A Guide to Battery Fast Charging (Part 1)

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What you'll learn:

• Basics of battery-charging systems.
• Regulating a charging MOSFET.
• Partitioning of the fuel gauge.
• Implementation of a battery-management system.
  

Inthiseraofmobiledevices,batterylifeisoneoftheprimaryfactorsthataffects theuserexperience.Thoughimplementingpower-savingtechnologyinsidedevices is important, it’s only one part of the solution. Given the increasing capabilities—andgreaterpowerrequirements—ofmobiledevices,originalequipmentmanufacturers (OEMs) also improve battery life by substantially boosting battery capacity.

Forexample,architectureslike1S2P(onecellinserieswithtwocellsinparallel)thatusetwo cells in parallel to increase overall capacity have become more popular. The downsideofhigherbatterycapacityisacorrespondingincreaseinchargingtime.

Tominimizechargingtime,improvementsinbatterytechnologyincreasecharge currentfrom 2C up to3C or6C(thatis,xCisxtimesthecurrentthatwould pass through the rated ampere-hours of a battery in an hour).For example, a 2000-mAh cell could utilize up to 12 A of charging current without negatively affecting battery reliability.

Highcurrentrequiresspecialcaretoensuresafecharginganddischarging.When usingcellsinparallel,developersalsoneedtotakecareofimpedanceandinitial capacity mismatches. Part 1 of this article series presents an overview of the challenges associated with implementing battery fast-charging capabilities for devicesof alltypes,including consumer,medical, andindustrialapplications.

We’ll also explore how to charge batteries in a 1S2P arrangement with high performance, aswellashowtopartitionthechargerandfuelgaugebetweenthe host and battery pack to increase system flexibility, minimize power dissipation, and improve the overall user experience.

ChargerBasicsandWhyFuel-Gauge Partitioning Matters

The key components of a battery-charging system are the charger itself and the fuelgaugethatreportsmetricssuchasthebatterystateofcharge(SOC),timeto empty,andtimetofull.Thefuelgaugecanbeimplementedeitheronthehostside or in the battery pack (Fig. 1).

When implemented in the battery pack, the fuel gauge requires nonvolatile memory to store battery information. MOSFETs on the power path monitor charging/dischargingcurrentsandprotectagainstdangerousconditions.On that front, adevicelikethe MAX17330 from Analog Devices is a battery fuel gauge with built-in protection circuitry and battery-charger capabilities (Fig. 2).

ThechargingMOSFETcanberegulatedwithfinegranularitytoimplementalinear charger that can serve as a standalone device when the charging source is limited to 5 V and the charge current is in the range of 500 mA. Since lithium-battery chargingexceeds3.6Vfor99%ofthechargecurve,powerdissipationis limited.

Ahigh-voltagechargingsourceandhighchargingcurrentcanbeaccommodated usingastep-downconverterinfrontofthechargertoregulateitsoutputvoltage (Fig.3).Thisalsominimizesdropoutandthereforereducespowerdissipation on the charging MOSFET (Fig. 4).

Implementing a fuel gauge in the battery pack enables the battery to become smart, allowing for advanced charging scenarios and capabilities. For example, the fuel gauge can store the charging profile suitable for the cell inside the battery packinitsnonvolatilememory.Thishastheaddedbenefitofoffloadingcharging fromthehostmicrocontrollerunit(MCU).NowthehostMCUonlyneedstomanage the ALRT signal coming from the battery pack to increase/decrease the output voltage of the step-down converter according to the alert type received.

Heat and MOSFET Limits

Limit settings:

• CP:heat limit→decreasethe voltage.
• CT:MOSFETtemperaturelimit→decreasethe voltage.
• Dropout:→increasethevoltage

CP is a flag that’s set when the current flowing in the protection MOSFETs can compromisethermaldissipation.CTisaflagthat’ssetwhentheMOSFETtemperature istoohigh.HeatlimitandMOSFETlimitsettingsareconfiguredusingthenChgCfg1 registerset.

Aprogrammablestep-downconverterliketheMAX20743 usesthePMBustoallow for fine regulation of the output current. Integrated MOSFETs in the step-down convertersupportchargingcurrentsupto10A.Inaddition,sincethePMBususes I²Casitsphysicallayer,asingleI²Cbuscanbeusedtomanageboththestep-down converter and fuel gauge.

The following example offers a way to charge a single 3.6-V lithium cell. Figure 5 shows the time-domain shape of the voltages and the currents in the charging system. Specifically, the graph shows the battery voltage, battery current, and output voltage of the buck converter.

As can be seen, the step-down converter output (V_PCK) is set to 50 mV above thebattery voltage. This output voltage is constantly increased to avoid dropout as well as to minimize overall power dissipation.

BatterySafetyManagement

Withthehighcurrentsinvolvedinfastcharging,OEMsmustbeabletoguarantee safecharging.Thus,asmartfastchargermustmonitorseveralimportantparameters as part of its overall battery management. For example, by monitoring battery temperatureandambient/roomtemperature,afastchargercandeterminewhen to reduce the charging current and/or lower the termination voltage to assure safety and improve the lifespan of the battery, according to cell manufacturer specifications and recommendations.

The voltage and current can be adjusted over temperature to comply with the six-zoneJEITA (Japan Electronic Information Technology Association)temperaturesettings(Fig.6) andwiththree-zonestep-charging based on the battery voltage.

Battery lifespan can be further improved using a step-charging profile that changes charge current according to battery voltage. Figure 7 shows a step-chargingprofilethatusesthreechargevoltagesandthreecorrespondingchargecurrents. Transitioningbetweenthestagescanbemanagedthroughastatemachine(Fig. 7, again).

Note that current, voltage, and temperature are all interrelated (Tables 1 and 2).

Parallel Charging

Parallel charging of multiple cells requires additional management. For example, the charger must prevent cross-charging when the two batteries’ voltages differ by more than 400 mV. Cross-charging can be tolerated for a limited time only when the lowest cell charge is too low to support system loading (Table 3 and Fig. 8).

InPart 2of thisarticle series, weexplore theimplementation ofa fast-charging systemwithparallelbatteries usingevaluationkitsand aRaspberryPi board.

Conclusion

Moving charging and fuel-gauge functionality from the host side to the battery packenablesindividualcontrolofeachbatteryina1S2Pconfiguration.Ratherthan requiringthehostMCUtofullymanagecharging,asmartchargeritselfcanmanage its own output according to an optimal charging profile. Because management onthehostsideislimitedtomanagingALRTsignalsgeneratedbythefuelgauge, systems can easily adapt to different battery packs.