Assessments - 4, GPA: 3.8
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The charger remains in the Charge Complete state and the charging process can be restarted if the voltage drops below some predefined level (8). The charging process can be terminated with an error if a total charge time-out or an operation charge time- out occurs, or if the battery voltage or charge current is higher than the charge termination voltage/current levels (4), (5). The charger from all states jumps to the Wait For Temperature state when the battery temperature is outside the allowed temperature range. For the Activation and Rapid states, the allowed temperature range is the charge range. For other states, the allowed temperature range is the discharge range (6). In the case of the charge range, when temperatures fall into the defined range with some hysteresis value, the charger goes to the Initialization state (7). Regardless of the state of the charger, it jumps to the Discharge state when the external power supply is switched off (9). If the external power supply is switched on, the charger goes to the Initialization state (10, 13). When the battery pack discharges completely (11), the charger switches to the Full Discharge state. If the system load resistance decreases and the battery pack voltage level re-establishes to the predefined voltage level, then the charger returns to the Discharge state (12). A two-cell battery charger firmware flowchart that corresponds to the state diagram is shown in Figure 9 on page 13 and Figure 10 on page 14. The invocation points of the cell-balancing procedures are also shown. The charge profile example is presented in the Appendix, Figure 13 on page 18. Figure 9. Two-Cell Battery Charger Firmware Flowchart Part 1 StartInit DeviceMeasure Vb1, Vb2, Ich, TbCalc Vbmin, VbmaxStateActivation Set Ireg=IACT; RegulateStateInitializationYesSet Activation StateCharge OnStart tACT, tCH, TimingOpen LOAD OutYesNoCheck For TimeoutsSet Error StateAnd Error Code Check Rapid Start ConditionVbmin>=VRSSet Rapid StateStart tRAP Time Counter YesYesNoStateRapid NoSet Ireg=IRAP; Vreg=VRAP; RegulateYesCheck For TimeoutsSet Error StateAnd Error Code YesNoCheck Charge Terminate ConditionSet Charge Complete StateYesNoSet Initialization StateNoNoSend Debug DataCheck Cell Balancing IntervalCell BalancingYesNoCheck Cell Balancing IntervalCell BalancingYesNo12Check For Negative IchYesNoState is not Error or Wait For Temperature YesNoCheck For Discharge Stop TemperatureSet Wait For Temperature StateYesNoCheck Full Discharge ConditionSet Full Discharge StateYesNoSet Full Discharge StateCheck for charge stop temperatureYesNoSet Wait For Temperature stateCheck For Current ErrorIch>=IMAXSet Error State And Error Code YesNoCheck For Voltage ErrorVbmax>=VMAXSet Error State And Error Code YesNo Figure 10. Two-Cell Battery Charger Firmware Flowchart Part 2 StateCharge CompleteCharge OffTimers OffYesNoStateErrorYesCharge OffTimers OffCheck Charge Restart ConditionSet Initialization StateYesNoStateWait For TemperatureCharge OffTimers OffYesNoTrueYesNoStateDischargeCharge OffOpen LOAD OutYesNoCheck For Negative IchSet Initialization StateYesNoCheck Cell Balancing IntervalCell BalancingYesNoStateFull DischargeCharge OffTimers OffClose LOAD outYesNoCheck For Negative IchSet Initialization StateYesNo12Cell Balancing ResetCell Balancing ResetCell Balancing ResetCheck For Discharge Stop TemperatureSet Wait For Temperature StateYesNoCheck For Negative IchYesNoSet Initialization StateCheck For Charge Restart TemperatureSet Initialization StateYesNoSet Wait For Temperature State Cell-Balancing Algorithm At first sight, the cell-balancing algorithm for a two-cell battery charger appears very simple. The criterion for the cell imbalance is the voltage difference between the cells. The cell with a greater voltage must be shunted. But this algorithm can lead to still more imbalance. During cell balancing only intrinsic cell voltage must be taken into account. The voltage portion contributed by the impedance of the cell leads to errors in cell balancing. In the deep discharge battery, where the internal resistance of the battery can be as high as several ohms, the I x R drop dominates the overall cell voltage. For this reason, cell balancing is not recommended when the battery pack is close to deep discharge. Cell balancing during this time can lead to greater imbalance than before cell balancing was conducted. During the 1-C rate charge, the battery has reached approximately 50 percent of the charged state when its voltage has risen above 3.9 volts. If the charging current is less than 1C, this threshold can be reduced. At this charge state, the internal resistance drops below 0.2. and the distortion level is within acceptable limits. Therefore, some cell-balancing methods can be executed if the cell voltage is above the predefined VMID value (voltage of middle charged state) and the minimum cell-balance parameter consists of the voltage measure error value plus the internal impedance error value. A better pr...
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