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Ni-Cd battery is the key component of the satellite energy system with the characteristics of long life and high reliability, and few or no failure occurs in its traditional life testing. In this paper, based on the performance degradation modeling method, we establish a dynamic degradation model to capture the evolution of the end of discharge voltage (EODV), which is a function of cycle number and temperature. Two methods are utilized to assess the reliability of Ni-Cd battery. Results show that the random effect regression method obtains a more conservative reliability estimation result that the pseudo life based method does. Introduction The Ni-Cd battery is one of the important components of the satellite and its reliability directly influences the reliability of the satellite. As for a product of long-life and high reliability, there often will be few failures or even “zero-failure” in the conventional life test and accelerated life test of the Ni-Cd battery [1]. The traditional reliability assessment methods of the batteries are usually based on the failure lifetime [2][3]. However, for the reason of long-life and high reliability, it is very difficult to obtain the failure time of the Ni-Cd battery by using life test or accelerated life test. In such case, the performance degradation modeling method makes it possible to evaluate the reliability of the Ni-Cd battery [4][5]. In this paper, we investigate the reliability assessment of the Ni-Cd battery. By studying the cycle life test data of a certain type of Ni-Cd battery, we find that the end of discharge voltage (EODV) is a key indicator which can reflect the performance of a Ni-Cd battery. During the cycle life test, EODV declines gradually with cycles, and shows a strong correlation with the ambient temperature. Firstly, we establish a dynamic degradation model of the EODV as the function of cycle number and temperature using the performance degradation modeling method. Based on the dynamic degradation model, we successfully predict the cycle life of the Ni-Cd battery. Then, the reliability of the Ni-Cd battery is evaluated based on the pseudo life and degradation path distribution, respectively. Cycle life test for Ni-Cd battery Nine Ni-Cd batteries connected in series are used to conduct the cycle life test to study their performance degradation over charge/discharge cycles. The total rated capacity of the nine Ni-Cd batteries is 80 Ah and the nominal voltage of a single battery is 1.2V. The depth of discharge (DOD) is set to be 15% to simulate the actual working process on the satellite. During the test, the EODV of each battery and the average temperature in each cycle are recorded by the computer. Testing results are shown in Figure 1. International Power, Electronics and Materials Engineering Conference (IPEMEC 2015) © 2015. The authors Published by Atlantis Press 1135 Figure. 1 EODV and Temperature variation vs cycles It is clear that the EODV of each battery degrades as cycle increases. EODV fade will affect the Ni-Cd working performance seriously. When the EODV decreases below a critical value, a battery cannot supply enough energy and fails. However, except for the cycles, temperature also affects the EODV apparently. The EODV of a battery in a cycle will be quite low when the cycle temperature is low, but it can go back to a high level in the next cycle if the temperature increases. Thus the EODV degradation model must consider the temperature effect. Ni-Cd EODV degradation model considering temperature effect J. Paatero [6] proposed a model of the Ni-Cd battery, which can describe the quantitative relation between EODV and temperature. In this model, temperature affects EODV by a linear term plus an exponential term, ( ) EODV exp( ) T a c T d T = + ⋅ + ⋅ (1) Where T is the temperature and a, c, d are the parameters of the model need to be evaluated. Note that the model in (1) does not consider the degradation of the battery. In practical applications, the EODV of the Ni-Cd battery degrades as the cycle number increases. Because the variation range of the temperature in our test is quite small, we assume that the temperature stress will not affect the degradation rates of the battery. Through the cycle life test of the Ni-Cd battery, we find that the EODV of the battery almost degrades linearly as the cycle number increases without considering the temperature influence. Thus, the EODV degradation model under time-varying temperature stress can be formulated as ( ) EODV , exp( ) t T a b t c T d T = + ⋅ + ⋅ + ⋅ (2) where t is the cycle number, T is the temperature and a, b, c, d are the parameters of the model need to be evaluated. Reliability Assessment of Ni-Cd Battery Reliability Assessment based on Pseudo Life The pseudo life based reliability assessment method is quite popular in engineering [7]. The step of this method is as follows: (1) Establish a degradation model to acquire the characteristics of the key degradation indictor. (2) Estimate model parameters with the degradation data of each sample. (3) Extrapolate the pseudo life of each sample with the degradation parameter estimators and a pre-defined threshold.

Reliability Assessment of Satellite Ni-Cd Battery based on the End of Discharge Voltage