Journal of Power Sources 162 (2006) 719–726 An electromechanical transfer circuit to measure individual battery voltages in series packs Abhiman Hande a,∗ , Sukumar Kamalasadan b a Erik Jonsson School of Engineering and CS, University of Texas at Dallas, P.O. Box 830688, EC33, Richardson, TX 75083, United States b Division of Engineering and Computer Technology, University of West Florida, Building 77, Room 131F, Pensacola, FL 32514, United States Received 25 April 2006; received in revised form 8 June 2006; accepted 9 June 2006 Available online 4 August 2006 Abstract A novel approach has been developed to measure the voltages of individual batteries used in electric vehicle (EV) battery packs using a unique selective battery measurement system. This system consists of a voltage measurement circuit that measures battery voltages using a set of electromechanical relays connected in a matrix formation. A 16-bit microcontroller was used for controlling the operation of the relay matrix circuit. The system was designed for a pack of 12 series connected 12 V dc lead–acid batteries. The proposed approach was found to be compact and is a universal one that can be used for any type of battery. Moreover, the method was very effective and produced good accuracy. In fact, test results over a wide temperature range of -20 to +40 ◦ C indicated that the method is very precise with voltage fluctuations less than ±30 mV. © 2006 Elsevier B.V. All rights reserved. Keywords: Electric vehicle; Voltage measurement; Lead–acid; Batteries; Relays 1. Introduction Several applications such as electric vehicles and uninterrupt- ible power supplies require the use of series connected battery packs for adequate power. These packs often need precise and autonomous voltage measuring schemes for accurate battery voltage measurements from time to time. The battery voltage is a good indicator of whether any battery is losing charge due to extraneous factors. Some of the factors that contribute towards reduction in battery life or charge retention include the type of battery cell design, ambient temperature, and length of usage/storage [1,2]. This means that if there are certain subtle differences between individual batteries in a pack, the batter- ies will not charge/discharge in a uniform manner, resulting in overcharge and excessive discharge in some units. All batteries must remain within a high and low voltage operating range to prevent damage. During the discharge cycle, batteries which are less efficient tend to go out of voltage bal- ance before the rest. This phenomenon limits the total battery ∗ Corresponding author. Tel.: +1 972 883 6563; fax: +1 972 883 2710. E-mail addresses: abhiman.hande@utdallas.edu (A. Hande), skamalasadan@uwf.edu (S. Kamalasadan). capacity. Also, during the charge cycle, batteries which are more efficient tend to get charged a little higher than the rest, resulting in an overcharge. Batteries such as nickel metal hydride (NiMH) and lead–acid when overcharged are subject to an oxygen recom- bination cycle at their negative electrodes, and this causes their cycle life to be significantly reduced over a period of time [1,2]. It is possible to obtain a fair idea of the state of health of a battery by keeping track of its terminal voltage. For example, the open circuit voltage (OCV) and state of charge (SOC) for a 12 V dc , 13 ampere-hour (Ah) EnerSys Genesis G13EP lead–acid battery have the following linear relationship for the 10–100% SOC range [16]: SOC = OCV - 11.6 0.0126 (1) Therefore, the SOC of each battery in series connected packs can easily be predicted by measuring its terminal voltage after some period of time at rest. This information is used to iden- tify weak batteries so that they can receive an additional boost charge to balance the pack. It is therefore imperative that battery voltage monitors be accompanied with some type of equaliza- tion scheme to maintain voltage balance [3–8]. In fact, these systems combine to form battery management systems (BMSs) that monitor several critical battery parameters, such as the volt- 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.06.014