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# Performance Metrics

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Things covered 1 Different kinds of voltages 2 Calculating battery capacity 3 Behavior of batteries away from equilibrium 4 Modified Peukert Plot 5 Ragone Plot Almost all examples in this class will be on a Li ion battery where the negative electrode is Li metal and the positive electrode is LiNi1 3Co1 3Mn1 3O2 The negative electrode reaction is The positive electrode reaction is A schematic of the battery is shown below The positive electrode material is made into a porous electrode The particles themselves are not very conductive so conductive additives like carbon are added to increase the conductivity The reactions occur at the electrode electrolyte interface and the Li enters the lattice of the oxide Different Kinds of Voltages and ways of measuring them You already know about equilibrium potential and overpotential Few more voltages 1 1 Open circuit voltage voltage when the battery is not connected to an external load Practically you have a voltmeter with a high resistance so that the current draw is very small and you measure the voltage of the battery This is taken to be the same as the equilibrium potential You can typically calculate the equilibrium potential by looking at the thermodynamic tables But for batteries this tends to be difficult to do because of the dependence on the activity coefficients 2 Closed circuit potential potential of the battery under a load 3 Mid point potential Potential of the battery when its discharged to 50 of its capacity 4 Cut off voltage Voltage when the battery discharge or charge is stopped For example cutoff voltage on charge is 4 3 V for a cell phone battery State of Discharge is defined as 1 0 when the battery is fully discharge and 0 0 when the battery is fully charged SOD 1 SOC or state of charge Below is an example for a LiNi1 3Co1 3Mn1 3O2 positive electrode material with respect to a Li metal anode The open circuit potential above was measured by first discharging the battery at C 30 to a specified State of

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