In this paper, a health auxiliary diagnosis method based on Sample Entropy (SampEn) is proposed for the lead-acid battery unit. The concept of the proposed method is that the discharging curve for a health battery unit is smooth; however, the degradation of battery unit caused by the internal shorts, opening of internal short or cell undergoing reversal will distort the discharging curve. Since SampEn can quantify the regularity of a data sequence, it can serve as an indicator for the state-of-health (SOH) of the lead-acid battery. The salient feature of the proposed method is that SOH of the battery is estimated automatically at the end of each discharging cycle by measuring the battery voltage and current of a battery unit, so that no complicated measurement is required. To verify the proposed health auxiliary diagnosis method based on SampEn, aging experiments on lead-acid battery are developed. The experimental data shows that the proposed health auxiliary diagnosis method provides the expected results. (C) 2009 Elsevier Ltd. All rights reserved.