and parallel to produce modules with much higher voltage than the traditional NiMH prismatic or cylindrical battery modules/sticks, such as a 32V Lithium module used in a Hyundai or Kia hybrid vehicle.

Hyundai-Kia Lithium battery module

Lithium technology: Basic vehicle level diagnostic application 

With six basic Lithium chemistries currently used in the market, and more on the way, it is vital that technicians understand why it is important to know the differences between the technologies and how each will react to testing and the associated diagnostic results. Although the OEMs provide some basic information on how to test the vehicle battery packs by using the scan tool, the testing necessary for determining the actual battery condition (power and energy testing) goes far beyond the scan tool. To test a vehicle battery for power and energy, the testing protocol would begin with using the standard OEM process of acquiring DTCs, Freeze Frame Data, etc. From this point there may be some limited testing that can be performed through the scan tool, although most OEMs don’t support enhanced battery pack testing via scan tool CAN output test messaging. 

After the initial DTC, Freeze Frame and output tests have been performed, the technician would typically determine whether or not to remove the battery pack for replacement or, if the OEM supports battery pack internal repair, replace large block components of the battery pack. However, there are additional tests that can be performed to assist the technician in determining battery pack power and energy. The additional tests use the scan tool and other off-board equipment to test, and determine if the battery pack needs to be “tuned” to ensure good performance and longevity.

Using only a scan tool, the technician could perform a battery stress test. A stress test is a testing method that our company (and some OEMs) have been using since the mid-1990s to determine if a battery pack has a reduction (or fade) in power or energy. A stress test is similar to a test performed by a cardiologist on the heart of a patient. In a stress tester performed by a doctor, the patient is placed on a treadmill that will gradually increase the load placed on the heart, by changing tread angle and speed. An electro-cardiogram and an echo-cardiogram measure heart activity to determine how well it performs under load. In automotive diagnostic term, the vehicle is driven with a specific drive cycle process to stress the battery pack for determining an overall performance and state of health (SOH) under load. This process is something currently not available via vehicle diagnostics (i.e., vehicle controller functional tests available through the scan tool). By documenting data before and after the stress test, and comparing and contrasting data, a solid picture of battery pack SOH can be determined. 

Also, the stress test can be used to evaluate battery pack SOH even if there are no DTCs. In more recent years, there are customers that are routinely bringing their hybrid vehicles to a repair business only because the fuel economy has dropped and/or there is a hesitation only during a more aggressive acceleration. This is a classic complaint for a high voltage battery pack-related power or energy problem. However, using a stress test is extremely hard on a battery pack. Therefore, prior to performing a stress test, you will need to know how to use the proper process and, more importantly, how to interpret the scan tool data once the test has been completed. This will ensure that no additional stress testing will be necessary that could lead to over-stressing the battery pack.

The stress test information (coupled with any DTC and Freeze Frame data if available) would provide enough information to determine how the battery pack performs during an extremely loaded condition. This would assist the technician in determining whether or not the battery pack would need to be removed for replacement or testing and rebuilding. If a shop is specializing in HEV/BEV repair, testing and rebuilding are additional steps that would require additional equipment and training to ensure that the battery pack is properly rebuilt, balanced, and has adequate power and energy.

Lithium technology:  Specific diagnostic application 
When working with the Lithium families of battery chemistries and performing stress testing, rebuilding or testing it is vital that a technician know which Lithium technology that is being used in the vehicle. For example, when working with the Lithium Manganese battery families, the diagnostics and any stress testing would yield data that would look very different from a Lithium Iron Phosphate battery family, due to how the discharge voltage signatures are generated by the battery cells. For example, the Lithium Manganese families have a very linear discharge signature (similar to a Lead Acid battery) vs. the Iron Phosphate families which discharge with an extremely flat discharge signature. This means battery capacity may or may not be easily interpreted for use in diagnosing battery pack module condition through recording battery module voltage data on the scan tool. Therefore, knowing how specific battery chemistries behave is critical in knowing how to interpret scan tool of discharging equipment data. Other battery chemistries have signatures that fall in-between the flat and linear discharge voltage signatures. Whether a vehicle is HEV, PHEV, BEV or FCEV powered, is vital that technicians understand the performance characteristics of the Lithium families (and NiMH) being used by the OEMs and how to interpret whether or not the performance of the battery pack is acceptable (whether a DTC is present or not). 

Other things to consider
Although both NiMH and Lithium technologies suffer from the effects of over temperature, cells drying out, etc., most of the Lithium technologies have one major disadvantage when compared to NiMH. Lithium must be kept within a very specific operating band during charging cell damage and thermal events can occur. This means that expensive electronic circuits and software controls must be used within the battery pack system to maintain cell operation within these bands and for maintaining the optimum top-balancing voltage. This results in a much higher cost to implement Lithium into a vehicle system when compared to a NiMH system. There are some Lithium families that are much more tolerant to overcharging than others (such as Iron Phosphate) when compared to other chemistries that may have narrower operating boundaries. Although most HEVs today continue to utilize NiMH battery technology, all mainstream PHEV and BEV programs use the Lithium technology to take advantage of its significant advantage of energy storage (mass and volume) when compared to NiMH.

In summary, it will vitally important that technicians begin to learn both NiMH and the Lithium family of battery systems. As battery technology continues to evolve, it is inevitable that there will be an overwhelming number of battery families and chemistries used to help the OEMs meet the target of 54.5 mile per gallon fuel economy and lower CO2 emissions that the must achieved by 2025. Lithium battery systems will be an integral technology of how the OEMs reach these mandate targets. The HEV and PHEV vehicles are becoming a higher percentage of OEM product offerings, and the BEV and FCEV have now become more than a mere novelty in the OEM vehicle product lineup. 

The time to start embracing the technology is now while there is still time to learn it without the pressure of trying to learn it on the fly. Wouldn’t it be easier to start learning these systems now rather than waiting until the first advanced technology vehicle rolls into your service bay?  

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