Battery Basics A guide to battery use in engineering projects

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Battery BasicsA guide to battery use in
engineering projects

• Thomas G. CleaverUniversity of
  LouisvilleDepartment of Electrical and Computer
  EngineeringSept. 19, 2008

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References

• This presentation was developed using the
  following sources
• T.E. Bell, Choosing the Best Battery for
  Portable Equipment, IEEE Spectrum, March, 1988,
  pp 30-35.
• Walt Kester, Joe Buxton, SECTION 5, BATTERY
  CHARGERS, available at http//www.analog.com/Uplo
  adedFiles/Associated_Docs/43241673Power_sect5.PDF
  search22batteries2022discharge20profiles222
  2
• Custom Power Solutions, available at
  http//www.mpoweruk.com/performance.htm
• New Technology Batteries Guide (1998), available
  at http//www.nlectc.org/rwscripts/rwisapi.dll/_at_JU
  STNET.env?CQ_SESSION_KEYCUANXLBUKMIRCQ_TPT_VIEW_
  DOCYESCQDOC_NUM2
• Green Batteries, available at http//www.greenbatt
  eries.com/libafa.html
• Steve Garland, Kyle Jamieson, Battery Overview,
  available at http//nms.csail.mit.edu/fun/batter
  y.ppt268
• Harding energy Inc, available at
  http//www.hardingenergy.com/
• BatteryUniversity.com, available at
  http//www.batteryuniversity.com/

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Battery Terms 1

• Capacity The charge a battery can hold in
  ampere-hours (Ah) or milliamperes-hours (mAh) or
  the energy the battery can hold in watt-hours.
• C Charge or discharge rate. Battery capacity
  in Ah or mAh divided by 1 hour. Also know as C
  rate.
• Discharge rate The maximum allowable load or
  discharge current.
• End voltage The voltage below which a battery
  will not operate satisfactorily. Also know as
  final voltage.
• Energy density The energy storage capacity of a
  battery compared to its mass or volume. The
  higher the energy density, the better.
• Memory effect The tendency of some rechargeable
  batteries to lose capacity when not periodically
  totally drained a particular problem in NiCd
  batteries.

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Battery Terms 2

• Primary battery A disposable battery.
• Polarity reversal The reversal of the polarity
  of an over-discharged cell of a rechargeable
  battery in a series connection. If one cell in a
  series string discharges before the others, the
  discharged cell may reverse polarity. If the
  current is maintained, the reversed cell may be
  permanently damaged.
• Secondary battery A rechargeable battery.
• Self-discharge The loss of capacity over time
  of a battery when it is unused.
• Service life The length of time a battery is
  expected to be usable.
• Shelf life The length of time a battery will
  retain useful charge when stored.

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Primary (Disposable) Battery Types

• Zinc-carbon
• Ordinary battery
• Voltage decreases steadily during discharge
• Zinc-alkaline
• Alkaline battery
• Better that zinc-carbon
• Voltage decreases steadily during discharge
• Zinc-air
• Button cell hearing aid batteries
• Voltage almost constant over useful life
• Lithium ion
• High energy density
• Voltage almost constant over useful life

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Secondary (rechargeable) Battery Types

• Sealed Lead-Acid (SLA)
• Automobile batteries
• Low cost
• Lead is toxic sulfuric acid is corrosive.
• Nickel-Cadmium (NiCd)
• Inexpensive
• Memory effect
• Cadmium is toxic.
• Nickel-metal-hydride (NiMH)
• Moderately expensive
• Voltage almost constant over useful life
• Lithium ion (Li-ion)
• Expensive
• High energy density
• Voltage almost constant over useful life
• Dangerous if overcharged

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Standard Sizes

• Button used in hearing aids and in other
  applications that require small size
• Cylindrical like AAA, AA, C, D all usually
  1.2 to 1.5 V
• Prismatic like 9 V batteries
• Li-ion does not come in standard cylindrical
  sizes.

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Discharge and Voltage

• The voltage of some batteries doesnt change much
  as the battery is discharged, for example, NiCD
  and NiMH.
• The voltage of others drops off as the battery is
  discharged, for example, zinc-carbon, and
  alkaline.

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Discharge and Current

• Battery capacity, usually expressed in mAh, is
  measured under specific conditions.
• The higher the current, the less the effective
  capacity.
• Example A battery rated at 1500 mAh may be able
  to deliver 1.5 A for 1 hour, but it may not be
  able to deliver 3 A for ½ hour.

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Peukert Curve(from http//www.batteryuniversity.c
om/partone-16a.htm)
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C Rate Calculations

• C Rated capacity/ 1 hour
• Example A 2800 mAh NiMH battery has a C of 2800
  mA.
• Batteries can be tested at various multiples of
  C.
• Example For the 2800 mAh battery, C/4 would be
  700 mA 3C would be 8400 mA.

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Voltage Dependence on Current

• Batteries are not ideal devices They have
  internal resistance.
• Vloss IRinternal

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Maximum and Optimal Drain
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Batteries in Series

• Batteries should be identical.
• Total voltage Voltage of each cell x number of
  cells
• When using rechargeable batteries in series,
  beware of deep discharge because of polarity
  reversal.

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Batteries in Parallel

• Batteries should be identical.
• Total current Current of each cell x number of
  cells
• Usually a bad idea
• Good batteries may discharge through bad battery.