Proper electric and thermal management of an HEV battery pack, consisting of many modules of cells, is imperative. During operation, voltage and temperature differences in the modules/cells can lead to electrical imbalances from module to module and decrease pack performance by as much as 25%. An active battery management system (BMS) is a must to monitor, control, and balance the pack. The University of Toledo, with funding from the U.S. Department of Energy and in collaboration with DaimlerChrysler and the National Renewable Energy Laboratory has developed a modular battery management system for HEVs. This modular unit is a 2 nd generation system, as compared to a previous 1 st

A Modular Battery Management System for HEVs

A selective equalizer for NiMH batteries

Due to the limited operating windows of lithium-ion batteries regarding temperature, voltage, and current and the dangerous situations that can arise if those operating windows are violated, a battery management system (BMS) is required to supervise and control the batteries in a multicell battery energy storage system. This chapter presents the requirements for an automotive BMS, the different possible topologies, and a typical implementation of a master/slave BMS for hybrid electric vehicle or battery electric vehicle applications, with a focus on the challenging design aspects. The chapter concludes with future trends and pointers to sources of further information.

High-voltage battery management systems (BMS) for electric vehicles

A unique selective equalization method has been developed to equalize nickel metal hydride (NiMH) battery packs. This technique uses a new selective boost equalizer that detects batteries either at a very low state of charge (SOC) or at an extremely high SOC. In this system a set of electromechanical relays is connected in a matrix to route boost current to the weaker batteries. A 32-bit microcontroller is used to control the relay switching, and the boost current is supplied by a separate boost charger. Once a weak battery is detected, it is scheduled for a specific boost time by a special "round robin" algorithm. The equalizer was tested on a pack of twelve series connected 12 V 93 ampere hour NiMH batteries. Test results show that the equalizer was able to rebalance an artificially unbalanced pack, and the capacity was increased by 27% within six charge-discharge cycles. The number of cycles required to rebalance the pack was significantly reduced by using the "round robin" algorithm.

A selective boost equalizer for series connected NiMH battery packs

An Op Amp transfer circuit to measure voltages in battery strings

Although this might seem like a fairly simple problem, obvious solutions prove to be inadequate for high performance batteries such as the lithium ion and nickel metal hydride packs being used in electric vehicles. Batteries of this type have higher energy and power densities than other available types, and they require very precise monitoring to insure safety and optimum performance. Requirements vary somewhat for each type of battery, but a typical voltage monitoring system should meet specifications similar to those listed below.

D.B. Garrett

Papers

Transfer circuit for measuring individual battery voltages in series packs