AE6PMRechargeable_Battery_Basics

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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-ZincRechargeable (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:

• 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

70-79 13.70 14.00 13.40 13.70

60-69 13.85 14.15 13.55 13.85

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

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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