Experimental study of the thermal behavior of a water cooled Ni–Cd battery Jean-Michel Mottard 1 , Ce ´cile Hannay 1 , Eric L. Winandy * Laboratory of Thermodynamics, University of Lie `ge, Campus du Sart Tilman-Ba ˆt. B49, B-4000 Lie `ge, Belgium Received 2 June 2002; received in revised form 19 December 2002; accepted 22 December 2002 Abstract In this paper, the Ni–Cd battery principle and exothermal processes are first recalled. It is pointed out that heat generation can be decomposed into irreversible losses (entropic effect), battery electric resistance and polarization effect. It is shown how these effects can be analyzed from overall battery heat balance. The calorimetric test bench is described. Battery cycles are composed of charge, overcharge, rest and discharge. Typically, the total heat losses range around 25% with an increase up to 33% at 200 A. Only during overcharge, gas losses are shown to be a high contribution. The paper shows that the calorimetric study permits not only further insight into local temperatures and heat losses, but also to deduce such parameters as entropy generation and, above all, total resistance (Joule effect and polarization). In this case, resistance was estimated to increase from 2 to 6 mO with depth of discharge from 0 to 100%. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Ni–Cd battery; Thermodynamics; Thermal behavior; Heat balance; Entropy generation; Electric resistance; Battery cycle; Experimental analysis 1. Introduction Battery thermal behavior is an important concern since it influences its longevity. Battery processes are indeed largely exothermic and the heat produced has to be well controlled in order not to destroy the battery due to temperature rise. Calorimetric studies are important to analyze the different modes of heat generation and how they can be influenced. For example, it helps to understand the gas production process during overcharge as well as to show hot spots inside the battery block and support the cooling network designer [1–4]. In this paper, it is shown that an accurate heat balance can be used to do more than a global analysis. It is shown how it is possible to deduce battery intrinsic parameters such as entropy generation and electrical resistance at different depths of discharge (DOD). The example taken here is the Ni–Cd battery. Ni–Cd and nickel-based rechargeable batteries are still expanding and still under research 30 years after their first commercializa- tion [5,6]. 2. Operating principle of the Ni–Cd battery The Ni–Cd battery tested here comprises a positive electrode of NiOOH and a negative electrode of Cd. The governing reactions occurring during the Ni–Cd battery charge and discharge processes are [7]: Positive electrode: 2NiOOH þ 2H 2 O þ 2e  @ discharge charge 2NiðOHÞ 2 þ 2OH  (1) Negative electrode: Cd þ 2OH  @ discharge charge CdðOHÞ 2 þ 2e  (2) Global reaction: Cd þ 2NiOOH þ 2H 2 O @ discharge charge CdðOHÞ 2 þ 2NiðOHÞ 2 (3) As the positive electrode is charged less easily than the nega- tive one [8], it results in a dioxygen release at the positive electrode, when the status of full charge is approached: 4OH  ! 2H 2 O þ O 2 þ 4e  (4) Thus, at the beginning of gas release, there is almost only dioxygen. Journal of Power Sources 117 (2003) 212–222 * Corresponding author. Tel.: þ32-4-3664802; fax: þ32-4-3664812. E-mail addresses: jmmottard@yahoo.fr (J.-M. Mottard), eric.winandy@planetmail.com (E.L. Winandy). 1 Tel.: þ32-4-3664800; fax: þ32-4-3664812. 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00004-1