Test of Battery Modules

3. August 2012  by Ningling Rao


WP6a Pre-Test of Technical Solutions

EDISON project focuses on the interaction between the electric vehicles and the power grid and seeks answers to the key question: how do we turn electric vehicles from a potential grid congestion risk to a grid supporting flexibility asset via smart charging?

The overall objective of WP 6a is to demonstrate such an answer in lab scale. To do this, we need firstly to build up an integrated test lab of EV charging infrastructure at SYSLAB, DTU Risø Campus, which should function as a solid technical platform capable of simulating the interplay between the electric vehicles, the charging station designs and the electricity grid allowing for testing of the new technologies developed in the whole EDISON project in lab scale.

Secondly, we have to develop a test plan covering all the critical issues in the implementation of an EV charging infrastructure based on the requirements and inputs from all other work packages. These issues are related to EV battery technology and modelling, communication interfaces between the vehicle/battery, charging post and the grid, the impact of EV charging on the grid especially in fast charging scenarios, and integration of the overall system.

Thirdly, we have to perform the POC tests of EV charging functionalities and control systems according to the test plan and provide operational experiences and recommendations addressing the above mentioned aspects based on the test results.

The main results of WP6a in these 3 areas are highlighted in the following sections.

 

I     Interplay Between EVs And Power grid Simulated In Lab scale

Batteries simulating EVs and PHEVs

A thorough survey of the state of the art lithium ion batteries used for electric vehicles was carried out in the first 6 months of the project (2009), which revealed that the most commonly selected lithium ion battery type for commercial electric vehicles is lithium ion manganese oxide spinel based (LMO) due to its overall balanced performance in energy/power densities, lifetime, cost and safety. This type has been chosen for both pure EVs and PHEVs, e.g. in pure EVs such as Mitsubishi iMiEV / Peugeot ION / Citroën C Zero, Nissan LEAF, Renault Florence Z.E. and GM’s PHEV Volt/Ampere. At the same time, new lithium ion battery types started to show competitive key features. Lithium iron phosphate (LFP) type was becoming a promising choice for PHEVs due to its high power/energy ratio favourable for HEV and PHEV as well as improved safety and cycle life. On the other hand, mixed oxide based lithium ion types such as NMC (Ni, Co and Mn based) showed high energy density, which is crucial for the driving range of pure EVs and started to be an interesting choice for pure EVs. Today, these observations from the state of the art survey from 3 years ago are still valid.

EV technologies are still relatively new and under strong development. Many different designs and choices of technologies have been pursued by the automotive OEMs, battery industry and EV research world. Although some standardized solutions are being promoted and becoming a new trend, there is an outstanding need for common platforms and sharing of knowledge and core technologies. As for pure EV and PHEV batteries, it is expected that LMO type will continue to dominate the commercial EV models and their price will decrease quickly thanks to the effect of economy of scale. LFP type will find larger market share in PHEVs and in some pure EVs as well, especially in Asia as the technology gets mature and the product quality improves. NMC or NCA (Ni, Co, al mixed oxide based) will find application in some niche EV models for their outstanding performance, but such batteries will remain expensive due to the high cost raw materials and required complex BMS controls as well as cooing needs for safety protection.

Based on such observations, 3 battery packs were purchased and installed in the SYSLAB test platform. Purchase of LMO battery type has not been possible. Today, such batteries are still proprietary products only exclusively produced for the commercial EV models and not accessible on the market.  A brief description of all the battery samples can be found below.

  • One NMC type battery pack of 26 kWh (355V/75Ah) representing a pure EV battery

  • 2 identical LFP packs of 16 kWh (320V/50Ah) mass produced for a commercial PHEV model, to represent a typical PHEV battery. The 2 identical packs should allow for flexibility in testing and could function as backup/replacement for one another, which turned out to be necessary.

  • 2 identical battery modules of each of the above chemistry types were also purchased as test samples for battery modelling purpose and have been used by WP1.5.

Battery

Size

Quantity

Delivery date

NMC battery system

(12X8S1P)

26.6 kWh

75 Ah / 355 V

1

June 3rd, 2010

NMC battery module

(8S1P)

2.2 kWh

75 Ah / 30 V

2

June 3rd, 2010

LFP battery system

(10X10S1P)

16 kWh

50 Ah / 320 V

2

July 7th 2010

LFP battery module

(10S1P)

1.6 kWh

50 Ah / 32 V

2

July 7th 2010

 

SYSLAB test setup

The focus of WP6a during the following 12 months up to the midterm EDISON public workshop was on setting up the test lab at SYSLAB of DTU. In close collaboration between WP4 and WP6a, three battery packs with individual bi-directional inverters have been specified, provided and integrated into the SYSLAB research testing platform at DTU Risø Campus.

Under this test setup, the SYSLAB power system has been operated as a micro-grid connected to the national grid, emulating a local part of a larger power system. In addition to the EV batteries, the micro grid includes wind power, solar power and flexible loads representing future consumers. The batteries can switch between being connected either to the micro grid or directly to the national grid through a dedicated grid connection – emulating grid connected state or driving state, respectively. During driving state, the batteries are loaded by realistic driving load patterns, based on real, measured driving patterns in combination with an EV load model. A schematic representation of the SYSLAB test setup and communication/control layout can be found in the WP6a final report (see the links at the bottom). 

II    The Test Plan

The detailed test plan to be followed by WP6a for POC testing was developed during the second year of the project based on several workshops hosted by WP6a, where selected  work packages or the whole consortium were involved as needed. During this process the requirements and recommendations from other work packages were collected based on the their technologies “in the making” and prioritized with respect to technical focus areas as well as the degree of flexibility in the scope of the plan to allow necessary adjustment naturally related to new technologies. The test plan was finalised in December 2010 and specifies 5 key focus areas:

·        Batteries, BMS and battery models; driving pattern simulation and charging controls; battery life test and prediction

·        Communication technologies and standardized interfaces to enable open network communication and interoperability

·        Grid impact of EV charging; the interaction between the charging inverter and the SYSLAB power grid

·        Fast charging capabilities and control algorithm; fast charging station design

·        System integration between the EV/battery, the charging point and the SYSLAB micro grid.

The detailed scope of each test categories is documented in the working version of the test plan and has been adjusted during the 3rd project year based on the new knowledge gained in the project and the actual capability of the test lab.

III    Key Findings from POC (Proof Of Concept) Testing

Battery characteristics

The different types of EV batteries have different characteristics regarding cost, energy density, safety, energy efficiency, degradation etc., making them suitable for different EV applications. On the EDISON POC test platform, we have tested two types of batteries: A 50 Ah / 16 kWh LFP battery for PHEV applications, and a 75 Ah / 26 kWh NMC battery, suitable for pure EV applications. The test results we have obtained from the performance characterisation of the batteries in general confirm the performance characteristics specified by the suppliers and reported generally in the literature.

Today, batteries are still a critical component in the EV industrialization due to their high price, technical complexity, limited records of long term operational data, etc. BMS functionalities are very important for the optimal use and handling of batteries; lack of knowhow towards these new types of batteries among EV academic and industrial professionals is an issue with respect to safe and optimal operation and correct handling of EV batteries. To validate battery aging models, one needs to understand the correlation between stress patterns and battery degradation mechanism; the lifetime prediction of the batteries can only be made generically and semi-quantitatively based on the battery type, charging and driving patterns, quality of the battery product and diagnostic measurements of SOH (State of Health).

Fast charging

EV fast charging (or alternatively battery swapping) infrastructure is expected to be requested for specific EV applications in the urban areas, like EVs in regular services (public transport, delivery vans etc.), and along the highways to compensate for the limited driving range provided by a fully charged battery (typically 150-200 km). Fast charging and battery swapping are expected to be provided from central stations with multiple charging posts or swapping stands.

In principle, fast charging in itself is not necessarily harmful for EV battery’s lifetime, but it does require sufficient understanding of the battery system and charging technology. The functionality of the BMS could play a critical role to make an EV battery robust or vulnerable towards fast charging. Temperature seems to be the critical factor, and cooling system is important to secure controlled fast charging without compromising the service life of the battery.

Power system impact

The bottom line is, the charging hardware connected to the grid has to live up to the existing grid code requirements for such devices and it is the responsibility of the device suppliers to secure this. The design of the charger will determine the grid impact of EV charging depending on the active/reactive power characteristics of the charging inverter.

Smart charging

One of the major conclusions of EDISON is that smart charging can turn electric vehicles from grid congestion threats to flexibility resources. The smart charging tests performed have demonstrated that even with a very simple control algorithm (without any forecasting) it is possible to provide the required charging and at the same time provide power system services in terms of regulation of the aggregated local power, reducing the peak power (reducing the power capacity of the power connection) and the total energy exchange (reducing the energy losses in the power connection line) of the power exchange with the national grid. This shows the potential to use EV batteries to successfully deliver power balancing services to the grid when the communication interfaces between the battery BMS, charging inverter and the control room of the grid are standardized for interoperability.

Standardization and compatibility

It is very important to implement standardized and compatible communication and control interfaces ICT platforms for charging infrastructure to support intelligent and controllable charging in an open network for optimal socioeconomic impact. Such technologies have been successfully developed and demonstrated in EDISON. At the “fast charging and batteries” demonstration stand on the Bornholm EDISON day in September 2011, we demonstrated in lab scale that EV charging can be intelligently and remotely controlled. Furthermore, the EDISON EV batteries have been used to stabilize the SYSLAB power grid with the 10 kW wind turbine, 8 kW PV panel and the 10 kW flexible load in both charging and V2G modes based on the control strategy selected for the SYSLAB grid. This demonstration provides the proof of concept for using EVs as flexible distributed energy resources (DER) to maximise the use of wind power and avoid unnecessary grid upgrade due to congestion in the distribution grid caused by uncontrolled EV charging in peak load time windows.

Please find more details about WP6a lab testing of the interplay between the EV batteries and the DTU Risø SYSLAB micro grid in these reports:

http://www.edison-net.dk/~/media/EDISON/Reports/EDISON_WP6a_Report_Part1_20120427_rev.2.2.ashx

http://www.edison-net.dk/~/media/EDISON/Reports/EDISON_WP6a_Report_Part2_20120430_rev1.3.ashx

Contact

Ningling Rao