Battery monitoring programs are basic enablers of various markets. Batteries play a key position in a variety of functions, from going the additional mile in electrical autos to storing renewable power for the good grid. The identical and related battery applied sciences are utilized in medical units for elevated security of operation and for having the liberty to maneuver devices round in hospitals. All these functions run on batteries that want correct and environment friendly semiconductors to watch, steadiness, defend and talk. This text will clarify how a state-of-the-art battery monitoring system, together with cell balancing and remoted communication networks, can exploit the advantages of recent lithium battery chemistries. Utilizing progressive built-in circuits permits greater reliability and a 30 % longer battery lifetime, particularly for giant scale power storage programs.
Batteries utilized in medical functions want to fulfill very excessive requirements for reliability, effectivity, and security in all functions the place they’re sometimes used: sufferers’ moveable programs reminiscent of chest compression programs, hospital emergency room gear, powered medical carts and beds, moveable ultrasound machines, distant monitoring, and the newcomer available in the market, power storage programs (ESS).
Vitality storage programs will not be instantly linked to sufferers, nor are they operated by medical doctors. They’re the following step ahead for uninterruptible energy provides (UPS). UPS have historically been used as backup energy for probably the most vital functions (for instance, emergency room units, IT community vital infrastructure). Vitality storage programs for hospitals are masking an increasing number of features, enabled by the brand new lithium-based batteries. They’re changing into absolutely built-in with the hospital energy grid, bringing benefits like:
- Full backup energy for total amenities, reasonably than only a small, vital subset of amenities, in addition to safety from blackouts, poor energy/voltage high quality from the grid, and decreased utilization of emergency diesel mills. With megawatt hour (MWh) scale ESS, hospitals can function even throughout extended blackouts, and so they can take part in grid stabilization.
- Financial advantages on the electrical energy invoice. With ESS, hospitals can instantly management the utilization profiles of electrical energy and cut back excessive energy peak calls for, which leads to decrease payments from the utilities.
Hospitals typically have sizable roof property, which is sweet for putting in photovoltaic (PV) programs to generate electrical energy. PV programs mixed with ESS permit for the storage and self-use of generated electrical energy, whereas additionally offering financial advantages and a decreased carbon footprint.
Vitality storage battery cells
Lithium-based chemistries at the moment are state-of-the-art for the batteries utilized in numerous markets, from automotive to industrial to well being care. Several types of lithium batteries have totally different advantages to higher go well with the ability necessities for a wide range of functions and product designs. For example, LiCoO2 (lithium cobalt oxide) has very excessive particular power and this makes it appropriate for moveable merchandise; LiMn2O4 (lithium manganese oxide), with its very low inner resistance, permits quick charging and excessive present discharging, which implies that it’s a sensible choice for peak shaving power storage functions. LiFePO4 (lithium iron phosphate) is extra tolerant to full cost situations and may maintain being saved at excessive voltage for a protracted time frame. This ends in it being the very best candidate for giant power storage programs that have to work throughout an influence outage. The downside is the next self-discharge fee, however this isn’t related within the above-mentioned storage implementations.
The differing wants of functions requires a wide range of battery sorts. For instance, automotive functions want excessive reliability and a superb charging and discharging velocity, whereas well being care functions necessitate excessive peak present sustainability for effectivity and a protracted lifetime. Nevertheless, the commonality amongst all these options is that the varied lithium chemistries all have a really flat discharge curve at a nominal voltage vary. Whereas in normal batteries we see a voltage drop within the vary of 500 mV to 1 V, in superior lithium batteries, reminiscent of lithium iron phosphate (LiFePO4) or lithium cobalt oxide (LiCoO2), the discharge curve exhibits a plateau with a voltage drop within the vary of 50 mV to 200 mV.
Lithium battery discharge profile
The flatness of the voltage curve has great advantages within the energy administration chain of ICs linked to the battery voltage rail: the dc-to-dc converters could be designed to function at a most effectivity level in a small enter voltage vary. Changing from a recognized VIN to a really shut VOUT, the ability chain of the system could be designed to have a really perfect responsibility cycle of the buck and enhance converters to realize >99 % effectivity all through all working situations. Furthermore, the battery charger can completely goal the charging voltage and the hundreds are dimensioned in accordance with a secure working voltage to extend the precision of the ultimate functions, reminiscent of distant monitoring or affected person in-body electronics. Within the case of previous chemistries or non-flat discharge curves, the dc-to-dc conversion operated from the battery will work with decrease effectivity, which leads to a shorter battery length (–20 %), or, when linked to medical moveable units, the necessity to cost them extra usually due to the additional energy dissipation.
The principle disadvantage of a flat discharge curve is that the state of cost (SOC) and state of well being (SOH) rankings of the battery are a lot more durable to find out. SOC have to be calculated with a really excessive precision to make sure that the battery is correctly charged and discharged. Overcharging can deliver questions of safety and generate chemistry degradation and brief circuits that result in fireplace and fuel hazards. Over-discharging can harm the battery and shorten the battery lifetime by greater than 50 %. SOH provides details about the standing of the battery to assist forestall changing good batteries and to watch the state of dangerous batteries earlier than a difficulty seems. The principle microcontroller analyzes the SOC and SOH knowledge in actual time, adapts the charging algorithms, informs the person concerning the potential of the battery (for instance, if the battery is prepared for a excessive present deep discharge in case of energy break), and ensures that, in huge power storage programs, the steadiness between batteries in dangerous situation and batteries in good situation is perfect to extend the full battery lifetime.
By imaging a really previous battery with a steep discharge curve, it’s simpler to calculate the state of cost of that battery by measuring the delta of the voltage drop in a small period of time and figuring out absolutely the worth of the battery voltage. For a brand new lithium-based battery, the accuracy required to make this measurement is orders of magnitude greater, for the reason that voltage drop is far smaller in a given time-frame.
For the SOH, previous batteries discharge in a sooner and extra predictable method: their voltage discharge curve turns into even steeper and the goal charging voltage can’t be reached. New lithium batteries will hold the identical good conduct longer, however ultimately can degrade with a extra distinctive conduct and quickly change their impedance and discharge curve simply when they’re shut to finish of life or being broken. Additional care have to be taken for temperature measurements, ideally at each single cell, to combine the SOC and SOH algorithms with this info to make them extra correct.
Exact and dependable SOC and SOH calculations assist lengthen battery lifetimes from 10 years to 20 years in the very best case and customarily lead to a 30 % lifetime enchancment, which reduces the full price of possession of the power storage system by larger than 30 % after together with upkeep prices. This, along with the upper accuracy of the SOC info, avoids overcharging or overdischarging situations that may shortly drain a battery, minimizes the prospect of brief circuits, fireplace, and different dangerous conditions, helps use all of the power in a battery, and permits charging batteries in the very best, most effective method attainable.
The LTC6813 battery administration answer (BMS) proposed on this article can be utilized in well being care units reminiscent of moveable ultrasound machines and in giant scale (megawatt/hours) power storage programs (for hospitals, factories, grid stabilization, electrical car charging infrastructure, and residential items), in addition to in industrial robots and autos. The portability of ADI expertise brings terrific benefits in reliability and security, because it’s designed to work in numerous, harsh environments and is compliant to varied purposeful security requirements, from the Automotive ASIL to Industrial SIL (for instance, VDE AR 2510-2/-50, IEC EN 61508, and others).
One new and distinctive answer for having probably the most environment friendly and dependable battery monitoring system includes the mixture of an 18-cell monitor and steadiness IC with a microcontroller to SPI slave remoted interface. A multicell battery stack monitor measures as much as 18 sequence linked battery cells with a complete measurement error of lower than 2.2 mV. The cell measurement vary of zero V to five V makes it appropriate for many battery chemistries. All 18 cells could be measured in 290 μs, and decrease knowledge acquisition charges could be chosen for prime noise discount. A number of stack monitor units could be linked in sequence, allowing simultaneous cell monitoring of lengthy, excessive voltage battery strings. Every stack monitor has an isoSPI™ interface for prime velocity, RF immune, lengthy distance communications. A number of units are linked in a daisy chain with one host processor connection for all units. This daisy chain could be operated bidirectionally, guaranteeing communication integrity, even within the occasion of a fault alongside the communication path.
The IC could be powered instantly from the battery stack or from an remoted provide. The IC consists of passive balancing for every cell, with particular person PWM responsibility cycle management for every cell. Different options embrace an onboard 5 V regulator, 9 general-purpose I/O traces, and a sleep mode the place present consumption is decreased to six μA.
LTC6813 software schematic
As a result of short- and long-term accuracy calls for of the BMS software, it makes use of a buried Zener conversion reference reasonably than a band hole reference. This gives a secure, low drift (20 ppm/), low temperature coefficient (three ppm/°C), low hysteresis (20 ppm) main voltage reference together with glorious long-term stability. This accuracy and stability are vital since it’s the foundation for all subsequent battery cell measurements and these errors have a cumulative affect on acquired knowledge credibility, algorithm consistency, and system efficiency.
Though a excessive accuracy reference is a vital characteristic to make sure superior efficiency, that alone is just not sufficient. The analog-to-digital converter structure and its operation should meet specs in an electrically noisy atmosphere, which is the results of the pulse-width modulated (PWM) transients of the system’s excessive present/voltage inverter. Correct evaluation of the state of cost and well being of the batteries additionally requires correlated voltage, present and temperature measurements.
To mitigate the system noise earlier than it could have an effect on the BMS efficiency, the stack monitor converter makes use of a ∑-Δ topology that’s aided by six person selectable filter choices to handle noisy environments. The ∑-Δ method reduces the impact of electromagnetic interference (EMI) and different transient noise, by its very nature of utilizing many samples per conversion, with an averaging filtering perform.
The necessity for cell balancing is an unavoidable consequence in any system that makes use of giant battery packs organized as teams of cells or modules, reminiscent of the large power storage items used to produce hospital microgrids and subgrids. Though most lithium cells are effectively matched when first acquired, they lose capability as they age. The growing older course of can differ from cell to cell on account of a number of components, reminiscent of gradients in pack temperature. Exacerbating the entire course of, a cell that may function past its SOC limits will prematurely age and lose extra capability. These variations in capability, mixed with small variations in self-discharge and cargo currents, result in cell imbalance.
To treatment the cell imbalance situation, the stack monitor IC instantly helps passive balancing (with a user-settable timer). Passive balancing is a low price, easy technique to normalize the SOC for all cells throughout the battery cost cycle. By eradicating cost from the decrease capability cells, passive balancing ensures these decrease capability cells will not be overcharged. The IC may also be used to manage energetic balancing, a extra difficult balancing method that transfers cost between cells by means of the cost or discharge cycle.
Whether or not carried out utilizing energetic or passive approaches, cell balancing depends on excessive measurement accuracy. As measurement error will increase, the working guard band that the system establishes should even be elevated, and subsequently the effectiveness of the balancing efficiency will probably be restricted. Additional, because the SOC vary is restricted, the sensitivity to those errors additionally will increase. A complete measurement error of lower than 1.2 mV is effectively inside system-level necessities for battery monitoring programs.
In power storage programs, a communication loop is necessary to attach all battery cells. This loop transmits knowledge from the system’s battery to a cloud-based power administration algorithm that tracks charging and discharging occasions to find out the easiest way to maximise battery use or to maintain the very best capability battery absolutely charged in case of an influence outage.
ADI’s LTC681x and LTC680x households symbolize the state-of-the-art for battery stack displays. The 18-channel model is known as LTC6813.
The battery stack monitor system wants to speak with the grasp unit the place a microcontroller or processor calculates the SOC and SOH values and regulates the charging and discharging profiles. Varied types of interconnection are attainable, the place the remoted communication channel is most well-liked for prime voltage functions, reminiscent of power storage programs (400 V to 1500 V) and moveable units with excessive capability batteries (40 V to 200 V).
The isoSPI characteristic constructed into the LTC6813 battery stack monitor, when mixed with an LTC6820 isoSPI communications interface, permits secure and strong info switch throughout a excessive voltage barrier. isoSPI is especially helpful in power storage programs that produce a whole lot of volts by way of series-connected cells, which require full dielectric isolation to reduce hazards to personnel.
LTC6813 remoted reference to an LTC6820
In these storage programs, the place greater than 18 cells are used, a number of LTC6813 BMS boards will must be interfaced collectively. Right here, a sturdy interconnection of a number of an identical PCBs, every containing one LTC6813, is configured for operation in a daisy chain. The microprocessor is situated on a separate PCB. To attain 2-wire isolation between the microprocessor PCB and the primary LTC6813 PCB, the LTC6820 help IC is used. When just one LTC6813-1 is required, it may be used as a single (non-daisy-chained) system if the second isoSPI port (Port B) is correctly biased and terminated.
The principle design problem for battery stack displays with balancing and communication features is to create a noise-free PCB structure design with vital hint routes removed from the noise sources, reminiscent of switching energy provides, giving clear alerts to the stack monitor. With ADI options, the stack monitor’s nice accuracy and precision will help optimize already-satisfactory designs. The batteries will then be effectively used, they may have a 30-percent longer lifetime, and they’ll function in a safer method.
To help clients in designing their closing merchandise, ADI gives a full vary of analysis programs and platforms for the battery monitor units, in addition to a whole portfolio of variants to adapt to all wants.
Concerning the Creator
Stefano Gallinaro joined Analog Devices in 2016 within the Renewable Vitality enterprise unit. He’s accountable for managing strategic advertising actions within the fields of photo voltaic power, electrical car, charging, and power storage, with a particular deal with energy conversion. Based mostly in Munich, he has worldwide enterprise accountability.
Stefano studied electronics engineering on the Politecnico di Torino, Italy (B.S.). He started his profession as an software engineer in STMicroelectronics Srl—DORA S.p.A., in Aosta, Italy. Earlier than becoming a member of ADI in 2016, he labored for 2 and a half years as a product advertising supervisor at Vincotech GmbH in Unterhaching, Germany.