By Bill Schweber for Mouser Electronics
Electronic engineers involved with electric vehicles (EV/HEV) generally have to work with the available battery technologies, which are usually some variant of lithium-ion based chemistry. However, they do have a significant challenge in managing the large assembly of cells which comprise the battery packs. The battery management system (BMS) which designers implement has three major objectives:
- Protect the individual cells and the complete battery assembly from damage
- Prolong the life of the batteries
These objectives translate to a lengthy list including functions such as cell protection, charge control, state of charge determination, state of health determination, and cell balancing. This article looks at one of the BMS functions – cell balancing – which is representative of the intense challenges that EV/HEV designers must address.
The reality of the overall EV/HEV design is that all BMS issues are interrelated to some extent rather than isolated (see Figure 1.) Therefore, there is a “ripple effect” as the battery condition or status changes and the BMS deals with them. One of the objectives of the BMS architecture is to separate these subfunctions to the best extent possible, so each one can be independently optimized and so contribute to a globally optimized design.
Further, as in most engineering decisions, there is no single “right” way to achieve a given goal. Each approach brings tradeoffs among factors of size, packaging, replaceable units, weight, data integrity, system confidence, and cost. The decision also depends on the objectives: longest range, longest battery-pack life, tolerance of weakness in individual cells in a pack, and safety, of course, to cite a few. The “best” solution is therefore dictated by the design priorities.
Cell Balancing: a Complex Issue
An unavoidable fact is that in multi-cell battery chains, there will always be small differences between cells due to production and operating conditions (especially temperature gradients, which can be significant for large battery packs).These differences are magnified with each charge/discharge cycle, with weaker cells becoming even weaker until they eventually fail, and cause premature failure of the larger battery pack.
Cell balancing compensates for weaker cells by attempting to equalize, or balance, the charge on all the cells within the pack. Various methods of cell balancing have been developed to address this problem. The approach is also a function of the battery chemistry: lithium-based batteries are more tolerant of the “micro” charge/discharge cycles associated with HEVs, but they are more affected by cell-to-cell differences. In contrast, there is some natural cell balancing by increasing the charging time with lead acid and NiMH cells, since the fully charged cells will release energy by out-gassing until the weaker cells reach their full charge.
The two most-common cell-balancing techniques are active and passive balancing; other approaches, such as charge shunting and lossless balancing are also used but, as always, there are difficult tradeoffs. Both start by monitoring the state of charge (SOC) of each cell. It is measured by “coulomb counting” of current flow into and out of the battery, sometimes supplemented by a battery-impedance measurement. In some situations, only the voltage across each cell is measured. Switching circuits then control the charge applied to each individual cell in the chain during the charging process to equalize the charge on all the cells in the pack.
In active balancing, charge is removed from higher-charged cells and passed to lower-charged cells. This is a time-consuming process since it must be done by assessing each cell in what may be a very large number (hundreds and thousands). Some active cell-balancing schemes are designed to stop charging a cell which is fully charged and continue charging the weaker until they reach full charge, thus maximizing the battery’s charge capacity.
In passive balancing, excess energy in higher-charged cells is automatically drained through a bypass resistor until the voltage or charge matches the voltage on the weaker cells. It is a low-cost option but wastes energy in the bypass resistors, and also bounds battery-pack performance by the weakest cells. Regardless of method used, squeezing out the last percentage points of capacity and performance will add greatly to BMS system complexity, BOM size, hardware size and cost, and software integration issues.
Targeting this BMS and cell-balancing challenge, the Maxim Integrated MAX14920 and similar MAX14921 battery measurement analog front-end (AFE) ICs accurately sample cell voltages for battery packs up to 16 cells at up to +65 V (the MAX14920 handles up to 12 cells, while the otherwise-identical MAX14921 is for 16 cells, see Figure 2.) The devices simultaneously sample all cell voltages for accurate state-of-charge and source-resistance determination, a time-saving feature for larger packs. All cell voltages are level-shifted to a ground reference at unity gain, which greatly eases the data-conversion ranging for the external analog/digital converter (ADC).
[‘pAccuracy is an issue with battery-cell monitoring, especially with chemistries which have fairly flat discharge curves. The high accuracy of these Maxim ICs makes them a good fit for cell chemistries with very flat discharge curves, such as lithium-metal phosphate. Their low-noise, low-offset amplifier buffers any differential voltages of up to +5 V, allowing monitoring of all common lithium-ion (Li+) cell technologies.
Combined with the internal self-calibration feature, the resulting cell voltage error is ±0.5 mV. The Maxim ICs are specified over the -40°C to +85°C extended temperature range, a necessity for the EV/HEV operating environment.
Don’t Ignore Connections and Amperes
For engineers whose exposure to “high power” is limited to a few hundred watts or under ten amps, the world of EV/HEV power interconnection needs a very different mindset. EV/HEV designs inherently deal with large flows of current between the various subsystems and assemblies, and at high voltages. Designers must select wire-to-board and wire-to-wire connectors which meet challenging criteria for power-handling, insertion/removal cycles, and mechanical ruggedness under very difficult conditions of vibration, stress, and temperature. As a result, there are unique considerations for power connectors associated with the battery subsystem. There can be no “cutting corners” or “we’ll worry about that later” attitudes when dealing with these high-current/voltage issues in the EV/HEV environment-they must be an early, prominent part of the design process.
Phoenix Contact’s E-Mobility Solutions are an example of a connector family meant for the EV/HEV platform, as the series offers a variety of body styles and contact arrangements (see Figure 4). Units in the family can handle up to 12 AWG and 16 AWG depending on contacts selected, at up to 25 A; size 12 contacts will accept wire ranges of 12-14 AWG; Size 16 contacts will accept 14-20 AWG.
The Combined Charging System (CCS) is a standard-compliant charging system for electric vehicles, which supports both conventional AC charging and fast DC charging. Both vehicle connectors fit into the CCS vehicle inlet. Phoenix Contact e-mobility offers a comprehensive range of charging plugs for all international standards for fast DC charging as well as for charging via AC connections. In addition to the comprehensive portfolio, Phoenix Contact also develops individual solutions for special customer requirements, even those not covered by standards.
Phoenix Contact offers the complete range of charging solutions, including connectors from a single source: Type 1, Type 2, and GB/T standard, as well as charging controllers for every e-mobility application from residential and commercial, to large, public-use EV charge controllers and beyond to custom solutions.
The path from the invisible but vital chemistry of battery cells and packs, to the highly visible and tangible connectors and contacts which route the battery-pack current of EV/HEVs requires careful, extreme attention to many details and subtleties. Issues which would be minor or modest in portable consumer devices are major ones in these applications, given the voltages and power levels, the operating environment, and user expectations. Battery-pack performance under normal extreme conditions, and especially under abnormal conditions, encompassing thermal management, cell balancing, connector IR drop, and connectors with solid retention but easy disengagement are a few of the many issues. These must be examined, reviewed, and resolved from many perspectives, with a clear focus on priorities, tradeoffs, and the interplay between solutions.