Building a power management system for a battery-powered MCU design - Embedded.com

Building a power management system for a battery-powered MCU design

Battery-fuel-gauge ICs, orgas gauges, are at the heartof modern battery-managementsystems. Theynot only maintain accurateestimates of the capacityremaining in the battery but alsocan serve as the host’s battery-data-acquisitionand -management system,primary battery-protection device,and cell-balancing system, as well asmaintain records of battery-use history.

Some gas-gauge systems comprisean analog-front-end IC that providesthe high-speed protection and voltage-measurement capabilities and thegas-gauge IC that maintains the capacityestimate and other more complexfunctions. Increasingly, one IC combinesthe analog-front-end and gas-gaugefunctions.

A range of fuel-gauge ICs is availableand targets use in a number of applications.These ICs include single-cellbatteries, multicell batteries with asmany as 13 cells in series, system-sidefuel gauges, and gauges with and withoutbuilt-in primary protection. GaugeICs are available from a number of largesemiconductor vendors, including Atmel,Intersil, Maxim Integrated Products,O2 Micro, and Texas Instruments(references 1 through 5).

Single-cell gauges usually have smallPCB (printed-circuit-board) footprintsfor tight circuit-layout situations. Thesetiny cell gauges target use with batterieswith only one cell in series, or 1S (one-serial)batteries in battery terminology.The battery may have as many parallelcells as necessary, such as the 1S1P(one serial/one parallel), 1S2P (oneserial/two parallel), 1S3P (one serial/three parallel), and so forth.

Examplesof these gauges include the TI bq275xx,the O2 Micro OZ8805, and the MaximDS278x series. Although some single-cellgauges have built-in protectionlogic, most require the use of a separateprotection IC (for example, the SeikoInstruments S-8211 or S-8241, Reference6). The low core voltages in ultra-portabledevices and the high voltageand energy density of lithium-ion cellscombine to produce an effective portablepower system.

Linear Technology’sLTC2941 and LTC2942 single-cell gasgauges implement a coulomb counter,which integrates the current into andout of the cell array, with a fast analogintegrator (Reference 7). This techniquemay allow accurate tracking ofpulsed load current, which is a challenge for sample-data-system coulombcounters.

A wide selection exists for gauge ICsfor multicell batteries of 2 to 4S (two tofour serial). They include the TI 20zxxseries and the O2 Micro OZ9310. The 3or 4S (three or four serial) battery configurationis popular for portable devicesbecause you can derive most corevoltages for complex portable electronicsdevices from the minimum voltageavailable from a 3 or 4S lithium-ionbattery using simple point-of-load buckor linear regulators—approximately 9Vfor a 3S battery and approximately 12Vfor a 4S battery.

Once the series cell configurationin the battery exceeds 4S, fuel-gauge-IC choices become limited. The relativelynew TI bq78PL114 and severalO2 Micro offerings can handle high-Scountbatteries. Some gauges supporthigh-S-count batteries using externalextension ICs.

High-S-count batteriesfind use in electric vehicles and otherhigh-energy motor-drive applications.In these applications, high battery voltageis necessary to avoid excessive currentin the motor-control circuits, andbatteries of several hundred volts arecommon. Many of these applicationsuse full-custom microcomputer-basedbattery-management-system circuits tohandle the highly complex managementand protection tasks. Figure 1 shows a typical 4S4P (four-serial/fourparallel)battery.

Designing  battery-management systems figure 1

Safety first

Battery safety must be a primary designconsideration. Always design multiplelayers of overvoltage, undervoltage,overcurrent, and overtemperature protectioninto all lithium-ion batteries,no matter how small. This protectionshould include PTC (positive-temperature-coefficient) devices for overcurrentconditions and TCO (thermal-cutoff)devices for overtemperatureconditions in series with the cells. Youshould also use active secondary- andprimary-protection circuits. Fuel-gaugeICs can provide primary protection, butthat protection alone is not enough.Secondary active protection that opensan electronically controlled fuse, suchas a Sony Chemical self-control protector,is often necessary (Reference 8).

Carefully analyze all circuit elementson the cell-array side of the protectioncircuits. It’s essential that no single-componentfault causes a short circuitacross one or more of the cells. For example,if a capacitor is necessary acrossa cell for bypassing EM (electromagnetic)noise, you should use two capacitorsin series to minimize the chance thatcomponent failure will short-circuit thecell.

Modern lithium-ion cells can deliverlarge currents for long times andcause “energetic events” on a PCB if acomponent fault short-circuits the cell.Do not depend on the cell’s embeddedovercurrent protection for this protection.Some cells lack such elements; onothers, the current trip point is so highthat it can damage the PCB before thecell opens. This consideration is especiallyimportant on high-parallel-cell-countbatteries in which the maximumcurrent from each cell can add to alarge maximum battery current.

Do not strike an electrical arc whenassembling the cell array to the battery-protectionelectronics. Such an arc cangenerate high-voltage transients thatcan damage the gas-gauge and protection-circuit elements. This damagemay allow the device to work properlyduring factory test and then fail in fielduse. Protection circuits may not alwaysbe failsafe and thus can cause the protectioncircuit to fail when an actualfault occurs. For this reason, you shoulddesign multiple layers of protection intothe battery.

Hosts and batteries

Most fuel gauges support either atwo-wire SMbus (system-managementbus), such as an I2 C (inter-integratedcircuit), or a one-wire HDQ (high-speed-DQ) interface for communicationwith the host device, which can bea portable device or a charger. SeveralMaxim gauge ICs support the proprietaryMaxim 1-Wire interface.

You canuse this interface to program the gaugeIC during manufacturing and communicatemany parameters with the portablehost device and charger. Most gaugesthat support SMbus communicationalso support the SBS (Smart BatterySystem) 1.1 list of standard battery parameters(Reference 9).

The low signal reference for thesedigital-communication interfaces carriesthe return current for the battery.Be careful that the voltage drop betweenthe gas-gauge reference to signal ground and the host-system ground isnot excessive at high battery current.

Digital signals may be unable to achievea valid low at either the gas gauge or thehost system during high-battery-currentsituations. This inability can be due tobattery-to-host system-contact resistance,wire resistance, shunt resistors,or even PCB-trace resistance. Watchout for pulse-current situations, such asinrush current during battery connection,start-up current for host devices,or high charger current. These conditionsmay cause communication dropoutsdue to signal-ground lift.

Cell balancing

Manufacturers recommend cell balancing,either in the fuel gauge or inthe protection IC, for 3 and 4S lithiumionbatteries and require it for 5S andlarger batteries, and many gauge ICshave this feature built in. Cell balancingis necessary because the capacity ofthe individual cells can diverge as thebattery cycles through charge and discharge.This situation is especially trueif the battery often deeply discharges.

The simplest cell-balancing method,passive balancing, shunts currentaround each fully charged cell in theseries stack until all cells in the stackhave the same capacity. Fuel gaugesthat keep track of the relative capacityof each cell in the stack perform thistask on each charge cycle. The LinearTechnology LTC6802-1 is a cell-monitoringIC that implements thistechnique.

TI’s bq78PL114 and some O2 Microproducts implement a more complexcell-balancing technique, active balancing.This method controls smallswitching power supplies at each cell.These circuits pump current into thecell to balance it with the others in thestack. Control and circuit design forthis method is fairly complex, but it optimizescharger energy and minimizescharge time.Connecting the gauge

The cell array, or core pack, of a high-S- and P-count battery can be complex.To ensure that the fuel gauge maintainsan accurate available-capacity measurement,you must carefully wire the gaugevoltage and current sense to the corepack. Also, many gas gauges require afirst-connection order—usually fromthe lowest to the highest voltage—duringmanufacturing to prevent damageto the IC.

When designing the battery, ensurethat little current flows in the voltage senseconnections between the gaugeIC and the core pack. This requirementusually calls for a separate sense wire, orKelvin connection, between the cell’spositive connection and the gauge IC.Also, be sure to follow the layout guidelinesfor the gauge IC you use, especiallybetween the current shunt and thegauge IC.

Capacity estimates

To remain accurate, coulomb countingrequires a known capacity startingpoint and precise current measurements.Most gas gauges reset their capacityestimate to the actual capacityof the cell array, or chemical capacity,when the battery is fully charged. However,the chemical capacity changes asthe battery ages, so the battery mustsupport some capacity-updating method.

You can update a battery’s chemicalcapacity by continuously discharging itfrom full charge to a low “training” voltage.This method, called conditioningthe battery, is inconvenient for mostbattery users because it can take severalhours and is usually a manual process.You can use conditioning chargers, butthe controls and discharge circuits addsignificant cost to the charger.

A few years ago, TI developed theImpedance Track algorithm, which usesa model of cell-impedance changeto update the cell’s chemical capacityduring normal battery use. The companyhas improved this algorithm severaltimes, and it works for many battery-usemodels.

Correct operation of the ImpedanceTrack algorithm requires thatduring the battery’s charge or discharge,two “relaxation” points occur at whichthe battery current is low and the batteryvoltage is in the flat portion of thedischarge curve—that is, neither at fullcharge nor close to full discharge. Youmust space these two relaxation pointsmore than approximately 40% apart inbattery capacity.

For example, if youfully charge your laptop computer’s battery,use the computer on battery for awhile, close the lid for a while, use it fora while longer, and then close the lidagain. The Impedance Track algorithmwill then likely have the information itneeds for a chemical-capacity update.

Some battery-use patterns do not allowthe Impedance Track algorithmto operate properly. One of these patternsis the backup-battery-use modelin which the battery almost alwaysremains at 100% charge, rarely undergoesshallow discharges, and rechargesimmediately after a discharge. TI offerssome white papers on its Web site aboutadapting the algorithm to this use model,but it’s a complex process.

Maxim has developed the ModelGauge algorithm, which uses a carefullydesigned model of the voltage-versus-temperature-versus-capacity characteristicsof cell types to update the cell’schemical capacity during normal batteryuse. Maxim is working with a smallgroup of battery integrators on the firstapplications of this technique.

O2 Micro uses high-resolution cell-voltagemeasurements and a model ofthe voltage versus capacity to estimatecell capacity. The flat voltage-versuscapacitycharacteristics of high-capacitylithium-ion cells limit this technique,especially in extremely flat LiFe (lithium-iron) PO4 cells, in which a 1-mVvoltage change can equal a 1% changein the state of charge. Fuel-gauge-ICcompanies are working on improvedvoltage-measurement capabilities becauseof this limitation.

Runtime estimation

Estimating remaining portable-deviceruntime is among the most complexand error-prone aspects of batteryuse. The gauge must know how muchpower to source from the battery andthe true chemical capacity of the cellarray to report remaining runtime.

Theamount of power the portable devicepulls from the battery may be inconsistentor unpredictable.For portable devices requiring maintenanceof accurate estimates of remainingcapacity, you should set up areserve capacity. When you program areserve-capacity value into a gas gauge,it offsets the reported capacity by thatamount.

So, the gauge would always reporta lower remaining capacity than isactually available from the cell array.This technique allows portable devicesto safely complete whatever transactionsthey are doing before poweringdown due to a low-battery indicationfrom the gas gauge. This approach is similar to having a reserve gas tank onan airplane, providing just enough capacityto land when the main tank isempty.

System, battery gauges

System-side gauges reside in the portablehost and must adapt to each batteryas you connect it. Battery-sidegauges reside in the battery and carrythe battery characteristics as the batterymoves. System-side gauges are more usefulin applications in which the batteryusually stays with the host—for example,laptop computers, PDAs (personaldigital assistants), and cell phones.

Ifyou replace the battery in a device witha system-side gas gauge, that gas gaugewill report erroneous information untilyou recalibrate it. Battery-side gaugeswork better in applications in whichthe battery is removed from the portabledevice for charging or moved betweenportable host devices.

System-side gauges must support a capacity-estimate-update algorithm thatruns during normal battery use. Otherwise,the gauge would not know thechemical capacity of the battery unlessyou run a conditioning cycle. Portablehosts integrate system-side gauges, minimizingbattery-electronics costs andeliminating the need for battery contactsfor the communication interface.

Battery-side gas gauges integrate analogthermistor inputs to get accuratetemperature readings from close proximityto the cells. Another issue withsystem-side gas gauges is that the distancebetween the thermistor and thethermistor’s input is greater. Hence, thethermistor’s reading at the system-sidegas gauge can be inaccurate.

Because battery-side gauges travelwith the cell array, they can refine theirchemical capacity estimate over time.They can also preserve capacity measurementsthat they completed during aconditioning cycle. However, the batterymust have one or two additionalcontacts to support the battery-to-hostcommunication interface.

Chargers and gas gauges

Battery chargers can be as simple asan ac-powered device, such as a cellphonecharger, or as complex as a multibaydevice with a display and communicationwith the batteries, such as thoseusers might employ to charge a bank ofportable military radios. Chargers generallycome in two flavors: Smart chargersinteract with the gas gauge in thebattery during charge, and dumb chargersuse only battery-terminal voltageand internally measured current tocontrol the charge cycle.

Lithium-ion battery chargers maintaina specific current and voltage profileon the battery as a charge progresses.During the initial portion of thecharge cycle, when the battery voltageis below the float voltage—that is, belowthe maximum for the type of cell and series arrangement—the chargersources a CCM (constant-currentmode) and allows the battery voltageto gradually increase. Once the chargerreaches the float voltage, the chargermaintains CVM (constant-voltagemode) and allows the current to taperoff until it reaches a preset minimumvalue, at which point the charge terminates.Unlike with lead or nickelcadmiumbatteries, you cannot tricklechargelithium-ion batteries—that is,once the battery achieves full charge,you must turn off the charge current.Trickle-charging can damage lithiumionbatteries.

Chargers that interact with the battery’sgas gauge have some advantages.The gas gauge measures the true voltageacross the cell array and can report thatvoltage to the charger. The charger canmeasure the voltage only at the batteryconnector, and that voltage is usuallyhigher than the cell array’s voltage dueto contact, wire, and current-shunt resistances.If the charger can control thegas gauge’s measured cell-array voltage,it can maintain CCM longer, reducingcharge time. Also, chargers that communicatewith the gas gauge can use theprecise current-measurement capabilityof the gas gauge, allowing the use of lessexpensive circuits in the charger.

Designing battery-management systems figure 2Figure 2 shows a typical voltage andcurrent profile for charging a singlecelllithium-ion battery. In this case,the battery voltage was measured insidethe charger, and the cell-voltage valuecomes from the gas gauge. Note theadvantage of maintaining CCM untilthe cell voltage reaches the 4.2V floatvoltage.

EM noise

Because battery-management systemscontain high-impedance measurementcircuits, they’re susceptible toEM-noise pickup. Battery-powered portablesystems, such as radio transmittersand motors in electronic vehicles, canthemselves generate EM noise, or theycan operate near an EM-noise source.The metal cans around the cells andthe cells’ interconnect strapping makeefficient antennas for high-frequencynoise.

Noise pickup in the cell array cancause reading noise in the gas-gaugevoltage- and current-measurement systemcomprising the ADC and signal conditioning components. Gas-gaugeICs use analog and digital noise filtersto reduce the problems this EM noisecauses, but it can still be an issue innoisy environments. EM-noise spikescan cause spurious protection trips inthe primary and secondary battery-protectioncircuits. These trips can be anuisance or, in the case of a secondaryprotection trip, may disable the battery.

Battery designers should follow goodEM-noise-reduction techniques whendesigning the battery-managementsystemelectronics. Careful PCB-tracerouting and extensive use of groundplaneareas in the PCB are essential.Carefully bypass power distributionfor the gas gauge and associated ICsbecause they receive their power directlyfrom the cells. Proper connectionsbetween the gas-gauge IC andthe current-measurement shunt areessential; consult vendor literature forrecommendations.


References
  1. 8- and 32-bit, low power high-performance MCU,” Atmel.
  2. Battery Management: Cell Balancing and Safety,” Intersil.
  3. Parametric Search: Battery Protectors, Selectors, and Monitors,” Maxim Integrated Products.
  4. Intelligent Battery,” O2 Micro.
  5. Battery Fuel Gauges,” Texas Instruments.
  6. Parametric Search: Lithium-ion Battery Protection ICs,” Seiko Instruments Inc.
  7. Battery Management,” Linear Technology.
  8. Self Control Protector,” Sony Chemical and Information Device Corp.
  9. SBS 1.1 specs: current,” Smart Battery System Implementers Forum.

David Gunderson is a senior electronics engineer at Micro Power Electronics. He is responsible for design electronics and embedded software for batteries and chargers. Gunderson holds a bachelor’s degree in electrical engineering, and his interests include composing and performing music and playing with his grandchildren.

This article has been previously published on EDN Magazine.

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