AE6PM

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• Rechargeable Battery
• Basics
• More than you ever wanted to know.
• Don Steinbach
• AE6PM
• Saratoga Amateur Radio Association
• April 6, 2011

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Typical Batteries

• Non-rechargeable (primary cells)
• Alkaline
• Carbon-Zinc
• Rechargeable (secondary cells)
• Ni-Cd (Nickel-Cadmium)
• Ni-MH (Nickel-Metal Hydride)
• Lithium-ion
• Lead-Acid
• Flooded
• Gel
• AGM

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Lead-Acid Batteries

• Oldest type of rechargeable battery.
• Invented in 1859.
• Consists of lead plates in the presence of an
  electrolyte.
• Plates are lead and lead oxide.
• May be alloyed with antimony, tin, calcium or
  selenium.
• Electrolyte is approximately 35 sulfuric acid
  and 65 water.
• Each lead-acid cell produces 2.14 volts.
• As the battery discharges, the electrolyte reacts
  with the plates, changing their surface to lead
  sulfate.

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Lead-Acid Batteries

• When the battery is recharged, the lead sulfate
  reforms back into lead and lead oxide, and the
  specific gravity of the electrolyte is restored.
• Over time, the lead sulfate converts to a
  crystalline form that no longer dissolves on
  recharging.
• This process is accelerated if the battery is
  left in a discharged condition.
• Sulfation can be avoided if the battery is fully
  recharged immediately after a discharge cycle.
• Excessive charging electrolyzes some of the
  water, producing hydrogen and oxygen (called
  gassing).
• Mixture is highly explosive.

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Lead-Acid Battery Types

• Flooded (Wet Cells)
• Electrolyte is a liquid.
• Battery is usually not sealed and not
  spill-proof.
• Usually vent hydrogen gas during charging.
• Valve-Regulated Lead-Acid (VRLA)
• Gelled Electrolyte (Gel Cell)
• Electrolyte is immobilized by a thickening agent.
• Absorbed Glass Mat (AGM)
• Electrolyte is absorbed in separators of matted
  glass fibers.
• VRLA batteries are hermetically sealed and
  designed to produce little or no hydrogen gas
  during charging.
• All use the same chemistry. Construction varies.

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Battery Service Types

• The three service types are Starting, Deep Cycle,
  and Marine
• Starting
• Sometimes called SLI, for starting, lighting,
  ignition.
• Typical automobile battery.
• Designed to produce very large starting current
  for a very short time.
• May last for thousands of cycles in normal
  starting use (2 - 5 discharge).
• Automotive batteries will generally fail after 30
  150 deep cycles.
• Plates are composed of a lead sponge.
• Gives very large surface area, but is quickly
  consumed if the battery is deep-cycled.

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Battery Service Types

• Deep Cycle
• Designed to be discharged as much as 80 many
  times.
• Maximum life if average discharge is kept at
  about 50.
• Plates are usually solid lead, not sponge.
• Less surface area, hence less instant power
  like starting batteries provide.
• It will not hurt a deep cycle battery to be used
  as a starting battery, but cannot supply as much
  cranking amps.
• Marine
• Hybrid.
• Compromise between starting and deep-cycle.

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Typical Specifications

• CCA Cold cranking amps
• The number of amps a battery can deliver at 0 F
  for 30 seconds and not drop below 7.2 volts for a
  12-volt battery.
• CA Cranking amps
• Same as CCA except tested at 32 F.
• Also called MCA Marine cranking amps.
• RC Reserve Capacity
• The number of minutes a fully charged battery at
  80 F will discharge at a 25-amp rate until the
  voltage drops below 10.5 volts.
• AH Amp hour
• The number of amps the battery can deliver for a
  number of hours until the voltage drops to 10.5
  volts. Typically for 20 hours. See Peukerts
  Law.

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Size Codes and Capacities

• For large batteries
• Six-volt golf cart batteries are typically 180
  225 AH capacity
• BCI Battery Council International

BCI Code Capacity Voltage
U1 34 40 AH 12 V
Group 24 70 85 AH 12 V
Group 27 85-105 AH 12V
Group 31 95-125 AH 12 V
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Battery State-of-Charge

• Open-circuit voltage vs. SOC for 12V lead-acid
  battery

Charge Flooded Gel AGM
100 12.60 or higher 12.85 or higher 12.80 or higher
75 12.40 12.65 12.60
50 12.20 12.35 12.30
25 12.00 12.00 12.00
0 11.80 11.80 11.80
Note Voltage under load at full discharge (0
charge) is 10.5V.
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Charging Algorithm

• Smart Charger uses a three-stage charging
  algorithm.
• For a 6-cell/12-volt flooded battery
• Bulk charge
• Constant current charge until battery voltage
  reaches 14.4 volts at which time the battery is
  80 charged.
• Absorption charge
• Constant voltage of 14.4 volts. Current
  decreases as battery charges to 98.
• Float charge
• Regulated voltage of not more than 13.4 volts and
  usually less than 1 amp.
• Battery eventually reaches 100 SOC.

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Charging AGM Gel

• AGM and Gel batteries have different charge
  voltage limits.
• 14.4 to 14.6 volts maximum at 68 F for AGM.
• 13.8 to 14.1 volts maximum at 68 F for Gel.
• Varies somewhat with manufacturer.
• See table on next slide.
• Check the specs for your battery.
• Automatic temperature-sensing voltage-regulated
  chargers are highly recommended.
• Voids or bubbles can form.
• Water cannot be replaced if continually
  overcharged.

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Charging AGM Gel

• AGM and Gel batteries have different charge
  voltages.
• AGM
• Gel

Temp F Optimum Charge Maximum Charge Optimum Float Maximum Float
70-80 14.30 14.60 13.40 13.70
60-70 14.45 14.75 13.55 13.85
Temp F Optimum Charge Maximum Charge Optimum Float Maximum Float
70-79 13.70 14.00 13.40 13.70
60-69 13.85 14.15 13.55 13.85
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Charging Flooded

• Flooded batteries are less critical because they
  are vented and water can be replaced.
• Automobiles typically charge at 13.8 to 14.4
  volts.
• For a 6-cell automotive lead-acid battery at room
  temperature
• Open circuit voltage at full charge 12.6 V
• Fully discharged 11.8 V
• Charge with 13.2 14.4 V
• Gassing voltage 14.4 V
• Continuous float voltage 13.2 V
• Note These voltages are for 20C and must be
  adjusted -22 mV/C for other temperatures.

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Notes

• Battery capacity is reduced as temperature goes
  down, and is increased as temperature goes up.
• At freezing, capacity is reduced by 20.
• Battery life is reduced at higher temperatures.
• Battery life is cut 50 for every 15 F above 77
  F.
• Deep-cycle battery life is directly related to
  how deeply the battery is cycled each time.
• Life at 50 DOD 2X 80 DOD.
• Life at 10 DOD 5X 50 DOD.
• Acid spills can be neutralized with ammonia or
  sodium bicarbonate (baking soda).

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Economics

• AGM and Gel batteries are more expensive than
  flooded batteries.
• AGM is a newer technology.
• Low self-discharge.
• Accepts higher charge rates than Gel.
• Costs 2 to 3 times as much as an equivalent
  flooded battery.
• Flooded deep-cycle battery vs. flooded SLI
  battery
• Additional cost probably not warranted for ham
  radio emergency power or Field Day application.
• Need to consider how many deep cycles the
  battery will encounter.
• Maybe 20 cycles over 10 years for Field Day.
• Maybe 10 cycles over 10 years for home use.
• Batteries will probably die of old age at 5 7
  years, long before cycle life limit is reached.

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• The End