the future of automotive battery testing · 2019-03-11 · © millbrook proving ground limited 2019...

www.millbrook.co.uk© Millbrook Proving Ground Limited 2019 1

The Future of Automotive Battery Testing.

28th February 2019

Dr Peter Miller


Overview


Automotive Battery Testing

• This presentation will focus on “traction”

batteries for HEVs, PHEVs and EVs.

• The key legal regulations are:

o UN “38.3” for transport

– This covers almost every country in

the world and transport in-between

by air, sea and land

o ECE Regulation 100

– This currently covers EU, Norway,

Russia, Ukraine, Croatia, Serbia,

Belarus, Kazakhstan, Turkey,

Azerbaijan, Tunisia, South Africa,

Australia, New Zealand, Japan,

South Korea, Thailand and

Malaysia


Legislative Tests for Automotive Battery Packs

Type of Test UN 38.3 UN 38.3

Duration

Reg. 100 Reg. 100

Duration

Altitude T1: 15000m ~6 hrs n/a

Thermal T2:+72/-40oC ~24 hrs +60/-40oC ~84 hrs

Vibration T3:7-200Hz <=2g ~ 9hrs 7-50Hz <=1g ~3 hrs

Shock T4: depends on weight

e.g. 100kg pack ~17g

~24 hrs Depends on vehicle type

for cars <=28g

~ 2 hours

Short circuit T5: <0.1 ohm ~ 7 hrs <5mOhm ~ 2 hrs

Overcharge T7: 1.2*rated voltage ~ 200hrs To twice rated capacity ~ 1 hr

Crush n/a 100kN ~ 4 minutes

Over discharge n/a Until 25% voltage ~ 1 hr

Over temperature n/a Use then heated in

climatic chamber

~ 24 hrs

Fire resistance n/a Pack over burning fire ~2 mins

• The Thermal and Vibration tests (especially for Reg. 100) could both be

considered life tests. The rest are best described as “abuse” tests.


Background: a Pack = Cells in Series

• 3 voltage sources in series

o The resultant voltage is Vs=V1+V2+V3

o If these are batteries, the total energy is ~ E1+E2+E3

– as long as E1=E2=E3, strictly it is 3* the smallest of (E1,E2,E3) as

charging must stop when the 1st cell (the one that can hold the least

energy) is fully charged

– Typical capacity tolerances for production li-ion cells are +/-2-5%.

• A typical automotive battery pack has ~ 100 cells in series to give ~ 370V DC.

- + - + - + +

-

Voltage V1

Energy E1

Voltage V2

Energy E2

Voltage V3

Energy E3Vs


Constant Current Constant Voltage (CCCV) Charging

• These voltages are typical for li-ion cells; most chemistries are charged in this way.

• This is (relatively) simple to achieve for a cell, but more complex for a pack as

due to manufacturing tolerances the cells in a pack do not always charge at the

same rate.

2.1Ah cell charged

at 4.2A (2C rate)

Total charge time

~50 minutes.


CCCV Charging – Pack

• When charging the pack all that can be adjusted is the pack voltage and

current. As the cells are in series the current is the same in all the cells.

• We can use the pack’s Battery Management System (BMS) to measure the

voltage of each cell and provide this (normally via CAN) to the “charger”

(battery pack cycler)

Cell

100

Cell 3

Cell 2

Cell 1

CyclerBMS

Current = I Amps

Voltage

=V VoltsCAN

Pack


Charging Holding the Pack Voltage Constant

• If the total pack voltage is constant then the average cell voltage is constant

(blue trace above)

• The maximum cell voltage will not be constant

o The highest cell reached 4.013V instead of the target 4V.


CCCV Using the Cell Voltages From the BMS

• Using the pack’s BMS and controlling the cycler to keep the maximum cell

voltage constant while still charging the pack gave the results shown above

o The BMS voltage measurement resolution (in this case 1mV)

o The average cell voltage in the CV region of this test was is very close to

the target 4V (3.99998V , which is just 0.02mV low).

o Note the pack voltage to achieve this is not constant (in this example it was

gradually reducing).


Does This Matter?

• The measured voltages on the last 2 slides would not cause a safety hazard

o But it is possible that an individual cells safety limit could be exceeded if only the overall pack

voltage was monitored.

• The cell voltage does have a significant impact on life

o 1mV above 4.2V for a “typical” NMC cell will reduce the cell cycle life by 2.2%

– So the measured 13mV ( data 2 slides back) could reduce the packs cycle life by over 25%, an

unacceptable error for a test.

o 1mV below 4.2V for an “typical” NMC cell increases the cell cycle life by 1% and reduces the energy

stored by 0.1%.

– The measured 0.02mV low on the last slide would impact cycle life by 0.02% and energy by

0.001%, acceptable errors for a test

– In practice, other things (like the accuracy of the BMS voltage measurement) would make more

difference.

• CCCV charging is fundamental to most battery tests as they normally need a defined State of Charge

(SOC = energy).

o E.g. Life tests normally consist of repeated charge/discharge cycles

• Achieving this level of control for pack CCCV charging requires a state of the art cycler and control system.

o For Millbrook’s new battery test chambers the cyclers were designed and manufactured to

Millbrook's exacting specifications and the control system was developed by Millbrook Revolutionary

Engineering.


How Can you Speed up Battery Pack Testing?

• The test chambers are the most valuable resource, but they are not normally an efficient

place for people to work

o Therefore having separate “prep” areas where packs to be tested are connected to a

standardised interface and checked for operation allows both higher utilisation of the

test chambers and faster preparation as these areas can be optimised for this task

• The time taken for the battery pack to reach the required test temperature is also “wasted

time” in terms of testing, so Millbrook’s new chambers are optimised for quick changes in

temperature, using what would normally be considered “oversized” heating and cooling

systems allows slew rates of up to 6 oC per minute

• Millbrook is also sponsoring two EngD students to research in this area and employs a

consultant who sits on the main automotive battery test standards committees.

Battery Prep area

(1 of 2)


Safety

• This could happen when a li-ion battery pack is severely damaged

o Impossible to control with hand held extinguishers

o It took nearly 15 minutes to completely extinguish the fire with water

from a fire truck


Impact of Different Chemistries

At present the only impact of different cell

chemistries is on UN “38.3” testing:

• For Lithium Ion :

• Cells require T1,2,3,4,5,6,8 (not 7)

• Packs require T1,2,3,4,5,7 (not 6,8)

• For Lithium Metal:

• Cells require T1,2,3,4,5,6,8 (not 7)

• Packs require T1,2,3,4,5 (not 6,7,8)

Safety is another consideration:

• Different chemistries may need

different approaches to control

thermal events and will produce

different hazardous gasses which

may require different sensors to

detect and different handling

precautions.

UN “38.3” tests:

• T1 - altitude

• T2 - thermal test

• T3 - vibration

• T4 - shock

• T5 - external short circuit

• T6 - impact/crush

• T7 - overcharge

• T8 - Forced discharge


Conclusions

• I’ve given a brief overview of battery testing and how its evolving, using

Millbrook's new, state-of-the-art battery test facilities as an example.

• This is still a relatively new field.

• Li-ion cells of the type now widely used were first available commercially in

1991. The patent for the NMC chemistry was not filed until 2001.

• To support the testing required to meet various government targets for vehicle

electrification we will require

– More test facilities

– More power (Millbrook’s new facility can consume a peak of over

7MW)

– Ongoing development of test techniques

• New chemistries are continually being developed

o For example, lithium-metal (Millbrook has already tested some of these).

Many other types are under development

o Some technologies (for example, metal-air and flow batteries) would

require more complex test systems


Dr Peter MillerChief Engineer – Battery

[email protected]

the future of automotive battery testing · 2019-03-11 · © millbrook proving ground limited 2019...

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