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Battery degradation

in electric buses

–

Danish Technological Institute

Title: Battery degradation in electric buses

Prepared for: Danish Transport and Construction Agency – journal id: TS200707-00017 Att.: Per Darger

Prepared by: Danish Technological Institute

Teknologiparken Kongsvang Allé 29 8000 Aarhus C Transport & Electric Systems

July 2016

Authors: Kjeld Nørregaard

Bjarne Johnsen Christian Hedegaard Gravesen


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Table of Contents

1. Purpose .......................................................................................................... 4

2. Short introduction to the BYD K9 electric bus ...................................................... 5

3. Preparation of test ........................................................................................... 7

3.1. List of equipment ..................................................................................... 7

3.2. Test setup .............................................................................................. 8

4. Characterization of vehicle battery via constant power discharge ........................... 9

4.1. Test procedure ........................................................................................ 9

4.2. Tested buses - overview ..........................................................................10

4.3. Test of Bus #1 .......................................................................................11

4.3.1. Bus #1 – first test, May 2014 .............................................................11

4.4. Test of Bus #2 .......................................................................................17

4.4.1. Bus #2 – First test, June 2014 ............................................................17

4.4.2. Bus #2 – Second test, December 2015 ................................................20

4.4.3. Bus #2 - comparison .........................................................................24

5. Basis for lifetime assessment ...........................................................................29

6. Lifetime assessment .......................................................................................31

7. Conclusion .....................................................................................................33

Appendix 1. Additional bus data from Bus #1 ......................................................36

Power curve ......................................................................................................36

Dynamo data .................................................................................................36

Appendix 2. Additional bus data from Bus #2 ......................................................37

Power curve ...................................................................................................37

Dynamo data .................................................................................................37

Appendix 3. Data from road test in Aarhus ..........................................................38


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1. Purpose

On behalf of the Danish Transport and Construction Agency, Danish Technological Institute

(DTI) is performing measurements of energy content in MOVIA’s BYD K9 electric buses

carrying around the citizens in the city of Copenhagen.

The main objective of this task is to verify the expected real battery lifetime of the electric

buses relative to the battery lifetime stated by the supplier – as best possible. For the

operator the battery life is a significant economic parameter for dissemination of electric

vehicles.

The buses were delivered to the traffic companies in November 2013. The buses were put

into operation during February and March 2014. The initial tests of the buses were per-

formed in Maj and June 2014 after a run-in period - A few thousand km and a number of

battery charging cycles ensures that all battery cells have found their normal balance. This

task only focus on the batteries in the buses, not range or other aspects like e.g. auxiliary

energy consumption.

Degradation of the battery is determined by two different methods. Primary method by

measuring any change in battery capacity and internal battery resistance at vehicle level

from start to end of test period (less than two years). Secondary method is a minor test

at battery cell level to validate consistence with BYD’s own test data at cell level. Expected

behavior of the actual Lithium Iron Phosphate chemistry is that a detectable energy con-

tent/capacity is lost with aging but only a marginal increase in internal resistance. The

actual rate of change vary significantly with the conditions of the application.

The energy content of the battery is determined by discharging and charging the battery

while measuring the amount of energy flowing to battery measuring current and voltage.

To be able to repeat the test, the discharge is performed at constant power. Temperature

has major influence on the batteries so to minimize impact on results all measurements

are performed in a room temperature laboratory on a heavy duty chassis dynamometer at

Danish Technological Institute in Aarhus.

Test at cell level is performed at a dedicated cell test facility at Danish Technological Insti-

tute in Aarhus using test setup similar to BYD. The measured results is compared to BYD

test data revealed to Danish Technological Institute only under a very strict NDA, that

prevent any publication of data or results.

For final lifetime assessment detailed logging of power, temperature, speed and distance

vs time was expected. The logging turned out to be too unstable, inconsistent and lacking

relevant battery data for any detailed prognosis. Instead general temperature observations

has been used.


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2. Short introduction to the BYD K9 electric bus

DTI tested two BYD K9 electric buses with the following specification:

Figure 1 BYD K9 electric bus specification.

The 324 kWh battery pack is actually three separate packs. One mounted behind the rear

axle, one mounted on the roof and one pack split in to two mounted over the front wheels.

Battery pack layout is illustrated in Figure 2.

Figure 2. Location of the batteries in the tested BYD electric buses.

Length 12000 mm

Width 2550 mm

Height 3360 mm

Wheelbase 5950 mm

Overhang (front/rear) 2600 mm / 3450 mm

Approach angle 7°

Departure angle 7°

Gradeability ≥15%

Top speed 70 km/h

Seats 29+2+1

Gross vehicle weight (GVW) 18000 kg

Type Permanent magnet synchronous in-wheel motor

Max power 2 x 90 kW

Rated power 2 x 75 kW

Max torque 2 x 350 Nm

Max speed 7500rpm

Ratio - final drive 17.7

BYD Fe battery FV200 A (540 V, 600 Ah)

Type Li-ion (Lithium Iron Phosphate battery)

Cell voltage 3.2 V

Capacity 324 kWh

Maximum charing power 2 x 30 kW

Charge time, approx. 4-5 h

Suspension Front & rear air suspension, ECAS system

Brakes Front & rear disc-braking, ABS

Steering Electric hydraulic power steering gear

Tires Michelin 275/70R 22.5

Chassis

Dimensions

Performance

Motor

Power Battery

Charger


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The electric motors are in wheel / wheel-hub motors integrated in the rear axle. See Fig-

ure 3.

Figure 3. Rear axle illustration. In wheel motors, disk brakes and final drive.


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3. Preparation of test

3.1. List of equipment

BYD standard wall-mounted charger for BYD K9 electric bus (delivered with bus)

- 3 phase AC, 50Hz, 400V, 125A, 60kW

Heavy duty chassis dynamometer

- Modified ELP 500 from AHS Prüftechnik Germany

- Modified Picaro data acquisition system from AHS Prüftechnik Germany

- Supplementary safety equipment

Analogue data acquisition:

- Agilent 34972A LXI data acquisition/switch unit (𝑓𝑠 = 0.5𝐻𝑧)

- Hameg differential voltage probe, 1 pcs.

- Fluke i1010 current clamp, 4 pcs.

Thermo elements:

- Thermocouple element type K, 4 pcs.

Forced cooling only on tractive wheels


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3.2. Test setup

To measure and calculate battery power and energy, battery current and voltage most be

logged simultaneously.

Current were measured at four different points, all current clamps were installed close to

the high voltage distribution box. Access to the high voltage distribution box was in the top

rear of the passenger compartment behind a cover. Current flow to the three individual

batteries were measured with the id’s I1, I2 and I3 in Figure 4 and the total current was

measured with the id I4.

Voltage was measured with an analogue differential voltage probe. The probe was installed

at the battery pack in the front of the bus, over the wheelhouses in the front. It was

possible to access this battery pack by removing two covers in the passenger compartment.

Figure 4. Block diagram of test setup.


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4. Characterization of vehicle battery via constant power discharge

4.1. Test procedure

The tests were carried out following this specific test procedure:

1. Preparation of the vehicle

a. Mounting of low friction tires with similar dimensions on the vehicle for safety

reasons

b. Preparation of vehicle in the chassis dynamometer (fixation, cooling etc.)

c. Locating suitable point for measurements

d. Mounting of equipment for data acquisition

e. Dynamometer compatibility test

f. Acclimatization of vehicle

g. Monitored charge until 100% state of charge

2. Performance

a. Calculation of load and speed for the constant power discharge test

i. 𝐶/4 = 324𝑘𝑊ℎ/4 = 81𝑘𝑊

b. Measurement of vehicle power curve

c. Measurement of drive train losses

d. Drivers training

e. Verification of acquired data

f. Measurement of battery step response

i. Accelerate to desired speed

ii. Shift to neutral

iii. Let vehicle roll-off and then stop for 10 seconds

iv. Accelerate with maximum power until desired speed

g. Stop driving

h. Monitored charge until 100% state of charge

3. C/4 constant power discharge – driving the vehicle at constant battery power until

the BMS ends the driving

a. Start driving at constant battery power

b. Measurement of battery step response at 98%

i. Shift to neutral

ii. Let vehicle roll-off and then stop for 10 seconds

iii. Accelerate with maximum power until desired speed

iv. Resume driving

c. Measurement of battery step response at 88%

d. Measurement of battery step response at 75%

e. Measurement of battery step response at 63%

f. Load calibration and break after driving 100km since start

g. Resume constant power discharge

h. Measurement of battery step response at 50%

i. Measurement of battery step response at 38%

j. Break after driving 200km since start

k. Resume constant power discharge

l. Measurement of battery step response at 25%

m. Measurement of battery step response at 5%

n. Driving until first BMS cut-off

o. Analysis of acquired data and calculation of energy content, capacity and

internal DC resistance.


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From an expectation of the driver being able to drive constantly throughout the whole test,

the test duration was prepared to finish after four hours of time, excluding two minor

breaks to let the tires of the vehicle to cool down and to hydrate the driver.

4.2. Tested buses - overview

Two BYD K9 electric buses were tested.

Keolis Danmark A/S (former: Citytrafik) operated bus #1, the bus had registration

XN94638. The first test was performed in May 2014. Due to a replacement of a battery

pack on this vehicle, the Keolis Danmark bus has only been tested once – no second test

was performed. DTI is informed that replacement was due to an unauthorized opening of

the battery packs. Sealing of the batteries have been broken and the batteries were sub-

sequently damaged by leaking water. This meant that BYD had to replace one of the bat-

tery packs.

Arriva Danmark A/S operated bus #2, the bus had registration XT93686. The first test was

performed in June 2014 and the second test was performed in December 2015.

Table 1. Bus id overview.

Bus id Registration Operator First test Second test

Bus #1 NX 94 639 Keolis Danmark A/S 2014 May Not performed

Bus #2 XT 93 686 Arriva 2014 June 2015 December


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4.3. Test of Bus #1

The constant power discharge test for Bus #1 was recorded according to the test described

in section 4.1.

The results from the constant power discharge test and the following charge are shown in

chapter 4.3.1 – only one test was performed for Bus #1. These values have been measured

at different points with focus on energy for comparison. The electricity meters were meas-

uring the energy drawn from the electric grid, which includes losses during charging.

4.3.1. Bus #1 – first test, May 2014

Test parameters listed in Table 2.

Discharge test Date 7th of May 2014

Odometer at test start km 10097

Number of active charge connectors # 2

Discharge test Driven distance km 212

Table 2. Test parameters: Bus #1, first test.

The values in Table 3 first column includes the base values for the battery efficiency, which

gives approximately 89% when calculating energy in and out of the battery and approxi-

mately 84% when calculating power drawn from the wall socket to energy of the battery.

Unit Measured at battery Measured inlet to charger

Discharge capacity Ah 531.2

Discharge energy kWh 282.8

Charge energy kWh 319.5 335.2 Table 3. Energy content and capacity measurements.

Figure 5 shows the complete constant power discharge. The discharge was performed at

constant speed and with a static load to match the desired battery power of 81 kW.

The discharge test was divided into parts of 100 km of driven distance and the total test

duration was approximately 6.5 hours. This included 1.5 hours of interruption for equip-

ment adjustments, preliminary test evaluation and lunch break.

The peaks in battery current in Figure 5 shows the battery step response.

The battery voltage was measured at the front battery inside the bus. The battery currents

were measured at the high voltage distribution box at the top rear end of the bus. The

battery temperatures were measured at the top of the battery cases, except with the roof

battery due to safety reasons.

The end of the graph indicates that the battery current did increase to keep the battery

power constant. This yields a constant vehicle performance, but a faster discharge at this

point.


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Figure 5. Overview of battery voltage, current and power during constant power dis-

charge.

All temperatures in Figure 6 increased during the test and decreased during breaks. The

rear battery temperature raised more than the other battery temperatures did, which was

probably caused by the proximity of the rear axle and motors.

Figure 6. Battery temperatures compared to discharge power.

The availability of the individual battery current at the high voltage distribution box made

it possible to measure the distribution of load to each battery. Figure 7 shows this distri-

bution throughout the discharge test.

Load steps Load steps


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Figure 7. Overview of distribution of load current between batteries.

In the first part of the discharge test, the battery currents were almost equal. Small vari-

ations were observed during load steps.

The second part of the discharge test showed abnormal behavior, as the rear battery cur-

rent was observed at 𝐼1 = 0𝐴, when resuming the test after the break, indicating that the

front and roof batteries must have drawn the full load without any warning. The graphs in

Figure 8 shows that the rear battery reconnected at 14:50 confirming that the rear battery

was disconnected. Suddenly a very high current (almost 550 A) was observed in the ana-

logue data. The current appeared between the three batteries as a result of different states

of charge between the batteries, which was caused by the disruption of the rear battery

for more than one hour. This data amounts to 𝑓𝑠 = 0.5𝐻𝑧, which does not include fast tran-

sients. This means that the actual value was probably even higher than the observed value.

Prior to the reconnection of the rear battery, the other batteries showed minor instability

in their currents, from the time 14:20.


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Figure 8. Distribution of load current between batteries. Zoom on driven distance 100 to

200km (middle part).

After the reconnection, the rear battery current was driving full load and charging the other

batteries for a period time, implying a rear battery current exceeding 100 A (Figure 9). At

the short break at around 15:30 the batteries had time balanced each other.

Figure 10 shows the last part of the test. The batteries are almost empty and beginning to

cut-out and in again, preserving full power until the end. Just after 15:53, all three batter-

ies disconnected at the same time, which was the end criteria.

With the throttle pedal still fully depressed for a few minutes, the driving was resumed five

times, not being able to drive more than a few meters. This indicated that the voltage in

each battery controlled the BMS cut-off. The cut-in/out-behavior may be a result of hyste-

resis in the voltage control.


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Figure 9. Battery reconnect and balancing charge between batteries.

Figure 10. Distribution of load current between batteries. Zoom on driven distance 200

to 212 km (last part).


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Figure 11. Distribution of energy and capacity in the over-night battery charge.

Results from data acquired during the over-night battery charge are shown in Figure 11.

The figure represents four separate battery capacity and battery energy measurements,

illustrated in Figure 4. The data indicates that the amount of electrons were equally trans-

ferred to each of the batteries. And the difference between the measured total and the

measured sum of rear, roof and front battery is well within the calculated tolerances.


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4.4. Test of Bus #2

The constant power discharge tests for Bus #2 were recorded according to the test de-

scribed in section 4.1.

The Bus #2 results from the constant power discharge and the following charge are shown

for the first test in chapter 4.4.1 and results from the second test in chapter 4.4.2. These

values have been measured at different points with focus on energy for comparison. The

electricity meters were measuring the energy drawn from the electric grid, which includes

losses during charging.

4.4.1. Bus #2 – First test, June 2014


Discharge test Date 17th of June 2014




Table 4. Test parameters: Bus #2, first test.

The values in the first column of Table 5 includes the base values for the battery efficiency,

which gives approximately 93% when calculating energy in and out of the battery and

approximately 89% when calculating power drawn from the wall socket to energy of the

battery.

Unit Measured at battery Measured before charger



Charge energy kWh 328.0 344.3 Table 5. Energy content and capacity measurements.

Figure 12 shows the view over the complete constant power discharge. The discharge was

performed at constant speed and with a static load to match the desired battery power of

81kW.

The discharge test was divided into parts of 100 km of driven distance and the total test

duration was approximately 5.5 hours. This included 1.5 hours of interruption for equip-

ment adjustments, preliminary test evaluation and lunch break.

The peaks in battery current in Figure 12 shows the battery step response performed dur-

ing the discharge test.


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charge.

All temperatures in Figure 13 increased during the test and decreased during breaks. The

rear battery temperature raised more compared to the other battery temperatures during

the test, which was probably caused by the proximity of the rear axle and motors.


The availability of the individual battery current at the high voltage distribution box made

it possible to measure the distribution of load to each battery. Figure 14 shows this distri-

bution throughout the discharge test.

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Vb_front [V] I_total [A] Power

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Bat

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Bat

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T_Front T_ambient T_Top T_Rear Power

Load steps Load steps


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Figure 14. Overview of distribution of load current between batteries.

Figure 15. Distribution of energy and capacity in the over-night battery charge.






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I_1 [A] I_2 [A] I_3 [A] I_total [A]


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4.4.2. Bus #2 – Second test, December 2015


Discharge test Date 17th of December 2015




Table 6. Test parameters: Bus #2, second test.

The values in Table 7 first column includes the base values for the battery efficiency, which

gives approximately 97% when calculating energy in and out of the battery and approxi-

mately 93% when calculating power drawn from the wall socket to energy of the battery.

Unit Measured at battery Measured before charger



Charge energy kWh 303.3 316.8 Table 7 Energy content and capacity measurements.

Figure 16 shows the view over the complete constant power discharge. The discharge was

performed at constant speed and with a static load to match the desired battery power of

81 kW.

The duration of the discharge test was approximately 4 hours. This test was performed in

one session. Immediately after disconnecting the charger, the discharge test was started.

The peaks in battery current in Figure 16 is the measured battery step response. Subsys-

tems such as the internal compressor did not run prior to the start of the test. At approx-

imately 10:20 in Figure 16 the compressor cuts off. The load therefore had to be adjusted

to maintain a total battery power output of 81 kW at constant speed. As the voltage de-

creases towards the end of the test the battery current increases to keep a constant power

output.


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charge.

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Vb_front[V] I_total[A] Power[kW]


of 41

Temperatures increased during the test and since there was no break after 100 km it was

therefore necessary to lower the ambient temperature.


Figure 18 shows the distribution of load to each battery throughout the discharge test.

Figure 18 Overview of distribution of load current between batteries.

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of 41

Figure 19 Distribution of energy and capacity in the over-night battery charge.







of 41

4.4.3. Bus #2 - comparison

Overview from the first and second test of Bus #2, see Table 8.

Bus #2

First test Second test

Discharge test date 17th of June 2014 17th of December 2015

Odometer at test start km 9233 53799

Measured discharge energy kWh 306.8 295.5

Measured charge energy kWh 328.0 303.3

Number of active charge connectors # 2 1

Charged energy from grid kWh 344.3 316.8

Discharge test Driven distance km 192 186

Table 8. Comparison overall.

The second test was performed 18 months after the first test. In that period the bus has

driven a little more than 44,500 km.

The first and second test has been conducted under similar conditions since the energy

consumed per km. on the chassis dynomometer match within 1%.

The batteries were charged with two charge cables active (rated by BYD: 2x30kW) in the

first test and with one charge cable active (rated by BYD: 1x30kW) in the second test. A

higher charge power will result in greater internal loss mainly due to the higher losses in

the batteries internal resistance. A higher charge power will force the battery chemistry to

react faster, which will imply a greater loss.

The consequence of the different charge setups are illustrated in Table 9. The battery round

trip efficiencies are calculated. The energy drain from the grid was more than 4% higher

when using two charge connectors compared to using one charge connector.

Active charge connectors

Comparison

1 2 2 plugs versus 1 plug

Measured discharge energy kWh 295.5 306.8

Charged energy from grid kWh 316.8 344.3

Round trip efficiency - 93.3% 89.1% Relative +4.5%

Table 9. Comparison of different charge setups.

The single measurement with a single charge connector indicates a potentially higher effi-

ciency when charging at lower power. A closer study seems relevant to assess an actual

saving potential when effects of temperature, tolerances and other aspects are taken into

account.


of 41

Adapted internal resistance

Figure 20 shows an example of the step response measurement. Directly from the dis-

charge test, the vehicle was shifted into neutral (to avoid regeneration), rolled off until 0

km/h and waiting for 10 seconds giving the batteries time to stabilize zero load voltage.

This method was used to make comparable results, in respect to the fact that it takes much

longer time to get fully stabile open circuit voltage. From here the vehicle was accelerated

with the throttle fully applied until discharge test speed and the constant power discharge

was continued, as indicated with yellow circles in Figure 20.

Figure 20. Example on load step. The battery step response reflects the resistance inter-nally in the battery. Indication with yellow circles which values were used for calculation the internal resistance – Left axis represents Voltage and right axis represents both cur-

rent and power.


of 41

Load steps were performed for every approximately 12.5% discharge in energy to deter-

mine the battery internal DC resistance, the results are shown in Figure 21. This value

varies with state of charge and therefore it needs to be recalculated along with the battery

discharge level.

Taking the gradient (dU/dI) between battery voltage and total current before and after

load step, the adapted internal resistance was calculated. Please see Figure 21.

Figure 21. Battery internal resistance compared to discharged energy.

Battery internal resistance is a very complex function and the actual measurements are

only a representation used for comparison under similar conditions – it cannot be used

directly to characterize the battery or to assess any specific change in the battery State

of Heath (SoH). The results from the adapted internal resistance test did not visualize

any significant change from first to second test. This indicates that the load step re-

sponse of the battery has not deteriorated from the first to the second test.


of 41

Battery voltage compared to discharged energy

The graph in Figure 22 shows the battery voltage progress versus discharged energy during

constant power discharge. The end value showed the energy content of the total battery

at the time of the measurement. The two tests performed under similar conditions on the

Bus #2 indicated a 3.7% degradation of discharged capacity. There is no clear explanation

for a higher cut-of voltage at second test. It may be caused by a single battery cell reaching

minimum level slightly earlier due to a marginal unbalance or difference in cell capacity.

The “constant voltage”-plateau was lower under the second test. The lower voltage plateau

indicates a slight increase in battery impedance. The combined effect indicates a degra-

dation in energy content of the batteries comparing between first and second test.

Figure 22. Battery voltage compared to discharged energy.

BYD expect the impedance to increase around 10% during the first few hundred cycles but

then the impedance stabilizes at a constant level until end of useful life. This can explain

the lower voltage during discharge at second test. The change in impedance is considered

less critical since it will stabilize at a nearly constant level.

The critical degradation is the capacity fade that will continue until end of life. The capacity

fade is expected to continue at a slightly lower rate after the first years.

The battery degradation is clearly measurable but not excessive compared to the expected

levels suggested from battery data. The measured degradation consists of different mech-

anisms:

1. Capacity degradation – less available energy from the battery. From the Voltage

versus Ah curve it can be seen that further capacity fade will result in shorter

range. Capacity degradation is primarily the combined effects of

a. Calendar aging -

Lithium Ion batteries are known to degrade throughout life independent

of use. The degradation rate depends on temperature and increase expo-

nentially to increased temperature.


of 41

b. Cycle aging -

Degradation resulting from the charge cycled through the battery.

i. Cycle degradation can increase with high C-rates, but the actual

electric bus has a large battery resulting in relatively low C-rates

both driving and charging. So adverse effects of high C-rate is

marginal.

ii. The cycle degradation is also influenced by the Depth of Discharge

(DoD) – the actual used range of the available range. It is nor-

mally recommended to keep DoD under 80% for long life applica-

tion and under 70% DoD should ensure a high number of available

cycles.

iii. There are no indication of a “State of Charge-window”. EV- manu-

factures are known to mask the real remaining battery capacity by

defining the battery empty when the nominal charge has been de-

livered rather than using minimum voltage as empty criteria. This

seems not to be the case in the bus.

iv. In the first test operation of the buses little more than half the

battery capacity have been used. As the battery capacity fade a

larger part of the available capacity will be used and this tend to

increase the degradation rate.

c. After an expected initial capacity fade of approximately 10% - 12% over

the first couple of years, the rate of the capacity degradation is expected

stabilize at a lower level around or even under 2% per year throughout the

remaining useful battery life, under stable load conditions and no exposure

to high temperatures for extended periods.

2. Increased DC impedance resulting in a slightly lower voltage during discharge.

a. The effect is negligible and only measurable consequence may be on effi-

ciency until end of useful battery life.

b. The rate of this degradation is expected to decrease to a stable level after

two to three years.


of 41

5. Basis for lifetime assessment

Rather than making a full degradation test in the laboratory, a more simple approach was

agreed between Danish Traffic Agency and Danish Technological Institute (DTI). Since BYD

kindly made some of their confidential test data available for DTI (only), these data was

applied as the basis data in the DTI lifetime model.

To validate consistence between DTI results and BYD data, four cells were setup in the

laboratory for a reduced lifetime test using same test method as BYD (DTI normal test

practice is similar to BYD). Battery cells of same type as used in the bus was requested for

the test, but actual cell capacity in the laboratory were measured to 230 Ah cells still

claimed to be of the same chemistry and packaging as the cells applied in the bus. The

results measured by DTI matched the BYD data quite well as they yielded a very similar

capacity loss (close to 1% larger than the BYD data).

Based on a visit to BYD factory and BYD test facilities, meetings with BYD research people

and consistent test results DTI has no qualified reason to question BYD test data provided

under a strict NDA. The dataset from BYD is made at room temperature and temperatures

above. BYD had no relevant cold degradation data available for the specific chemistry at

that time but planned to start tests. No data from cold tests has been received at the

conclusion of this task.

Hence, the lifetime calculations are made on the basis of extrapolated data at temperatures

below room temperature.

In addition, the production, test and lifetime test facilitates at the BYD battery factory in

Shenzhen, China was assessed by DTI. The facilities appeared professional with a high

degree of automatic processing and quality assurance. The lifetime test facilities were well

equipped and capable of performing the lifetime tests – most equipment from well reputed

international high class equipment manufactories.

In the bus the total current drawn from the battery is not necessarily equally drawn from

each package (Top, front and rear) – it depends on the individual battery packs SoC (State

of Charge) and SoH (State of Health) and any possible bypass operation. Under one test

one of three packages was switched off by the bus battery management system for some

time while operating only on two packages. The load current is still relative low even with

a battery package switched off.

The total current drawn from the battery was measured during a real drive cycle and max-

imum was less than 300 A which implies a current draw less than 0.5C (1C = 600 A drawn

in one hour = total battery capacity). At a relatively low peak load like this, the C-rate has

only a negligible impact on the battery degradation, which is also confirmed by the BYD

lifetime data. Current draw example see Figure 23.


of 41

Figure 23. Current draw during driving.

Public temperature data for Denmark provided by DMI (Danish Metrological Institute) for

the period 2001 to 2010 (http://www.dmi.dk/vejr/arkiver/normaler-og-ekstremer/klima-

normaler-dk/vejrnormal/) has been used as climate reference.

http://www.dmi.dk/vejr/arkiver/normaler-og-ekstremer/klimanormaler-dk/vejrnormal/

http://www.dmi.dk/vejr/arkiver/normaler-og-ekstremer/klimanormaler-dk/vejrnormal/


of 41

6. Lifetime assessment

For a very rough lifetime assessment, some assumptions about the use pattern were made

initially.

The bus is used for 15 hours every day driving 200 km. Afterwards the batteries are

charged for 4.5 hours, and then resting for 5.5 hours. Charging take place when the bus

return to the depot and the batteries still have residual heat from driving. The resting is

done at evening and night times when the temperature is low.

Feeding the BYD data, the temperature data and the conditions above into the DTI lifetime

model yields a lifetime prediction as shown in Table 10.

Front package Top package Rear package Battery package

7 years 6 Years 7 years 6½ years Table 10. Estimated lifetime to 80% retained capacity – the uncertainty is substantial

due to limited data.

The number of years express the time when the total battery capacity is reduced to 80%

of the initial capacity if put into use from day one. The bus will still be able to drive on after

this point, but the range will decrease as the capacity decays further. The estimated ca-

pacity loss for the whole battery package is plotted in Figure 24, where it can be seen that

the degradation rate is larger in the first year compared to the following years.

Figure 24. Estimated battery package capacity loss versus lifetime – the uncertainty is

substantial due to limited data.


of 41

Four uncertainty factors should be mentioned here:

The empirical data input to the lifetime model is extrapolated from the accessible

dataset and supported by other DTI experiences.

The temperature profile applied in the lifetime model is an average profile. If the

battery packs are exposed to a more moderate temperature profile, the lifetime will

be longer than estimated and if the battery packs are exposed to a more extreme

temperature profile, the lifetime will be shorter.

The temperature distribution inside each package was not measured by DTI and

some battery cells may be significantly warmer than the average temperature

measured. If so, the lifetime would be shorter than estimated.

The BYD dataset is based on full charge & discharge lifetime tests. Normally a dy-

namic load pattern, as the one the battery packages in the bus is exposed to, will

degrade the battery less severe than the full charge & discharge test. Therefore,

the actual lifetime is expected longer than the lifetime estimated.

Charging under cold conditions has not been considered but there might be a risk of in-

creased degradation or significant longer charging time. E.g. if Smart Grid control is con-

sidered for postponed charging of the bus it might be wise to assess risk of cold degradation

or need for preheating of batteries before charge.


of 41

7. Conclusion

By request of Trafik- og Byggestyrelsen (The Danish Traffic Agency), the expected battery

lifetime of two BYD K9 electric buses has been assessed by DTI. The two buses is leased

from BYD by MOVIA and operated by Keolis Danmark A/S (former: Citytrafik) and Arriva

Danmark A/S respectively. The buses have been tested to assess whether the expected

battery lifetime stated by BYD seems realistic.

BYD statement on lifetime to MOVIA:

Warranty 8 years or 500.000 km

Degradation is expected to be higher the first year but decreasing to a stable low

level after a couple of more years

Two different approaches have been used to estimate expected lifetime of the batteries:

1. Measurement at vehicle level by DTI - Battery capacity, early (first) and late (sec-

ond) measurements.

2. Verification of battery technology and BYD data including stressed cycle tests at

cell level by DTI.

Battery capacity were measured on the buses shortly after the vehicles were put into op-

eration. Comparison tests were planned at the end project period for both buses. One bus

received a new battery pack halfway into the test period due to a failure of the housing.

Only the Arriva Danmark A/S bus was tested second time at the very end of the test period,

just before handing it back to BYD. First tests were performed during Maj and June 2014.

The second test was performed 18 months later in December 2015. In that period the bus

has driven a little more than 44,500 km. DTI focus has been on battery condition only and

no other criteria like e.g. range, efficiency etc. has been addressed.

BYD has provided limited test data to DTI under a strict NDA. DTI has found no qualified

reason to question the BYD data provided after the following activities:

a visit to BYD battery factory with extensive automated production and high level

of integrated quality assurance

a visit to BYD test facilities were extensive testing takes place to all major global

norms and standards using high quality test equipment.

meetings with BYD research people presenting their data and taking questions.

a few selected accelerated cell tests at DTI showing results consistent with BYD’s

data.

Lifetime assessment from analysis of BYD data and cell testing suggest an expected lifetime

of nearly 7 years to 80% remaining capacity in Danish temperature conditions.

The critical degradation is the capacity fade that will continue until end of life. The capacity

fade is expected to continue at a slightly lower rate after the first years.


of 41

The battery degradation is clearly measurable but not excessive compared to the expected

levels suggested from battery data. The measured degradation consists of different mech-

anisms:

1. Capacity degradation – less available energy from the battery – will result in shorter

range. Capacity degradation is primarily the combined effects of

a. Calendar aging -

Lithium Ion batteries are known to degrade throughout life independent

of use. The degradation rate depends on temperature and increase expo-

nentially to increased temperature.

b. Cycle aging -

Degradation resulting from the charge cycled through the battery.

i. Cycle degradation can increase with high C-rates, but the actual

electric bus has a large battery resulting in relatively low C-rates

both driving and charging. Adverse effects of high C-rates is mar-

ginal.

ii. The cycle degradation is also influenced by the Depth of Discharge

(DoD) – the actual used range of the available range. It is nor-

mally recommended to keep DoD under 80% for long life applica-

tion and under 70% DoD should ensure a high number of available

cycles.

iii. Discharge seems to be stopped on a minimum voltage criteria

meaning that the full capacity seems to available for driving – ex-

cept for a marginal emergency charge. There are no indication of

a “State of Charge-window”.

iv. In the first test operation of the buses little more than half the

battery capacity have been used. As the battery capacity fade a

larger part of the available capacity will be used and this tend to

increase the degradation rate as the DoD increase.

a. After an expected initial capacity fade of approximately 10% - 12% over the

first couple of years, the rate of the capacity degradation is expected stabi-

lize at a lower level around or even under 2% per year throughout the re-

maining useful battery life, under stable load conditions and no exposure to

high temperatures for extended periods.

2. Increased DC impedance resulting in a slightly lower voltage during discharge.

c. The effect is negligible and only measurable consequence may be on effi-

ciency until end of useful battery life.

d. The rate of this degradation is expected to decrease to a stable level after

two to three years.

The resulting total degradation is expected to continue at a slightly lower but stable level

until end of useful life. The degradation after three years is expected to be on available

capacity only.

Even though predictions on such a limited information carries a substantial uncertainty,

operation of the buses in a similar load pattern down to e.g. 65% remaining capacity (with

a slight degradation of available range) seems realistic, since the degradation rate seems

fairly stable with no rapid trend towards end of life. The degradation rate must be expected

to increase if a larger share of the available remaining capacity is used.


of 41

Essential factors increasing uncertainty of life assessment:

The empirical data input to the lifetime model is extrapolated from a very limited accessible

dataset but supported by other DTI experience.

Actual temperature for each of the three battery packs in each bus affects life and perfor-

mance significantly. The temperature profile applied in the lifetime model is only a Danish

average profile. Battery packs exposed to more extreme temperatures may experience

shorter lifetime. Different locations of the three battery packs on the bus makes it very

complex to assess temperature dependence – one pack on the roof are cooled well by wind

speed but heated by sun; one pack under floor are exposed to low temperatures but heated

by driveline and heat from road with poor cooling conditions. Third pack is inside the bus

will see a stable room temperature but has poor cooling conditions.

The temperature distribution inside each battery package could not be measured by DTI

and some battery cells may be significantly warmer than the average temperature meas-

ured. If so, the lifetime would be shorter than estimated.

The BYD dataset is based on full charge & discharge lifetime tests. Normally a dynamic

load pattern, as the one the battery packages in the bus is exposed to, will degrade the

battery less severe than the full charge & discharge test. Therefore, the actual lifetime is

expected longer than the lifetime estimated.

Data logging in the buses has not been as useful as expected due to lack of relevant battery

related data, and driveline data like e.g. power, speed and distance. Even attempts on

analysing available sample data has been overly labour intensive. Still detailed data logging

will be needed to compute better lifetime estimation on specific driving patterns.

Charging under cold conditions has not been considered but there might be a risk of in-

creased degradation or significant longer charging time. E.g. if Smart Grid control is con-

sidered for postponed charging of the bus it might be wise to assess risk of cold degradation

or need for preheating of batteries before charge.

The charge efficiency is generally good. During the second vehicle test, only one of two

charge plugs was active. The round trip efficiency of charging may be slightly affected by

doubling the charge power. Further measurements will be needed to decide statistical var-

iation and assess potential energy saving by lower charge power.


of 41

Appendix 1. Additional bus data from Bus #1

Power curve

Dynamo data

Figure 2 Measurement of drive train loss. Power vs. speed.

Figure 1 Propulsion power (upper part) and regenerative power (lower part).


of 41

Appendix 2. Additional bus data from Bus #2

Power curve

Dynamo data

Figure 4 Measurement of drive train loss. Power vs. velocity.

Figure 3 Propulsion power (upper part) and regenerative power (lower part).


of 41

Appendix 3. Data from road test in Aarhus

Figure 25. Road test in Aarhus, part 1 of 4.


of 41


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