hund ultra battery utility testing

The 18th International Seminar on Double Layer Capacitors and Hybrid Energy Storage Devices12/08

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Ultrabattery Test Results for Utility Cycling Applications

Tom Hund, Nancy Clark, and Wes Baca Power Source Component Development Department

Sandia National Laboratories* Albuquerque, NM 87185-0614

Abstract. The Ultrabattery and an absorbed glass matt (AGM) valve regulated lead-acid (VRLA) battery have been tested using a high-rate-partial-state-of-charge (HRPSoC) cycle profile designed to simulate the ancillary regulation services of a utility and a wind farm energy smoothing application. The Ultrabattery is a hybrid energy storage device which combines an asymmetric electrochemical supercapacitor in parallel with a lead-acid battery designed to improve cycling and power performance. In both ancillary services and wind farm energy smoothing the battery is required to source and sink energy for only a few minutes per cycle. The HRPSoC cycle test uses a 10% discharge cycle at the 1C1 to 4C1 (C1 = 1 hr Ah capacity) discharge and charge rates at near 50% state of charge (SOC) to provide the maximum power and energy performance. Cycle-life performance was measured by capacity at the 1C1 rate after a full charge on the energy storage device. The capacity measurement was triggered when the high voltage limit was reached or when the number of HRPSoC cycles reached the predetermined limit of 100 or 1,000 cycles. In addition to the cycle testing the Ultrabattery was also characterized using capacity, ohmic resistance, float current, and end of charge current measurements to indicate ageing effects from cycling. The test results show that the Ultrabattery cycled in excess of 15,000 HRPSoC cycles with less than 20% capacity loss and was able to cycle at the 4C1 rate. The VRLA battery using this test procedure could only cycle at the 1C1 rate, required a recovery charge at about 100 HRPSoC cycles, and at 1,100 HRPSoC cycles lost more than 20% of its capacity. In summary, the Ultrabattery was capable of about 13 times more HRPSoC cycles and more than 10 times the number of cycles between recovery charges compared to the VRLA battery.

Introduction

This work was conducted as part of the Energy Storage Systems Program of the U.S. Department Of Energy (DOE/ESS) through Sandia National Laboratories (SNL). The DOE Energy Storage Program provides support to develop and evaluate integrated energy storage systems involving batteries, superconducting magnetic energy storage (SMES), flywheels, super capacitors and other advanced energy storage devices. In addition to energy storage devices, the DOE program supports improvements in multi-use power electronics, controls, and communications components. The Ultrabattery, as manufactured by Furukawa Battery in Japan and designed by Dr. Lam at the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) as seen in Figure 1, is a lead-acid hybrid energy storage device with a valve regulated lead-acid (VRLA) battery and a parallel connected asymmetric electrochemical supercapacitor in the same cell without supporting

*Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. This work was funded by the DOE Energy Storage Program.

http://www.ecoult.com/technology/ultrabattery/


http://www.ecoult.com/applications/microgrids/

http://www.ecoult.com/case-studies/hampton-wind-farm-australia-wind-smoothing/

http://www.ecoult.com/



http://www.ecoult.com/technology/ultrabattery-energy-resource-uber/


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electronics. The asymmetric supercap supports the load of the lead-acid battery to minimize heating and negative plate sulfation at high-rate-partial-state-of-charge (HRPSoC) cycling. One of the challenges in achieving a successful asymmetric configuration is to properly match the operational voltages of both the battery and negative capacitor electrode. If the battery and capacitor voltages are matched properly, then it is possible to achieve low hydrogen gassing rates, higher capacity, and longer cycle-life at a relatively low cost. The second challenge that must be over come is the minimization of the negative plate sulfation that occurs in VRLA batteries during HRPSoC cycling. This is achieved by the addition of both the supercap in parallel with the negative plate and the addition of carbon in the negative active material of the battery [1,2,3]. Based on the manufacturers 2006 cost projections, the Ultrabattery should cost in the range of $220 for 1 kWh of energy storage [1]. Hybrid electric vehicle (HEV) testing as of January 2008 in a Honda Insight at the Millbrook Proving Ground near London has completed over 100,000 miles of road tests [4]. In addition, laboratory HEV cycle tests have demonstrated comparable HRPSoC cycle performance to NiMH HEV batteries [2]. The Ultrabattery is now expanding from the small HEV battery packs to a large format 1,000 Ah cell [5,6]. East Penn Manufacturing in the United States has obtained manufacturing rights for the large format stationary cells. The large cells would allow entry into the mega-watt scale markets including utility, solar, and wind applications. At present, the large format cells have only been manufactured by Furukawa Battery and should be available for testing very soon. There are a number of high value utility ancillary service applications for energy storage that range from regulation services to peak shaving/load leveling to power quality and improvements in reliability to deferments of new or upgraded T&D infrastructure [7]. The regulation service requires energy to source and sink in time intervals of less than five minutes and on an hourly basis. With the proven performance in HEV tests, the Ultrabattery could economically support the regulation services application. This market can potentially generate a considerable revenue stream [8]. In Figure 2 is a plot of Pennsylvania-Jersey-Maryland Interconnection’s (PJM) regulation power requirements over a two-day period. In addition to ancillary services, wind farm energy smoothing is also an application that may grow dramatically to assist utilities in managing the irregular nature of wind energy over short time periods. This wind application would be similar to the regulation services in time and in the nature of the cyclic energy requirements.

Figure 1: UltraBattery at Sandia National Labs


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The Ultrabattery is uniquely suited to the above applications because it was designed for HRPSoC cycling. The VRLA battery failure modes of water loss, negative plate sulfation, and grid corrosion are minimized in the Ultrabattery. When using the Ultrabattery in a HRPSoC application, cycle-life is improved because very little time is spent on charge at the gassing voltage, carbon additions to the negative active material minimize the negative plate hard sulfation, and the parallel supercap enhances power performance while minimizing operating temperature. Therefore, the Ultrabattery can cycle well beyond conventional VRLA batteries and thus be economical for these new applications.

Test Procedure Five tests were developed to characterize the Ultrabattery for the utility HRPSoC pulsed cycling environment. The five characterization tests are itemized below:

1) Capacity Test – Establishes an initial and final capacity. 2) DC Ohmic Resistance – Establishes an initial and final

resistance. 3) Float Current – Establishes a float current.

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Figure 2: PJM Ancillary Services Power Requirements – Prepared by

C. Koontz, WPS.


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4) Power Density and Specific Energy Density – Measures the power and energy density.

5) Utility HRPSoC Cycle Test – Measures the ability of the cell to HRPSoC cycle for utility and wind farm energy smoothing applications.

The Ultrabattery capacity was measured at the 1C1 (7.0 A, C1 = 1 hr Ah capacity) rate using a charge voltage of 14.7 V. On reaching the charge voltage, the current was allowed to taper while maintaining the charge voltage until the change in current was less than 0.1 amp for at least 45 minutes. The taper charge time was usually about 1 hr and 45 minutes at the beginning of a test sequence. The DC resistance was measured with a high current discharge pulse of at least 2C1 at 100% SOC. An oscilloscope was used to measure the voltage drop and related current ramp. Ohmic resistance was measured by the slope of the voltage drop as a function of the increasing current. The float current measurement was on a fully charged battery after 24 hr on float at 13.62 V (2.27 vpc). Power density and specific energy were measured with capacity measurements near the following rates using an end voltage of 10.02 V for 1C1 rates and higher and 10.5 V for rates below 1C1:

1 0.1C1, 10 hr 2 0.2C1, 5 hr 3 1C1, 1 hr 4 2C1, 0.5 hr 5 4C1, 0.25 hr 6 10C1, 0.1 hr

Using the capacity data above, the power and energy density were calculated. The utility HRPSoC pulsed cycle test in Figure 3 uses charge and discharge pulses between 6 and 1.5 minutes in length at discharge rates between 1C1 (7A) and 4C1

(28A). The cycle profile in this test is illustrated in Figure 3 and consists of the following steps:

1 Charge battery at 1C1 rate until voltage reaches the charge voltage limit (14.7 V).

2 Keep voltage at charge voltage until the change in current is less than 0.1 A for 45 minutes.

3 Rest for 30 min. 4 Discharge at 1C1 rate to end voltage (10.5 V).


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5 Rest for 30 min. 6 Recharge battery as in step 2. 7 Discharge at 1C1 rate to 50% of the Ah capacity. 8 Rest for 5 min. 9 Discharge at 1C1 to 4C1 rate for 6 to 1.5 min. (10% Discharge). 10 Rest for 5 min. 11 Charge at 1C1 to 4C1 rate for 6 to 1.5 min. (10% Charge) 12 Rest for 5 min. 13 Repeat steps 9 through 12 for 100 or 1,000 cycles or until the low or

high voltage limit is reached (10.5 or 14.7 V). 14 Measure available capacity as specified in steps 3 through 5. 15 Repeat cycle profile to end of life (~80% of initial capacity). 16 Evaluate battery performance and determine if higher power levels

are possible and if the HRPSoC cycle interval can be extended to 1,000 cycles.

17 Additional testing may be conducted at the 2C1, 3 min. or 4C1, 1.5 min. rates and times.

Test Results

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Figure 3: Ultrabattery Utility HRPSoC Cycle Test


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In Figure 4 and 5 are the voltage and capacity (Ah) data for the recovery charge and discharge on the Ultrabattery and absorbed glass matt (AGM) VRLA battery. The Ultrabattery data is from before the first HRPSoC cycle, after 500 cycles, and after 16,740 HRPSoC utility cycles. At an initial capacity of 7.8 Ah, this battery exceeds the manufacturer’s specified capacity of 6.67 Ah. After 500 HRPSoC pulse utility cycles, the capacity increased to 8.1 Ah, an increase of 4%. The capacity after 16,740 cycles was 5.8 Ah, a loss of 26%. The most significant aspect of this data was the increase in voltage on charge and the increased time spent in taper charge from 1.75 to 3.5 hours at the 1 C1 charge rate. The taper charge step limit was 6 hours. The increased voltage and time on taper charge is characteristic of sulfation resulting from HRPSoC cycling. Other than the loss of capacity and apparent sulfation, the discharge curve remains relatively constant.

In Figure 5 are similar curves for the AGM VRLA battery showing an increase in voltage and time spent on taper charge. In this case, the taper charge time increased from 3.5 to 6 hr at a 0.25C1 charge rate. The 6 hr taper charge was the manufacturer’s time limit for the step. The extended time on taper charge, the capacity loss, and a low end of charge current (~0.10 A) are an indication of hard sulfation in the negative plate of the VRLA battery and other possible degradation mechanisms that can result in capacity loss. The negative plate sulfation is clearly shown in Figure 6 where the negative plate voltage is measured using a mercury reference electrode. The negative plate voltage increases in the negative direction

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Cycle # 0

Charge 7A

Discharge 7A

Figure 4: Ultrabattery Capacity Curve at 0, 500, and 16,740

HRPSoC Utility Cycles.


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during HRPSoC cycling and during the taper charge portion of the recovery charge.

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Figure 5: VRLA Battery Capacity Curve at 0, 100, 1,000, 2,000

and 2,500 Utility HRPSoC Cycles.

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Figure 6: VRLA Battery Reference Electrode Cell Voltage During

Utility HRPSoC Cycles.


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The reference electrode indicates that most if not all of the negative plate sulfation is recovered during the recovery charge by the return to normal negative plate voltage. Also, the reference electrode shows no significant change in the positive electrode voltage during HRPSoC cycling or during the recovery charge. It is clear from Figure 5 and 6 that the VRLA battery does experience negative plate sulfation from HRPSoC cycling, but capacity loss may be due to other mechanisms. An elemental scan of the negative plate would be necessary to identify residual negative plate sulfation after recovery. The Ultrabattery clearly demonstrates a much faster and more complete recovery charge after the HRPSoC cycling. (No reference electrode data is available for the Ultrabattery because of the non-disclosure agreement restrictions required before testing.) The Ultrabattery manufacturer’s specifications are shown in Table 1, as well as the measured float current, measured DC ohmic resistance, initial and final 1C1 capacity, and initial and final battery weight. As seen, there was a 5 mohm increase in ohmic resistance from 20 to 25 mohm, a 26% loss in capacity from 7.8 to 5.8 Ah, and a 17 gm weight loss from 3.787 to 3.770 kg after 16,740 HRPSoC cycles. Table 1: Ultrabattery Specifications.

Specifications Ultrabattery #FTZ12-HEV

Measured After 16,740 HRPSoC Cycles

Operating voltage window (V) 14.7 to 10.5 Max voltage (V) 16 Discharge end voltage at 1C rate (V) 10.02 Float Voltage (V) 13.62 Float Current, after 24 hr at 13.62V (ma) 9.3 Charge Regulation Voltage (V) 14.7 Full Charge Termination (V, I, Time) 14.7, I ≤ 0.1A, 24 h or

c/d = 115%

Maximum Charge Current (A) 20 Maximum Pulse Current (A) 80 Maximum Constant Current (A) 20 DC Ohmic resistance (mohm) 25 0С

20 25

Capacity, 1C rate to 10.5V (Ah) 7.8 5.8 Energy stored in operating voltage window, (Wh)

30 Wh/kg 77 Wh/L

Overall dimensions, (mm) 110 (H) x 87 (W) x 150 (L)

Weight, (kg) 3.787 3.770 Operating temperature, (0С) 30> x <60 Chr

30> x <60 Dch

Storage temperature, (0С) 25 Cycle-Life, (cycles) 250 000 – 300 000

cycles under EUCAR profile test

16,740 HRPSoC Utility Cycles


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The power density and specific energy measurements shown in Figure 7 and 8 are similar to the conventional VRLA battery except for an increase in specific energy and power as seen in Figure 8. It is assumed that the added carbon for the asymmetric supercap and negative plate active material improves the specific energy and power.

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Power.

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Figure 7: Ultrabattery And VRLA Battery Energy And Power

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The utility HRPSoC pulsed cycle test results are shown in Figure 9 through 11. There are a number of cycling characteristics that are unique to the Ultrabattery. These include the accelerated rise in the end of charge voltage after the first 1,000 cycle sequence (Fig. 9 and 10), the recovery of end of charge voltage after an extended rest period (~24 hr), and the fluctuation in capacity measurements between cycle sequences (Fig. 9 and 11). The accelerated rise in end of charge voltage after the first cycle sequence was the most unusual characteristic because it would not occur if the Ultrabattery cycle sequence was terminated and the battery was allowed to rest, then restarted. If the battery was deep-cycled for an extra capacity measurement, then there was still no reduction in the rise of end of charge voltage. The test termination with a rest period was repeated many times with the same improvement in end of charge voltage. The fluctuation in capacity measurement, as seen in Figure 11, indicates that this effect predominately occurred after 7,500 HRPSoC cycles.

The temperature increase due to cycling was low for the Ultrabattery. At the 1C1 rate the temperature always stayed below 28C, at the 2C1 rate the temperature stayed below 29C1, and at the 4C1 rate the temperature stayed below 32C. The VRLA battery operated below 34C at the 1C1 rate and was not tested at higher rates because of increased temperature and voltage. All testing was conducted at an ambient temperature of 22 to 25C.

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Figure 9: Ultrabattery HRPSoC Utility Cycle Accelerated Rise In

End of Charge Voltage, At 2C1 Rate For 3 min.


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As the Ultrabattery approached end of testing, Figure 10 shows how the end of charge voltage increased during HRPSoC cycling. This effect reduced the power capability of the battery and could shortened the cycling interval. Based on the shape of the rise in end of charge voltage in Figure 10, the effect looks like the result of aging rather than just the result of lower battery capacity, which should just increase the end of charge voltage without changing the shape of the curve.

In Figure 11 are the HRPSoC cycling capacity measurements for the VRLA battery and the Ultrabattery. The results for the VRLA battery show a steep constant decrease in capacity except for a capacity recovery charge that was implemented with a constant current (2 A) charge at 400 cycles into the test. The recovery charge resulted in a capacity increase from 77% (24.8 Ah) to 100% (31 Ah), which is the manufacturer’s specification at the 1C1 rate. This capacity recovery may be the result of hard sulfation that develops on the negative plate and other capacity loss mechanisms, such as passivation layers at the grid paste interface. Traditional constant voltage charging at 14.7 V (2.45 vpc) could not recover the capacity. An elemental scan of the negative plate would be required to verify if negative plate sulfation was the cause of the capacity loss. Continued cycling after the recovery charge results in a repeat of the downward capacity trend. The VRLA battery drops to 80% of its recovered capacity in only 1,100 cycles. The Ultrabattery HRPSoC cycle performance is substantially better and actually increases in capacity from 100 to 2,500 cycles. From 2,500 cycles to 7,500 cycles the capacity

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Figure 10: Ultrabattery HRPSoC Utility Cycle Aging Effect

Between 2,500 and 16,740 Cycles, At 1C1 Rate For 6 min.


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slowly drops off to 85% of its initial value. After 7,500 cycles, the capacity tends to become more erratic and jumps up to 99% at 8,500 cycles. The erratic capacity behavior continues until the end of the test at 16,740 cycles and 74% of the battery’s initial capacity. After completion of the HRPSoC cycling, a constant current (0.3 A) recovery charge was conducted on the Ultrabattery to help determine if capacity loss was permanent. The capacity could only be partially recovered to 6.5 Ah, or 83% of the initial capacity. This is an indication that the capacity loss was due at least in part to permanent capacity loss mechanisms. Also, the constant current recovery charge resulted in an increase in end of charge current from 0.10 to over 0.5 amps at 14.7 V. The VRLA battery was always tested at the 1C1 rate (30A) while the Ultrabattery was initially tested at the 1C1 (7A) rate to 2,500 Cycles, at the 4C1 (28A) rate to 7,500 cycles, at the 2C1 (14A) rate to 14,740 cycles, and finally at the 1C1 rate to 16,740 cycles. The Ultrabattery cycled well at the 4C1 rate at a temperature below 32C, but at this rate the voltage limit (14.7 V) would be reached in about 100 cycles into the second HRPSoC cycle sequence. The first cycle sequence would always complete 1,000 cycles at the 4C1 rate.

Conclusions

The Ultrabattery has demonstrated its ability to cycle well in a utility HRPSoC cycling environment compared to AGM VRLA batteries. Using the utility

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Figure 11: Ultrabattery And VRLA Battery 1C1 Capacity After

HRPSoC Cycling.


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HRPSoC cycling profile at the 1C1 to 4C1 rate, the Ultrabattery cycle performance was about thirteen times greater (>15,000 cycles) than the AGM VRLA battery (1,100 cycles). The Ultrabattery was also able to cycle at a HRPSoC for more than ten times the number of cycles as compared to the AGM VRLA battery (1,000 vs. 100). In addition to the cycling performance, the Ultrabattery cycling temperature at 28, 29, and 32C at 1C1, 2C1, and 4C1 rate was much lower than the AGM VRLA battery at 32C at the 1C1 rate. The HRPSoC cycling also identified an aging effect and an accelerated end of charge voltage rise. The aging effect was seen as the increase and change in shape of the end of charge voltage between the initial and final cycles. Because the shape of the voltage rise curve changed, it is unlikely that this effect was just the result of lower capacity as the testing proceeded. The accelerated end of charge voltage increase occurred after the first cycle sequence. Into the second cycle sequence the end of charge voltage would quickly rise to the limit voltage (2C1 and 4C1) and trigger a capacity measurement. If after the first cycle sequence, the battery was allowed to rest for an extended period of time, then the end of charge voltage would be in line with the first cycle sequence and the battery could cycle for a full 1,000 HRPSoC cycles. Both of these effects could require modifications or limits to the use of the Ultrabattery in utility regulation service or wind farm energy smoothing applications.

References 1. L.T. Lam, R. Louey “Development of ultra-battery for hybrid-electric vehicle applications,”

Journal of Power Sources 158 (2006), pp. 1140-1148. 2. L.T. Lam, et al., “VRLA Ultrabattery for high-rate partial-state-of-charge operation,” Journal

of Power Sources 174 (2007), pp.16-29. 3. The Furukawa Battery Co., Ultrabattery,

http://www.furukawadenchi.co.jp/english/rd/nt_ultra.htm. 4. Advanced Lead-Acid Battery Consortium, “Lead-Acid Breakthrough Could Propel Hybrid

Electric Vehicle Market,” News Release, http://www.csiro.au/news/UltraBattery.html, http://www.batterypoweronline.com/images/BPPTMA08.pdf 1/15/2008 .

5. Overview of Technology Innovation in Australia, http://www.rega.com.au/Documents/2007%20Forum/Presentations/John_wright.pdf, 2007. .

6. CSIRO Invests in Hybrid Energy Storage System Start-Up, http://www.greencarcongress.com/2007/11/csiro-invests-i.html 10/26/07.

7. D.K. Nichols, Steve Eckroad, “Utility-Scale Application of Sodium Sulfur Battery,” Battcon, 2003, http://www.battcon.com/PapersFinal2003/NicholsPaperFINAL2003.pdf

8. Charles Koontz, “Value of Ancillary Services,” Presentation, Intergrys Energy Services, (614) 844-4324, 12/12/2005.

http://www.furukawadenchi.co.jp/english/rd/nt_ultra.htm�

http://www.csiro.au/news/UltraBattery.html�

http://www.rega.com.au/Documents/2007 Forum/Presentations/John_wright.pdf�

http://www.greencarcongress.com/2007/11/csiro-invests-i.html 10/26/07�

http://www.battcon.com/PapersFinal2003/NicholsPaperFINAL2003.pdf�