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Several nonintrusive, continuous battery monitoring devices for lead-acid batteries in standby applications, such as uninterruptible power systems (UPS), are available today on the market. Some of them are using impedance measurements on every bloc of a battery string for determining the state of health of each bloc. The paper will present the results of an extensive validation test, comparing the results provided on the one hand by low cost monitoring devices and on the other hand by a full scale, laboratory impedance spectroscope. For the validation tests, accelerated ageing on many different leadacid battery products have been performed. The batteries vary in terms of size (Ah), bloc voltage, battery brands and technologies. The targeted standby applications often use large batteries consisting of many single cells or monoblocs with six or twelve volts each connected in series. These batteries grow old during their utilization and show several ageing phenomena, which could lead to a reduced bridging time and, therefore, unforeseen failure of the system connected to the UPS in case of blackout. Nonhomogenous ageing of the cells or monoblocs is much more the normal case than homogenous ageing of all elements.