Rechargeable Battery
Basics
More than you ever wanted to know.
Don Steinbach
AE6PM
Saratoga Amateur Radio Association
April 6, 2011
AE6PM
Rechargeable_Battery_Basics
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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 Peukert’s Law.
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Size Codes and Capacities
• For large batteries:
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
• Six-volt golf cart batteries are typically 180 – 225 AH capacity
BCI = Battery Council International
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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
Temp °F
Optimum Maximum
Charge
Charge
Optimum Maximum
Float
Float
70-80
14.30
14.60
13.40
13.70
60-70
14.45
14.75
13.55
13.85
Gel
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Temp °F
Optimum Maximum
Charge
Charge
Optimum Maximum
Float
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 20°C 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
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