care and feeding of rechargeable batteries

Care and Feeding ofRechargeable Batteries

Introduction

Rechargeable (or secondary) Can be charged and discharged many times,

versus one time use of "primary" batteries. Charging is done electrically.

Voltage varies significantly in both charge and discharge phases


"C" rating is the number of ampere-hours of charge the battery can hold, usually based on a current of C/20 amps, i.e. a 20 hour rate.

Most current specifications are normalized to multiples or fractions of C.

These batteries are all available in sealed configurations, which are recommended for safety and ease of handling


Main Battery Chemistries

Sealed Lead-Acid (SLA) e.g. Gel cell, etcElectrolyte: sulphuric acidOriginal lead acid cells developed in 1860’sSealed Lead-Acid developed in 1970’sCharacteristic Discharge Curves shown in later slide


Nickel-Cadmium (Nicad)Electrolyte: Potassium hydroxideOriginal development 1890’sSealed nicad cells developed in 1940sMemory effect – especially in older designs


Nickel-Metal-Hydride (NiMH)Electrolyte: potassium hydroxideDeveloped in 1980’s


Lithium-Ion (Li-Ion) Developed in 1990’sDo not confuse with conventional lithium batteries. Look for the full name and nominal 3.6 or 3.7 v/cellDifficult to purchase individual Li-Ion cells.


Characteristics (all at room temperature)

Characteristic SLA NiCd NiMH Li-Ion

Vmax/cell 2.3 1.45 1.45 4.2

Vnom/cell 2.1 1.2 1.2 3.6

Vmin/cell 1.75 1.0 1.0 3.0

Cycles: 100% DOD 300 1000 300 500

Watt-hours/C$ 0.96 0.26 0.34 0.12

Life WH/C$ 288 260 102 60

Watt-hours/oz 0.96 0.98 2.37 5.2

Watt-hours/cu in 1.24 1.51 4.77 7.4

Self dischg %/mo 5 50 25 5


Discharging

Should not be discharged below "Vmin" (lithium-ion can be damaged)

Use of a fuse between battery and load is strongly recommended. A short circuit can damage the battery and cause a fire.

Available amp-hours depend on discharge rate and temperature


Load should be able to work over full range of battery voltage. Load should be automatically switched off at "Vmin" to protect battery

“Memory effect" in Nicads can be prevented by:- avoiding repeated, small discharges, followed by recharge- periodic full discharge and recharge


Charging

See manufacturer's recommendations for charging The charging technique is different for each

battery chemistry Options:

1) Buy a charger designed for that particular battery’s

- chemistry- number of cells in series, i.e. voltage- amp-hour rating


2) Design your own, using information from battery manufacturer or charge control integrated circuit supplier.

- Ensure that battery cannot discharge into charger if power source goes off.

- Charging current should be limited, with a fuse backup.


Do not:

- Use a power supply as a charger unless the charging is monitored continuously- Use a charger designed for a different type of battery- Exceed the maximum allowable battery voltage- Overcharge battery


Storage for more than one month

Follow manufacturer's instructions for storage Fully charge battery before storage, except

lithium-ion should be 50% charged Store at room temperature or below Provide trickle charging or periodic recharge if

there is significant self discharge (keep battery well charged)

Perform one full discharge/charge cycle before use


Safety

Fuses should be used in the charging and discharging paths

Battery is sealed, but has pressure relief valve which will open if internal pressure exceeds safe value (for example, due to a short circuit or overcharging). Do not operate in a sealed container.


Battery may vent gas and electrolyte if abused. The electrolyte is corrosive. If this material gets into the eyes, flush with water and get immediate medical attention.

Use caution when working on a battery and its wiring. Do not wear rings, especially when working on high current batteries.

Main Battery Chemistries

Battery Failures

Sulphation is the primary cause of failure in lead-acid and SLA cells

Self-destruction results in Li-Ion cells from v/cell greater than 3.0 volts

Surplus batteries, with unknown storage history, may be failure prone.


Manufacturers and Dealers

Batteries and Chargers- Electronic parts suppliers (Digi-Key, Electrosonic, Newark, Active, etc.)- Battery suppliers (incl local) and manufacturers

Panasonic produces a full line of batteries

Charging circuit components- Unitrode and Benchmarq (both part of Texas Instruments) plus others for charging control ICs


References

Application notes and spec sheets from battery manufacturers

Application notes and spec sheets from charger and control circuit manufacturers

Book "Batteries in a Portable World" Second edition, by Isidor Buchmann, Cadex Electronics, Inc., Richmond, BC ($52.96 Canadian) (The "bible" for serious battery users)


Questions?


Thanks for your attention.

Enjoy your visit.

73

de

Dick Bonnycastle - VE3FUA

care and feeding of rechargeable batteries

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