Highly accurate monitoring and protection functionality for Li-ion battery packs

Author : Finn Lange, ROHM Europe

31 January 2014

Li-ion batteries have proven to be an efficient high performance power source; however, safety functions such as voltage monitoring, overcharge and overdischarge detection of each battery cell, cell balancing and temperate measuring play a crucial role in order to secure a stable and safe operation and an extended lifetime.

These rechargeable battery packs are commonly used in hybrid electric vehicles or electric vehicles (HEV/EV), Power Tools and E-Bicycles, application areas which are expected to rapidly grow over the next few years. Exactly addressing the needs of those target applications, ROHM has developed a complete multi-cell control and protection solution consisting of an analogue front end (AFE) and low power microcontroller (MCU). This chipset can handle a system using lithium-ion batteries, which requires multiple protection functions.

Li-ion batteries have gained their share as they are highly energy-efficient but aiming for higher performance, they are also sensitive towards unbalanced cell-charge. Therefore, a lot of surveillance functions are needed in order to ensure proper operation of the system and keep the performance level and lifetime high. In multi-cell battery stacks, even small charge differences between the cells tend to increase with each charge and discharge process. Lower cells are overstressed through charging, becoming even weaker until they might fail and cause a failure of the whole battery. So there must be a continuous monitoring of the battery status and in case of any abnormal state, the chipset can prevent a deterioration of single cells. This means, that when an overcharge voltage is detected, the charging of the cell has to be stopped. Also, in case a low voltage is discovered, the further discharge of this cell has to be terminated. There have been different models to meet these needs and balance the charges.

Within conventional models, a microcontroller runs complicated algorithms which result in cell status checks and then turning to action. Unbalanced differences in cell voltages during charge or discharge are then usually corrected via a cell-balancing model that includes by-passing cells with higher voltage. Compared to this, complex compensation algorithms are necessary in a sequential cell monitoring approach in order to achieve the required voltage and current measurement across the entire cell stack. In the ROHM solution, the monitoring takes place locally, enabling a more accurate, streamlined and faster cell management without relying on complicated communication links, host administration or software set-ups. The built-in 100mA cell-balancing switches reduce connection pins, keep the impedance low, reduce the degradation risk and eliminate the need of an external circuit for cell balancing. The passive cell balancing function automatically discharges all cells to the same voltage level so that a long and stable lifetime of the whole pack can be guaranteed.

For temperature measuring ML5238 has the option to connect up to 8 thermistors to detect the temperature at different locations in the battery pack. Based on this temperature measurement, there are four temperature different states so that different temperature ranges for charging and discharging can be specified, which is related to the fact that battery manufacturers allow charging and discharging only in dedicated temperature windows. These ranges are settable between -30°C and +85°C.

The voltage detection accuracy is typically at +/- 10mV and the charge/discharge measurement accuracy at +/-0.5A. The overcharge voltage can be set between 3.3 and 4.4V so that different Li-Ion battery types can be supported. The overdischarge voltage is in the range of 2.0 to 3.0V. To comply with the voltage ratings of new generation MCUs, the output voltage of each cell is only half of the battery cell. The short-circuit detection function is also defined by parameters. The detecting threshold voltage can be set to 0.1V to 0.4V in 0.1V steps. The detecting delay time is set by external capacitor. With this function the sensitivity of the short current mode can be designed to the needs of the application.

The maximum rating voltage is at 86.5V due to the high voltage wafer process.
Furthermore, the battery remaining charge display function is easily configured using up to five external LEDs. The LED driver for driving this gas-gauge LEDs resp. the battery balance LEDs at a maximum of five levels (20% steps) are already integrated into the AFE. The ICs also integrate the Gate driver for an automatic ON/OFF control of the external N-ch FETs for charging and discharging. This helps designing isolated charge and discharge paths. It is also possible to detect individual charger connection and load connection with separated pins.

For optimum power efficiency, the quiescent current consumption of the control IC needs to be kept low as well. The operating current consumption is similar to that of the conventional models but the current consumption in the power-down mode is minimised to around zero in order to especially reduce the load on the battery pack during long-term storage. At normal mode it typically features 250µA, at power save mode 80µA and at power-down mode 0.1µA. In the power save state a basic protection function (VREG and VREF outputs, FET driving and short circuit detection is still working).

The chipset has two different memory modes for evaluation and final product. During evaluation, the parameters can be stored in external EEPROM so that an easy change of the parameters can be done. For the final product, the parameters can be stored in the integrated Flash of the MCU to save system cost.

For a complete protection solution, ROHM offers the external MCU ML610Q486P which can be used in combination with the ML5238. The MCU is an 8bit high-performance controller based on the low power Microcontroller series ML610Q4xx with integrated Flash memory to store the register settings (see also the above). The monitoring IC can provide with its internal regulator the 3.3V power supply to the MCU so that no external voltage regulator is needed for this purpose. For evaluation of the chipset, ROHM can provide a complete evaluation system consisting of all electronic components needed for the battery pack. Together with the supplied monitoring software running on the connected PC the designer of the electronics can test and evaluate all functions of this chipset. It also helps to develop the application based software for series production of the battery pack electronics.

The IC includes additional self-diagnosis functions, which are activated and analysed by the external MCU. So it is possible to detect open and short cell connections as well as communication errors between the MCU and the AFE, monitoring the internal state by a UART interface. These features are especially needed in high-reliability applications like the electric vehicle. Based on the results of the voltage control of each cell or the entire battery pack, as well as the monitoring of the charge and discharge current, it can perform corresponding actions to protect the batteries and to prevent the system to get into an irregular state. With this set-up, the ROHM solution can be adapted to various systems according to the customer’s needs. This kind of system is also a perfect solution for uninterruptible power supplies (UPS), electrical bicycle and battery driven power tools and medical devices with higher current demand.


Summary
ROHM achieves to secure battery safety and longevity via providing reliable battery monitoring and fault detection while simultaneously facilitating design needs (including software aspects) and delivering a highly power efficient solution, ideal for applications where run time, reliability energy efficiency and service lifetime are essential.


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