To realize a specific voltage range on the battery pack level, which reduces the current that is drawn for a given power value, cells have to be connected in series (Figure 2
c), while parallel connections increase the capacity (Figure 2
b). In today’s systems, one possible variant is to use multiple cells with small capacity in parallel to form modules with higher overall capacity, which are then connected in series to increase the voltage (Figure 2
e), also see example Tesla Model S
below). Another variant is the usage of battery cells with a high capacity, which are connected in series (Figure 2
c), also see examples Mitsubishi i-MiEV
, or smart fortwo ed
below). Both variants are the most reasonable ones in terms of BMS complexity. A parallel connection of multiple strings of battery cells (e.g., for special redundancy requirements) would increase the expenditure for cell voltage monitoring, balancing, etc., by a factor of the number of parallel strings, while this way only one voltage measurement channel per parallel connection of n
cells is needed.
In some special cases, where size, weight and power consumption are very critical, battery modules are even built without single cell monitoring and balancing. Examples are the European Space Agency (ESA)’s Mars Express and Rosetta probes. According to [20
], there are three battery strings (composed of Sony 18650HC (Tokyo, Japan) [21
]), each with individual DC/DC converters, which connect them to a main bus. Single cell monitoring for the strings is not mentioned, however. In [21
], the authors claim that single cell monitoring is not necessary for the described application, given that a careful selection and testing of the cells are guaranteed. In [22
], it is stated that the cells should originate from the same lot, which means an uninterrupted unchanged production run with the same raw materials. Other research, however, shows that, although the cells that were examined seemed equal at the time of selection, their ageing behaviour can differ substantially (see Section 5
]). Thus, it is questionable if single cell monitoring really can be omitted, even when carefully selected cells of the same lot that seem to be totally equal at the time of manufacturing are used. Admittedly, the benefit of carrying battery monitoring electronics would be limited, as there would be no way to exchange damaged cells, once the spacecraft has been launched. Furthermore, a failure of these electronics could lead to the whole system failing. Apart from that, omitting single cell monitoring can be less critical for small systems with only a few cells in series. Especially when for a string the total voltage remains in a region, where theoretically (assuming identical voltages per cell) each cell remains way below its maximum allowed voltage and above its minimum allowed voltage, the probability of one cell becoming critical is comparatively small.
To implement the basic monitoring functionality needed for a safe operation of the batteries, several off-the-shelf Application-specific integrated circuits (ASICs) are offered by well-known semiconductor manufacturers. As there is a wide variety of available components, only representative examples, which are available or in use at the time of writing, are provided here.
For small personal electronic devices (smartphones, e-book readers, audio players, etc.) that often contain only one single battery cell, so-called “fuel gauges” ICs exist. These components represent the simplest form of a BMS ASIC. They provide functions like voltage monitoring, current measurement and temperature measurement for one cell. In addition, often some kind of SOC estimation, based on the measured values, is integrated. Some of these devices also contain further functional elements, like charging regulators.
For applications in need of higher power and/or with greater energy demand, the battery pack has to consist of several cells. ICs are offered for these kinds of systems that provide monitoring for several cells at once and also provide means for balancing, which is not needed in one-cell-systems. In these kinds of systems, usually more advanced functions are implemented in one central module or Electronic Control Unit (ECU), sometimes known as “BMS-Master”. Figure 3
shows the structure of a typical system of this class. As examples for tasks the master cares about, sophisticated SOC estimation, or power prediction algorithms, which need a certain amount of processing power, can be named. An overview can be found in [12
The modules carrying the front-end ICs are then often referred to as “BMS-Slaves”. They are used for basic functions like signal acquisition, filtering, etc., which are carried out by the monitoring ICs named above. (see e.g., [24
] or the examples in Section 3.3
). As examples, Texas Instruments’ bq76PL536A [9
] (used in Tesla Model-S (Palo Alto, CA, USA)
and possibly in smart fortwo ed
, see Section 3.3
), Linear Technology’s (Milpitas, CA, USA) LT6802G-2 [25
] (used in Mitsubishi Motors i-MiEV (Tokyo, Japan)
), Maxim Integrated’s (San Jose, CA, USA) MAX11068 [3
] (used in VW e-Up
), or AMS’ (Premstätten, Austria) AS8506C [26
] can be named. The TI bq76PL536A, MAX11068 and LT6802G-2 provide passive balancing, while the AMS device can be used for passive balancing topologies, but also provides active balancing capabilities using an external transformer. More on balancing can be found in Section 5
. At the time of writing, there are already successors available for most of the named ICs, e.g., TI bq76PL455 [27
] or LTC6811 [28
]—also see Section 4.2
. As mentioned in the section on requirements, a certain level of redundancy regarding voltage monitoring is usually desired. Thus, so-called secondary protection ICs often can be used in combination with the described ICs (or are even contained in the same package) to provide an additional level of safety. Another possibility would be the usage of a completely redundant BMS, which, however, would largely increase the costs.
3.3. Real-World Examples
As part of a research project conducted at the authors’ institute, the traction batteries of several commercially available electric vehicles have been disassembled, in order to analyse the cell ageing after a certain time of usage. As a side-product, the authors of this work were able to analyse and compare the different systems, while the disassembly was done. Thus, in the following, four real-world examples can be provided. (The data provided regarding the Tesla Model-S battery are based on a presentation by AVL [29
] and our own analysis of a single module, purchased separately. The other batteries were available and disassembled completely.).
Example (1) Mitsubishi i-MiEV
The first example is the traction battery of a Mitsubishi i-MiEV
(initial registration: February 2014), shown in Figure 4
a. It contains 10 Modules of eight cells and two modules with four cells, which leads to a total amount of 88 prismatic cells, all connected serially using screwed contacts. On top of each of the modules, a PCB is mounted, which—among other things—contains an LTC6802G-2. This IC is designed to monitor up to 12 lithium-ion cells, which are connected in series. The same PCB design is used for the module versions with four and eight cells. When used with four cells, the PCB is not fully populated, as four of eight available channels are not needed. The eight-cell modules use a second PCB to connect the second half of the module to the four remaining channels. The PCB on top of the modules is called the Cell Management Unit (CMU) in the official service manual for the car [30
]. In addition to voltage measurement, each PCB contains three temperature sensors, which are connected to a controller located next to the Linear Technology BMS IC.
Apart from the cell modules, the battery housing contains contactors (separate ones for the connection to the inverter and for DC charging), fuses, a service plug, an LEM (Freiburg im Üechtland, Switzerland) current transducer, an insulation monitor and a fan to extract cooling air. The service plug splits up the pack into two sections when it is removed. In the car, it can be found below the left hand seat. The main fuse also splits the pack in the middle. The cooling air originates from the vehicle’s air conditioning system and is partially guided by short plastic pipes and forwarded to the outside by the mentioned fan.
Not contained in the battery housing is the BMS master, or Battery Management Unit (BMU) [30
], which communicates to the rest of the vehicle. It is located below the rear bench seat of the vehicle. The contactors, the current transducer and the insulation monitor are connected directly to the BMU. The CMUs on top of the cell modules are connected to each other and to the BMU via an internal CAN bus. Some of the signals on the battery internal CAN bus can also be found on the car’s main CAN bus in lower resolution. Compared to the smart’s battery, which is described below, there is a lot of free space in the iMiEV’s battery housing, which may be a side effect of the air cooling.
Unfortunately, for the remaining vehicles/batteries, no documentation from the manufacturers was available, thus the information provided on the other systems is less detailed than for the i-MiEV.
Example (2) Smart Fortwo Electric Drive (Dt. Accumotive)
The second example is a battery pack taken from a smart fortwo electric drive
(third generation, initial registration: September 2014), built by Deutsche Accumotive
, which is shown in Figure 4
c). It consists of 90 pouchbag cells, connected in series by welded connections. The cells are mounted in plastic frames, organized in three rows positioned side by side with cooling plates for a liquid cooling system mounted on top.
The basic monitoring tasks are performed by ICs of TI with anonymized product code. A comparison of the PCB found in the battery pack and the specifications of bq76PL536A, however, showed that the used product is at least very similar to bq76PL536A, which is also built into the Tesla Model-S battery that was available for analysis (see below). Each of the three PCBs contains six monitoring ICs and one microcontroller with a galvanically isolated connection to the rest of the system (most likely CAN). TI’s bq76PL536A can be daisy-chained in order to need only one communication connection to the system, which can also be observed here. For cell voltage measurement, there are connections of flexible PCB between the cells’ terminals and the BMS PCBs located between the terminals. In addition, several temperature sensors can be observed. A module that seems to be the BMS master (Bosch) can be found next to the pack’s communication signal connector. The main contactors and a fuse are placed next to the power connector.
This battery showed the highest integration efforts of all specimens examined in this work’s context. The whole BMS is located in the battery’s case, it is built very space efficient and uses few cables. Altogether, it appears to be designed holistically, having the complete system in mind, as opposed to the next example.
Example (3) VW e-Up
Another battery pack that has been disassembled originates from the Volkswagen e-Up
(2013, initial registration: February 2014). It is shown in Figure 4
b. The pack contains 17 serially connected modules (Figure 5
b), each consisting of six serially connected pairs of two prismatic cells.
The e-Up battery does not have a cooling system or a service disconnect, dividing the battery into two halves. The BMS modules are centralized, and the white box on the left side of Figure 4
b) contains the measurement ICs (or BMS slave) for the whole battery pack. To the right of it, below a black cover, the contactors, fuse and current measurement can be found. The other white box contains some kind of BMS master. A large amount of voltage measurement wires connects the individual cells to the slave module, where Maxim’s MAX11068 [3
] is used for the measurement and balancing tasks with MAX11081 [8
] as a secondary protection device. A closer look at the PCB shows that nine MAX11068 (12 voltage measurement channels each) are daisy-chained via I2C, which is also used to connect the last IC to the rest of the system. There is no microcontroller converting to a more robust field bus, like e.g., CAN. Apart from that, the slave’s PCB is filled with a large amount of balancing resistors taking up most of the space.
Example (4) Tesla Model-S
An exception among the named examples is the Tesla Model-S battery. Here, only one module was available for analysis (see Figure 5
a). According to [29
], the whole battery consists of 7104 single 18,650 cells, divided into 16 modules in a 96s74p configuration, which means that each module contains a series connection of 6 × 74 parallel cells. The individual cells are connected via bond-wires, which also act as fuses. The nominal voltage is 355 V and a liquid cooling system exists. For the BMS’ cell-monitoring functions, TI’s bq76PL536A-Q1 is used, which is placed on a PCB mounted to one side of the module. The cell voltage measurement is performed using wires welded to the connecting plates of the parallel connections.
Comparing the different battery packs shown here, it can be determined that there are significant differences in the construction, especially regarding the level of integration observed. While the smart ed pack features large scale integration, the batteries of VW e-Up and Mitsubishi i-MiEV show only very moderate integration efforts. The former pack contains comparatively little cabling. The cell monitoring ICs are mounted on oblong PCBs on top of the battery modules in a very space saving manner, while the BMS-master is also located inside the pack, next to the signal connector to the car. In the i-MiEV battery, the cell monitoring ICs are also mounted on PCBs on top of the battery modules, taking a bit more space. The BMS-master that also controls the contactors is not contained in the pack’s housing, which is why a certain amount of cabling is necessary to connect the components to each other. A part of the cables found in the i-MiEV battery could be reduced by putting the BMS-master into the pack and by organizing the power supply cabling more reasonably. The e-Up pack finally is an example for a central monitoring unit. Every cell voltage is acquired using long voltage measurement cables, which connect cell modules to the monitoring unit in a star layout.
Apart from these differences, the BMS components are very similar in terms of their functionality. All mentioned examples use specialized monitoring ICs to implement the basic functions, which are supervised by a central ECU (BMS-master) for higher functions, or central functions like contactor control.