TECHNOLOGY NEWSLETTER No.7 June 2012
COLD START MEASUREMENT BY MEANS OF QUICK TEST DEVICES As is well known, the open circuit voltage of a starter battery provides information regarding its charging status. Its determination by means of quick test devices is a standard feature of the logistics chain. As a byproduct, the cold start current is also frequently determined at the same time and compared with a set value from an EN specification. However, in order to interpret the result, one should be aware of the factors influencing the measured value. Problem. The physical-chemical rules prevailing in a lead-acid battery result in the problem that a complete assessment cannot take place within a short space of time. A quick judgement regarding the battery’s state of health (SOH) is also impossible. A reliable result can still only be obtained following a laboratory test in accordance with the EN 50342-1 standard. This “real” cold test takes place with a battery temperature of minus 18°C and a real load with the declared cold start current for 10 seconds. Under these circumstances, the starter battery must always demonstrate a minimum of 7.5V. The hit rate of the results from quick test devices, which often function according to the impedance method (= internal resistance measurement), is far removed from 100 per cent and a certain degree of doubt is always given. Influential factors. In the main, the measurement result is affected by the charging status, battery temperature and design. Charge status. The corresponding performance value (cold start current)
varies in line with the charge status (open-circuit voltage). This means that the lower the charge status, the lower the cold start value. However, this does not represent an accurate statement with regard to how good the battery really is. Consequently, comparisons with rated EN current specifications are only of value when the battery is fully charged. A difference of 0.1V creates a roughly 3% deviation in the measurement value. This rule applies in the 12.45 – 12.75V corridor. In the case of lower voltages, the drop in the measured value decelerates, while incorrect measurements can occur at higher voltages owing to acid stratification or surface tension directly after charging.
of 680A is estimated. Calculation: 620A x 1.06(temperature) x 1.03 (voltage). Battery design. The basic design of a starter battery also has an influence on measurement values. For example, thick separators with a glass web covering, like those employed for the Buffalo Bull SHD product line, provide excellent cycle resistance, but also increased internal resistance, which reduces the cold start value shown.
Similar results derive from the battery design. A large number of electrode plates generate excellent test values, but whether the life of such as battery is longer cannot be determined using quick test devices. Many other factors such as Internal battery temperature. active mass density, alloy quality and Measurements with quick test devices general craftsmanship levels also play a are heavily dependent upon the internal major role. temperature of the battery. The colder the battery is, the lower the measured Measurement precision. Last, but not value. As a rule, 10°C means an approx. least, quick measurements lack precision 3% difference in the measurement value. and repeatability. For the shorter a This rule of thumb applies in the corridor measurement is, the greater the spread between 0° and 20°C. In sub-zero of the results during repeats. temperatures, the measurement value falls to a greater extent owing to the Banner customer promise. All Banner increasing inertia of the electro-chemical Batteries leave the company plant with a system. However, in higher minimum voltage of 12.70V. The cold temperatures, the value measured does start current measured using quick test not increase significantly. Accordingly, devices amounts to at least the EN value some quick test devices are fitted with on the label given for 12.70V and a room temperature compensation, but this does temperature of 20°C. Batteries supplied not entirely counteract the from Banner branches have a minimum aforementioned effects. voltage of 12.60V. Example. In a 72Ah/640A battery cold This customer promise clearly underlines start current of 620A is determined at a the quality of Banner batteries and the voltage of 12.60V and 0°C. In a fully company’s logistics chain. charged condition (>12.70V) and at room temperature, a measurement value
Banner BBT 205
Banner GmbH, A-4021 Linz/Austria, Postfach 777, Banner Strasse 1, Tel. +43/(0)732/38 88-0, Fax Sales +43/(0)732/38 88-21599, E-mail: firstname.lastname@example.org Author: Günther Lemmerer, Marketing and PR