Batteries for HTM. Basic Battery Parameters:

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1 Batteries for HTM Key Points Batteries: - chemistry; know the characteristic cell voltages of common chemistries: NiCd/ NiMH 1.2V Hg 1.35V Zn Alkaline 1.5V Ag Oxide 1.55V Pb 2.0V Li 3.0V LiIon/ LiPo 3.6V - know which ones are, which ones are - be very comfortable with volts times amps equals watts - understand capacity, measured in Ah - understand C-rate for battery discharge - know how to recycle depleted batteries - understand that specific battery chemistries require matching chargers D. J. McMahon rev cewood Basic Battery Parameters: Chemistry: The most common battery chemistries are based on Zinc, Lead, Nickel and Lithium. (Silver & Mercury are also used.) Each chemistry has a characteristic Cell Voltage and a characteristic Energy Density. Cell Voltage: Individual cells have a nominal voltage; however, the open circuit voltage (OCV) on a fully charged cell is 5-7% higher. Battery Voltage (V): Battery voltage is the cell voltage multiplied by the number of cells in series. C-rate: The C-rate specifies the speed a battery is charged or discharged. At 1C, the battery charges and discharges at a current that is on par with the marked Ah rating. For example, a 5 Ah battery discharged at 1C is discharged at 5 Amps. At 0.5C, the current is half and the time is doubled, and at 0.1C the current is one-tenth and the time is 10-fold. That 5 Ah battery at.1c would be discharged at 500 ma and would last for 10 hours. Capacity (Ah): Capacity represents specific energy in ampere-hours (Ah). Ah is the discharge current a battery can deliver over time. You can install a battery with a higher Ah than specified and get a longer runtime; you can also use a smaller battery and expect a shorter runtime. Specific power: loading capability. Batteries for power tools, for example, are made for high specific power. They support a high C-rate. Load current: battery output current under a load. (Real) Power (W): (On a purely resistive load) Watts = Volts x Amps (also called VA) Physical size and dimensions: will it fit? Energy Density: watt-hours per kilogram. Thanks to, and further information from:

2 Primary Cell Batteries irreversible chemistry discarded after use from this excellent reference Zinc-Carbon cell Alkaline Cell (1.5v) Zn + Zn2+ + 2e- e- + NH4+ + MnO2 MnO(OH) + NH3 called Dry Cell uses MnO2 to absorb the NH3 usage consumes the Zinc case obsolete (1.5v) Uses ZnO2 and Zn alkali used as an electrolyte commercially most common

3 Mercury Cell (1.35v) Zn + HgO + H 2 O Zn(OH) + Hg higher current capacity flatter voltage curve expensive Silver Oxide Cell (1.55v) Zn + AgO 2 ZnO + 2Ag very stable output, low current common watch battery Lithium Cell (3v) Multiple chemistries Lithium has the highest negative potential Li cells have the highest energy densities (Do not confuse with Lithium Ion or Lithium Polymer cells!)

4 Secondary Cells: Rechargeable Batteries i.e. chemistry is reversible Lead - Acid (2.0v cell) Pb + (HSO4) - Pb(SO 4 ) + H + + 2e - 2e - + PbO 2 + 3H + + HSO4 - Pb(SO 4 ) + 2H 2 O standard car battery (3 or 6 cells = 6 or 12v) can be Sealed Lead-Acid (SLA) can be activated glass mat (AGM) can be Valved Sealed Lead-Acid High current output, durable Heavy, some hazards FR in size code: Flame Retardant fully (freshly) charged: 13.8 volts (2.3v per cell) fully discharged: 10.5 volts (1.75v per cell, < 76% of full) Valve-Regulated Lead-Acid Batteries or Sealed Lead- Acid (SLA) batteries Same chemistry as a classic lead-acid, but uses less acid, and recombines gas products to form water. Includes a pressure relief valve to vent gases if overcharged or overheated. Has a higher energy density than standard lead-acid batteries. Commonly used in medical electronics. Nickel-Cadmium (1.2v cell) NiCad batteries 2NiOOH + Cd + 2H 2 O 2Ni(OH) 2 + Cd(OH) 2 High efficiency, high current Develop a discharge memory Cadmium is highly toxic Virtually extinct now fully (freshly) charged: 13.8 volts (2.3v per cell) fully discharged: 10.5 volts (1.75v per cell, < 76%)

5 Nickel Metal Hydride Cell NiMH cell H2O + M + e (1.2v cell) OH + MH Replaced Ni-Cad Higher energy density than NiCd No memory issues M is an alloy of Ni and another metal, formulated for best current delivery. Lithium-Ion Cell (3.6v cell) (Do not confuse with Lithium cell!) Li+ + e- + LiCoO2 D Li2O + CoO Large proportion of consumer products Highest energy density No memory effect Low self-discharge per month Capacity fails with high current draw or heat lithium-ion polymer battery LiPo has the same chemistry in a soft pouch format Lithium-Ion Boeing 787 grounded: Jan-May 2013 NTSB: scorched battery cells, likely cascading cell failure

6 Smart Battery Rechargeable battery that includes a microchip that communicates information to the charger and the user. When good batteries go bad: Watch for: corrosion on terminals 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. Regular batteries paired with Smart Chargers are becoming much more common. corrosion damage to device leaking batteries bulging cases! cell voltage reversal!

7 Charging Lead-Acid Batteries Charger needs to be constant voltage only Lead-acid takes 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 current capacity Do NOT mix or swap battery types in chargers! Charging Ni-MH Batteries: 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 Expired batteries should be sent to recycling Protect terminal connectors from shorts with insulators Don t leave in a charger more than 24 hrs Keep at room temp after charging (NiCd needed periodic full discharge, but memory is no longer a concern)

8 Pull one or both terminals from a dead battery when discarding or wrap terminals with tape Major vendors of batteries for medical devices: Suggested reference: Isador Buchman Part of Cadex company (