Batteries for HTM © D. J. McMahon 2014 rev

Basic Battery Parameters: Cell Voltage: default voltage of a cell, under design load Battery capacity: Ampere-Hours (AH) Energy Density: watt-hours per kilogram Load current: battery output current under a load

Primary Cell Batteries > irreversible chemistry > discarded after use

Zinc-Carbon cell (1.5v) Zn → + Zn e - e - + NH MnO2 → MnO(OH) + NH 3 > called “Dry Cell” > uses MnO 2 to absorb the NH 3

“Alkaline” Cell (1.5v) > Uses ZnO 2 and Zn > alkali used as an electrolyte > commercially most common

Mercury Cell (1.35v) > Zn + HgO + H 2 O→ Zn(OH) + Hg > higher current capacity > flatter voltage curve > expensive

Lithium Cells (3v) > Multiple chemistries > Lithium has the highest negative potential > Li cells have the highest energy densities

CRxxxx identification: (‘coin’ cells) First two digits = diameter in mm Second two digits = height in mm x 10 eg: CR2016 is 20mm in diameter, 1.6mm thick

Silver Oxide Cell (1.55v) > Zn + AgO 2 → ZnO + 2Ag > very stable output, low current > common watch battery

Secondary Cells: “Rechargeable” Batteries i.e. chemistry is reversible

Lead - Acid (2.0v) Pb + (HSO4) - → Pb(SO 4 ) + H + + 2e - 2e - + PbO 2 + 3H + + HSO4 - → Pb(SO 4 ) + 2H 2 O > standard ‘car battery’ > “Sealed Lead-Acid” (SLA) > “Valved Sealed Lead-Acid” > High current output, durable > Heavy, some hazards > FR in size code: Flame Retardant

Valve-Regulated Lead-Acid Batteries or ‘Sealed Lead- Acid” (SLA) batteries Same chemistry as a classic lead-acid, but use less acid, and recombine gas products to form water. Include a pressure relief valve to vent gases if overcharged or overheated. Have a higher energy density than standard lead-acids. Commonly used in medical electronics.

Nickel-Cadmium (1.2v) > 2NiOOH + Cd + 2H 2 O ↔ 2Ni(OH) 2 + Cd(OH) 2 > High efficiency, high current > Develop a discharge ‘memory’ > Virtually extinct now

Nickel Metal Hydride Cell “NiMH” cell (1.2v) > Replacing Ni-Cad’s > Higher energy density than NiCd > No “memory” issues

Chemistry of the NiMH cell: “M” is an alloy of Ni and another metal, formulated for best current delivery.

Lithium-Ion Cell (3.6v) (Do not confuse with Lithium battery) Li + + e - + LiCoO 2  Li 2 O + CoO > Large proportion of the consumer items > No memory effect > Low self-discharge per month > Capacity fails with high current draw or heat

“Smart Battery”: > Rechargeable battery that includes a microchip that communicates information to the charger and the user. > Usually use the ‘System Management Bus’ (SMBus): a 2-wire interface that signals the battery voltage, temperature, clock data, and the battery’s type, birthdate, serial number, and cycle count as binary data. > More expensive, but more efficient.

Thermal fuse: Fusible link:

When good batteries go bad: Watch for corrosion on terminals

Cell voltage reversal !

Charging Lead-Acid’s: > Charge rate can be higher than for Ni-Cads > Charger needs to be constant voltage only > Lead-acids take longer to fully recharge (~12 hours), and current should be reduced gradually > Assume 2.4 volts per cell, and charge initially at 20% to 30% of capacity Do NOT mix or swap battery types in chargers !

Charging Ni-MH’s: > Charge rate should not exceed 1/10 th of the battery’s amp-hour rating. > Chargers need to be constant voltage and constant current. > Modern chargers drop down to a “trickle charge” when the battery is at 90% of capacity.

Battery Maintenance > Protect terminal connectors from shorts with insulators > Don’t leave in a charger more than 24 hrs > Periodically, discharge fully ** > Keep at room temp after charging ** depends on battery’s chemistry

Expired batteries should be sent to recycling

Pull one or both terminals from a dead battery when discarding or wrap terminals with tape

Suggested reference: Isador Buchman Part of Cadex company (

Major vendors of batteries for medical devices: