Lightweight lead acid battery with high power

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 whe

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SAE TECHNICAL

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

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ISSN 0148-7191

Copyright © 2004 SAE International

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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

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

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 5 to 50

A/ft2 A smooth, pinhole free and

uniform Pb plating was obtained

between current density of 15A-30

A/ft2

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

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

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 group-contained 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 1 hour and constant voltage charge at 28.8V for

2 hrs

4.0 Results and Discussion:

4.1 Battery assembly:

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-on ccast-onnecticast-on 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 2 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 2 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

200

400

600

800

1000

1200

TIME (SECONDS)

Specification (RT)

Al Grid at RT

Pb Battery (B) (RT)

Figure 1 High rate 14V test at room temperature

0

100

200

300

400

500

600

700

800

Time (seconds)

Specification (-26C)

Al Grid (-26C)

Pb Battery (A) (-26 C)

Figure 2 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

2 hours The cycle life of first battery

with Pb plated Al grids in negative

electrode are shown in Figure 3

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 4 The battery shows loss of 1 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 5 shows the cycle data after voltage decay measurements

10

20

30

40

Cycle Number Charge/Discharge Capacity (Ah)

Charge Discharge

Figure 3 Cycle life of battery with Pb coated Al negative electrode

0

5

10

15

20

25

30

Number of Days

Figure 4 Voltage decay of battery after 85 cycles

5

10

15

20

25

30

35

40

45

Cycle Number

0

Charge Cap Discharge Cap

Figure 5 Cycle life of battery after voltage decay measurements

0

5

10

15

20

25

30

35

40

45

Cycle Number

Figure 6 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 6 shows the plot of

charge-discharge capacity as a function

of cycle number The battery delivered

112 cycles before it started receiving

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 7

0

10

20

30

40

50

60

70

Cycle Number

Figure 7 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 1 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

The second battery was assembled using

Al grids plated with lead in a sulfonic

acid solution Figure 8 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 1 The minimum and maximum savings indicates either electrode is replaced or both electrodes are replaced with Pb coated Al grids

0

10

20

30

40

Cycle Number

Figure 8 Cycle life of second battery with Pb coated Al positive electrode

Table 1 Comparison of battery weights with conventional Pb grid and Pb coated Al grids

Weight Savings with

Al Grids

Weight of Al grid Battery

Battery Type Weight Pb Grid

Battery (lb)

Min (lb) Max (lb) Min (lb) Max (lb)