Lightweight lead acid battery with high power

SAE TECHNICAL PAPER SERIES 2004-01-3208 Lightweight Lead Acid Battery with High Power Ramesh Bhardwaj, Chhaya Bhardwaj and John Timmons Concorde Battery Corporation Sue Waggoner NSWC Bill Johnson Naval Air System Command Power Systems Conference Reno, Nevada November 2-4, 2004 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE For permission and licensing requests contact: SAE Permissions 400 Commonwealth Drive Warrendale, PA 15096-0001-USA Email: permissions@sae.org Fax: 724-772-4891 Tel: 724-772-4028 For multiple print copies contact: SAE Customer Service Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-1615 Email: CustomerService@sae.org ISSN 0148-7191 Copyright © 2004 SAE International Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE The author is solely responsible for the content of the paper A process is available by which discussions will be printed with the paper if it is published in SAE Transactions Persons wishing to submit papers to be considered for presentation or publication by SAE should send the manuscript or a 300 word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE Printed in USA 2004-01-3208 Lightweight Lead Acid Battery with High Power Ramesh Bhardwaj, Chhaya Bhardwaj and John Timmons Concorde Battery Corporation Sue Waggoner NSWC Bill Johnson Naval Air System Command Copyright © 2004 SAE International 1.0 Abstract: The aircraft industry demands high power batteries for auxiliary power unit (APU) and engine start The power demands for these applications are met lead acid battery, however, these batteries are bulky and heavy One of the important criteria for selecting an aircraft battery is its weight There is an acute need for a lead acid battery, which can supply the power and energy demands of the application but also be lightweight In this paper we present the use of lead coated Aluminum (Al) grids which result in a lightweight lead acid battery The battery with lead (Pb) coated Al grids are 15% lighter in weight when compared to conventional lead acid battery The results on capacity, high rate and cycle life of batteries made from lead coated Al grids is presented and discussed in detail in this paper 2.0 Introduction: The aircraft industry demands high power and high-energy batteries for various applications such as auxiliary power unit (APU) start, direct engine start, computer backup and emergency DC power for avionic or emergency lighting Conventional lead acid batteries are capable of providing the demanded performance at temperatures as cold as –40°C, but they are bulky and heavy The high power demands are met by increasing the opposed surface area of the electrodes by incorporating more plates per cell in a battery This increases the weight of the battery The cast or expanded lead (Pb) grids used as the current collector have manufacturing limitations in terms of thickness, weight, high resistivity and current carrying capability The idea of using low weight thin grids was initiated at Concorde Battery Corporation in the year 2000 when the possibility of using lead plated Aluminum (Al) grids was successfully implemented to make a lightweight lead acid battery [1-5] The30Ah/24V batteries with Pb plated Al grids have exceeded the high power capability and cycle life required of typical military aircraft batteries The batteries with Pb plated Al grids have shown weight savings of 10-15% when compared to the conventional lead acid battery This paper presents the use of Pb plated Al grids in a lead acid battery for aircraft application The results on capacity, cycle life and high rate testing are presented and discussed in detail The weight savings on 5Ah to 30Ah, 24 V batteries are presented and compared with conventional lead acid batteries 3.0 Experimental: Aluminum grids were punched from 0.025-0.038 cm thick Al 5052 alloy sheets They were cleaned, degreased, deoxidized and etched using Oakite and acid etch solutions Lead cannot be plated on aluminum alloy directly due to its poor adherence Concorde battery has developed a two-step proprietary activation process, which takes 30 seconds, where thin layer of metal covers the Al before Pb can be plated on the aluminum grid This activation layer allows good coverage on Al against corrosion and provides adherence of Pb on the Al grids Electroplating of lead was carried out using conventional lead plating bath Plating was carried out at different current densities of to 50 A/ft2 A smooth, pinhole free and uniform Pb plating was obtained between current density of 15A-30 A/ft2 into 30 Ah/24 V batteries Two types of batteries were assembled One group of batteries used lead plated Al grids in the negative electrode and the second groupcontained batteries with lead plated Al grids in the positive electrode The batteries were tested for their capacity to 20 V at discharge current of 30A Immediately after capacity test, batteries were conditioned as per military specification and tested for high rate at constant voltage of 14 V The current at 14 V was monitored every 0.1 Second for 60 seconds The high rate test was conducted at room temperature as well as at –26 C The life cycle tests were conducted at 30A discharge for hour and constant voltage charge at 28.8V for hrs Two types of plating solutions were used for lead plating on he surface of activated Al grids The first type of plating solution was lead fluoborate made in fluoboric acid, which provided excellent lead covering but was environmentally unfriendly We also performed lead plating to study the environmentally friendly second solution, which contained methane sulfonic acid The results indicated that the second solution was not only environmentally friendly but also provided more pinhole free, uniform and robust covering of lead on the surface of activated Aluminum grid Four 30A/24V batteries were assembled Two batteries contained lead plated Al grids in the negative electrode with the positive being the conventional lead grid electrode Two batteries were assembled using positive Al grids and negative being the conventional lead grid electrodes The assembly involved stacking positive plates with glass mat separator with negative plates on both sides of positive plate A special fixture for cast-on-connection was designed to connect all negative and positive plates to make a single cell A lead-tin alloy was used for the cast-on-connection to use the differential melting temperature between the plated lead and cast-on connection lead The melting point of the Pb-Sn alloy is 610°F while the melting point of the electroplated lead on Al is around 620° F This temperature difference allowed us to make the cast- The Pb plated Al grids were washed with water and pasted with a conventional positive and negative active material (PAM and NAM) The pasted plates were cured and assembled 4.0 Results and Discussion: 4.1 Battery assembly: on connection without melting the electroplated lead from the Al grid Twelve cells were connected in series to make 30A/24 V batteries All batteries were filled with sulfuric acid of correct concentration and formed at low current for extended period of time The extra acid is removed after conditioning cycles and battery is capped with a pressure relief valve vent valve Batteries after formation are provided with conditioning cycles for acid gravity adjustment 4.2 Battery testing: The batteries are tested for their capacity and compared with conventional grid batteries 4.2.1 Capacity Test: The capacity test is performed at 30A discharge until 18 V and recharged at 28.8 V for hrs We found that Aluminum grid battery and conventional lead grid batteries delivered exactly the same capacity of 40-42 Ah at tested rate and Aluminum grid batteries shows no improvement or degradation in the capacity performance 4.2.2 High Rate Test: The batteries are charged completely after the capacity test and tested for high rate at a constant 14V discharge for 60 seconds and current is monitored every 0.1 seconds The results of high rate test at room temperature are shown in Figure1 Note that both batteries met the military specification but Al grid batteries in negative electrode delivered slightly better current compared to conventional battery The better results can be attributed to higher conductivity of Al base material compared to the lead grid Both batteries with Al grids provided similar data indicating that Al grid batteries are equal or better than the conventional lead grid batteries The batteries were charged completely after room temperature high rate test and kept in the freezer for 24 hours at –26 C The batteries were taken out after 24 hrs and immediately tested for high rate performance The high rate performance of control battery and battery with Al negative grid is shown in Figure The batteries with Al negative electrode met the all-military specification requirements and were better than conventional battery especially after 45 seconds Conventional battery missed two specification points at –26 C while Al grid battery current was higher than specification at all recorded times 1200 Specification (RT) Al Grid at RT Pb Battery (B) (RT) Current (Amp) 1000 800 600 400 200 0 10 20 30 40 50 60 70 TIME (SECONDS) Figure High rate 14V test at room temperature 800 Specification (-26C) Al Grid (-26C) Pb Battery (A) (-26 C) Current (Amp) 700 600 500 400 300 200 100 0 10 20 30 40 Time (seconds) 50 60 70 Figure High Rate tests at -26 C 4.3 Life Cycle Test: The life cycle test was performed as per military specification The batteries were discharged at 30A for one hour followed by constant voltage charge at 28.8 V for hours The cycle life of first battery with Pb plated Al grids in negative electrode are shown in Figure 4.3.1 Battery with Al Negative Grid: The first battery was cycled till it completed 85 cycles The fully charged battery was removed from cycle tester and left at room temperature for voltage decay measurement for two months The voltage decay of the battery as a function of time is shown in Figure The battery shows loss of Volt in about 160 days, which is 0.62 mV per day The voltage decay curve indicates that there was no unusual loss of voltage in a battery with lead coated Al grids The battery was charged back after 160 days and tested for capacity The battery recovered all capacity and delivered 65 additional cycles before it reached to 80% of its initial capacity Figure shows the cycle data after voltage decay measurements Charge/Discharge Capacity (Ah) 40 30 20 Charge Discharge 10 0 20 40 60 80 100 Cycle Number Figure Cycle life of battery with Pb coated Al negative electrode 30 Voltage (V) 25 20 15 10 0 50 100 Number of Days Figure Voltage decay of battery after 85 cycles 150 200 Charge/Discharge Capacity (Ah) 45 40 35 30 25 20 15 10 Charge Cap Discharge Cap 10 20 30 40 50 60 70 Cycle Number Charge/Discharge Capacity (Ah) Figure Cycle life of battery after voltage decay measurements 45 40 35 30 25 20 15 10 0 20 40 60 80 100 120 Cycle Number Figure Cycle life of second battery with Pb coated Al negative electrode The second battery with lead coated Al grid was cycled till it reached 80% of its capacity Figure shows the plot of charge-discharge capacity as a function of cycle number The battery delivered 112 cycles before it started receiving Charge/Discharge Capacity (Ah) greater charge Battery was removed once the charge acceptance increased at about 110 cycles It is important to note that voltage decay of battery with Al grids and all Pb grids after 85 cycles was similar indicating that Pb plated Al grids did not get exposed to sulfuric acid even after 85 cycles The cycle life data before and after voltage decay measurements clearly indicates that Pb plated Al grids can be used in a lead acid batteries in a negative electrode providing the advantage of substantial weight savings for aircraft as well as for other applications where weight saving is important 4.3.2 Battery with Al Positive electrode: Two 30Ah/24V batteries with Al positive electrode were assembled First battery contained positive electrode of lead coated Al grids, which were plated by lead in fluoborate solution The Al grids for the second battery were plated with lead in methane sulfonic acid solution The cycle life of first battery with Al positive electrode is shown in Figure 70 60 50 40 30 20 10 0 20 40 60 80 100 120 Cycle Number Figure Cycle life of battery with Pb coated Al positive electrode Note that the battery started receiving more charge after few cycles and continue accepting increased amount of charge compared to discharge Although battery did not fail to deliver required amount of energy (30A for hr) till 110 cycles but charge acceptance was abnormally high The greater charge acceptance can be attributed to exposure of Al grid underneath the lead It is stated that lead plated parts in a fluoborate solution might contain microscopic pinholes not visible to the naked eye Dipping the lead coated parts in boiling peanut oil can seal the pinholes Although we were aware of the technique, but decided to use the lead plated Al grids without sealing It was decided that these microscopic holes might be responsible for higher charge currents Charge/Discharge Capacity (Ah) The second battery was assembled using Al grids plated with lead in a sulfonic acid solution Figure shows the cycle life of battery The battery delivered 112 cycles The battery was removed after 112 cycles for other tests The discharge capacity increased from 30-40 cycles, which is normal in new lead acid battery but it, remained fairly constant until 100 cycles The charge increased after about 100 cycles but much less compared to that observed in first battery with Al positive grids The improved results indicated that the methane sulfonic acid produces better Pb plating compared to fluoborate solution and does not require extra step of sealing the pinholes The implementation of Al grids in batteries can result in weight savings as shown in Table The minimum and maximum savings indicates either electrode is replaced or both electrodes are replaced with Pb coated Al grids 40 30 20 10 0 20 40 60 80 100 120 Cycle Number Figure Cycle life of second battery with Pb coated Al positive electrode Table Comparison of battery weights with conventional Pb grid and Pb coated Al grids Battery Type Weight Pb Grid Weight Savings Weight of Al grid Battery Battery with (lb) Al Grids Min (lb) Max (lb) Min (lb) Max (lb) D8565/5-1 80 4.0 11.0 69.0 76.0 D8565/4-1 26 2.3 4.0 22.0 23.7 D8565/7-2 64 4.5 10.0 54.0 59.5 D8565/15-1 90 5.0 13.4 76.6 85.0 RG-18 19 2.0 3.7 16.3 17.0 RG-206 36 4.0 8.0 28.0 32.0 Conclusions: The work clearly demonstrates that Pb plated Al grid can be used successfully in lead acid batteries delivering 100-120 deep discharge cycles and are capable of reducing the weight of current aircraft batteries by 10-15 % The high rate data shows that Al grid in negative electrode improves the high rate performance at room temperature as well as at –26°C The improvement is more noticeable at –26°C The batteries with Al grids in both positive and negative electrode are being assembled and tested and results will be published in the future Acknowledgement: Authors would like to thank Steven Fagan, Bill Johnson, Susan Waggoner, Allen Goodman, and NAWC for continuing support and financial assistance for this research under contract # N68335-03-C-0063 References: Bhardwaj R C., Timmons J B., and Orsino J.A., US patent # 6,566,010, May 20, 2003 Timmons J B., Bhardwaj R.C and Orsino J.A., US patent # 6,316,148, Nov 13, 2001 Timmons J B., and Orsino J.A., Bhardwaj R.C., US patent # 6,447,954, Sept 10, 2002 Bhardwaj R C., Timmons J B., and Orsino J.A., US patent #6,586,136