Battery-powered functions have develop into commonplace during the last decade, and such gadgets require a sure degree of safety to make sure protected utilization. The battery administration system (BMS) displays the battery and potential fault situations, stopping the battery from conditions through which it could actually degrade, fade in capability, and even doubtlessly hurt the consumer or surrounding atmosphere. It’s also the accountability of the BMS to offer an correct state-of-charge (SoC) and state-of-health (SoH) estimate to make sure an informative and protected consumer expertise over the lifetime of the battery. Designing a correct BMS is crucial not solely from a security viewpoint, but in addition for buyer satisfaction.
The principle construction of an entire BMS for low or medium voltages is often made up of three ICs: an analog front-end (AFE), a microcontroller (MCU), and a gas gauge (see Determine 1). The gas gauge could be a standalone IC, or it may be embedded within the MCU. The MCU is the central factor of the BMS, taking info from each the AFE and gas gauge whereas interfacing with the remainder of the system.
Determine 1 Block diagram of a BMS’s structure.
The AFE gives the MCU and gas gauge with voltage, temperature, and present readings from the battery. For the reason that AFE is bodily closest to the battery, it is suggested that the AFE additionally controls the circuit breakers, which disconnect the battery from the remainder of the system if any faults are triggered.
The gas gauge IC takes the readings from the AFE, then makes use of complicated cell modeling and superior algorithms to estimate key parameters, such because the SoC and SoH. Much like the AFE, a few of the gas gauge’s duties will be included within the MCU code; nevertheless, utilizing a devoted gas gauge IC presents a number of benefits:
- Environment friendly design: Utilizing devoted ICs to run complicated gas gauge algorithms permits designers to make use of MCUs with decrease specs, lowering total price and present consumption.
- Improved insights and security: A devoted gas gauge can measure the person SoC and SoH of every collection cell mixture within the battery pack, which permits extra exact measurement accuracy and growing older detection over the lifespan of the battery. That is essential as a result of cell impedances and capacities can diverge over time, resulting in run-time and security implications.
- Quick time-to-market: Gasoline gauge ICs come absolutely examined for a wide range of conditions and check circumstances. This reduces the time and value of testing complicated algorithms, whereas concurrently enabling sooner time-to-market.
Bettering SoC and SoH accuracy
The principle aim when designing an correct BMS is to ship a exact calculation for the battery pack’s SoC (remaining runtime/vary) and SoH (lifespan and situation). BMS designers might imagine the one strategy to obtain that is to make use of a really costly AFE with exact cell voltage measurement tolerance, however this is only one issue within the total calculation accuracy. Crucial elements are the gas gauge cell mannequin and gas gauge algorithm, adopted by the flexibility of the AFE to ship a synchronous voltage-current studying for the cell resistance calculation.
The gas gauge makes use of its inner algorithm to run complicated calculations that convert the voltage, present, and temperature measurements into SoC and SoH outputs by analyzing the connection of those values with the precise cell mannequin saved in its reminiscence. The cell mannequin is generated by characterizing the cell throughout totally different temperature, capability, and loading situations to mathematically outline its open-circuit voltage, in addition to resistive and capacitive elements. This mannequin permits the gas gauge’s algorithm to calculate an optimum SoC based mostly on how these parameters change throughout totally different working situations. Subsequently, if the gas gauge’s cell mannequin or algorithm are usually not correct, the ensuing calculation isn’t correct, irrespective of how exactly the AFE’s take measurements. In different phrases, implementing a extremely correct gas gauge has the biggest impact on the BMS’s SoC accuracy.
Voltage-current synchronous studying
Though virtually all AFEs present totally different ADCs for voltages and currents, not all of them present precise synchronous present and voltage measurements for every cell. This characteristic, referred to as voltage-current synchronous studying, permits the gas gauge to precisely estimate the cell’s equal collection resistance (ESR). For the reason that ESR modifications throughout totally different working situations and over time, estimating the ESR in actual time permits for extra correct SoC estimates.
Determine 2 reveals how the SoC error with synchronous studying is considerably decrease than the error with out synchronous studying, particularly after just a few discharge cycles. These outcomes had been extracted utilizing the MPF42791, which integrates ESR detection and thermal modeling.
Determine 2 SoC error comparability with and with out synchronous studying.
AFE direct fault management
As talked about beforehand, a very powerful position the AFE performs within the BMS is safety administration. The AFE can instantly management the safety circuitry, defending the system and the battery when a fault is detected. Some programs implement the fault controls within the MCU, however this leads to an extended response time and requires extra sources from the MCU, growing firmware complexity.
Superior AFEs use their ADC readings and consumer configurations to detect any fault situations. The AFE reacts to faults by opening the safety MOSFETs to make sure true {hardware} safety. AFEs are additionally absolutely examined, which makes it easy to ensure a strong security system. On this method, the MCU can be utilized as a secondary safety mechanism for the next degree of security and robustness.
The MP279x household integrates each types of safety management. This enables designers to pick whether or not the fault responses and/or protections are managed by way of the AFE or MCU.
Excessive-side vs. low-side battery protections
When designing a BMS, you will need to contemplate the place the battery safety circuit-breakers are positioned. Typically, these circuits are applied with N-channel MOSFETs since they’ve a decrease inner resistance in comparison with P-channel MOSFETs. These circuit-breakers will be positioned both on the excessive aspect (constructive terminal of the battery) or the low aspect (damaging terminal of the battery).
Excessive-side architectures be certain that the bottom (GND) is all the time effectively referenced, which avoids potential security and communication points when there are quick circuits. As well as, a clear, fixed connection to GND helps cut back reference sign fluctuations, which is essential for exact MCU operation.
Nonetheless, driving the gate of the N-channel MOSFETs when they’re positioned within the battery’s constructive terminal requires voltages greater than the battery pack voltage, which makes the design course of tougher. In consequence, devoted cost pumps built-in into the AFE are generally used for high-side architectures, which will increase the general price and IC present consumption.
For low-side configurations, the cost pump isn’t essential because the safety MOSFETs are positioned on the battery’s damaging pole. Nonetheless, it’s harder to implement efficient communication in low-side configurations as a result of there isn’t any GND reference when the protections are open.
The MP279x household makes use of a high-side structure which gives sturdy safety whereas minimizing the BOM. As well as, high-precision cost pump management permits for an N-channel MOSFET tender turn-on perform, which doesn’t require any further pre-charge circuitry, additional minimizing BOM measurement and value. Comfortable turn-on is achieved by slowly growing the safety FET’s gate voltage, permitting a small present to circulate by way of the protections to pre-charge the load (see Determine 3). A number of parameters will be configured to make sure a protected transition, corresponding to the utmost allowed present, or the time till the safety FETs are closed with out triggering a fault.
Determine 3 Comfortable turn-on scheme for the MP279x household.
Cell-balancing to increase battery life
Battery packs that energy bigger programs (e.g., e-bikes or vitality storage) are made up of many cells in collection and parallel. Every cell is theoretically the identical, however as a consequence of manufacturing tolerances and chemical variations, every cell typically has barely totally different conduct. Over time, these discrepancies develop into extra important as a consequence of totally different working situations and growing older, severely impacting the battery efficiency by limiting its usable capability or doubtlessly damaging the cells. To keep away from these hazardous conditions, it’s essential to periodically equalize the cell voltages in collection by way of a course of referred to as cell balancing.
Passive balancing is the commonest strategy to equalize cell voltages, and it requires discharging probably the most charged cells till all of them have equal cost. Passive cell-balancing in AFEs will be completed externally or internally. Exterior balancing permits for bigger balancing currents, but in addition will increase the BOM (see Determine 4).
Determine 4 Block diagram of exterior cell-balancing.
Then again, inner balancing doesn’t improve the BOM nevertheless it typically limits the balancing present to a decrease worth as a consequence of warmth dissipation (see Determine 5). Contemplate the price of the exterior {hardware} and the goal balancing present when deciding between inner and exterior balancing.
Determine 5 Block diagram of inner cell-balancing.
One other essential side of cell-balancing is the bodily connections. For instance, the MP279x AFE household makes use of the identical pins for each voltage sensing and balancing. This considerably reduces the IC measurement, however signifies that consecutive cells can’t be balanced concurrently, growing the time it takes to carry out the cell balancing. Utilizing devoted balancing pins reduces the balancing time, however considerably will increase the IC measurement and total price.
AFE Security Capabilities
As defined all through this text, the AFE controlling the system’s protections and fault responses is extraordinarily essential in BMS designs. Previous to opening or closing the safety FETs, the AFE should be capable to detect these undesirable situations.
Cell- and pack-level faults, corresponding to over-voltage (OV), under-voltage (UV), over-current (OC), short-circuit (SC), over-temperature (OT), and under-temperature (UT) faults ought to all be monitored. Nonetheless, there are different helpful protections and functionalities that AFEs can supply for sure functions. For instance, self-testing permits the IC to detect whether or not its inner ADC is malfunctioning, which prevents the system from incorrect measurements. Enhanced watchdog timer functionalities additionally guarantee robustness and safety when the primary MCU isn’t responding.
The MP279x household gives the faults protections listed above with a excessive diploma of configurability, enabling the consumer to outline totally different thresholds, deglitch occasions, and hysteresis for every fault. These gadgets additionally depend on two totally different comparators for SC and OC fault situations to attenuate the response time. They provide configurations for fault auto-recovery as effectively, which means that they’ll robotically get well from most faults with out requiring any motion from the MCU.
Conclusion
The BMS displays the battery pack to guard each the battery and the remainder of the system. A substandard BMS not solely reduces the system’s security, nevertheless it additionally gives inaccurate battery SoC administration. These inaccuracies have a really important impact on the product’s remaining high quality, as they may end up in doubtlessly harmful faults, or faults that negatively impression consumer expertise. To mitigate these points, this text defined what designers ought to count on and search for when designing their BMS. Be taught extra about how battery administration programs work and the best way to design them with MPS’s BMS analysis kits.
Tomas Hudson is an software engineer at Monolithic Energy Programs.
Miguel Angel Sanchez is a battery software engineer at Monolithic Energy Programs.
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