Traditional Float Charges: are They Suited to Stationary Antimony-free Lead Acid Batteries?

Abstract

The self-discharge phenomenon caused by side reactions such as corrosion, water decomposition and recombination is unavoidable in lead acid batteries. Self-discharge rates depend on several factors such as the state of charge, the temperature, grid alloys, etc. These rates are not equal between negative and positive electrodes and are independent of each other. In leadacid batteries with antimony, the self-discharge rate of the negative electrode is several times stronger than the self-discharge rate of the positive electrode, depending mainly on the battery age and the antimony content in the alloy of the positive grid. In order to compensate for the self-discharge of lead-acid batteries, a current whose value exceeds the self-discharge of the negative electrode has to be supplied. This is traditionally achieved by applying a constant voltage whose value is slightly higher than the battery open circuit voltage. This is called “float charge”. It has to be kept in mind that the float polarization increases gas evolution rates at both electrodes. This float polarization voltage is adjusted to ensure that the battery is not under-charged. At typical float polarization voltages of the order of 120 mV/cell, batteries are permanently overcharged. The float current is then about 10 stronger than the self-discharge current at open circuit. In antimony-free lead-acid batteries, e.g. VRLA batteries, the self-discharge rate of the negative electrode is largely reduced and is about a half of the self-discharge rate of the positive electrode. The question is: are traditional float charges really suited to antimony-free lead-acid batteries? Experimental results say no. In this paper, a new method of maintaining the charge of antimony-free lead-acid batteries using low currents and periodic charges is presented. This is called “Low-current” method. Low currents are about 5 to 10 times smaller than traditional float currents. This method reduces overcharge, thus decreasing water loss and keeping the battery voltage in a minimum corrosion zone. Laboratory experiments are focused on studying rates of self-discharge at open circuit and state of charge evolutions of antimony free lead-acid batteries in three different cases: float charge, low-current charge and open circuit. The new Lowcurrent method, associated to an improved storage architecture system is now used in several UPS systems at Electricite de France (EDF). This association is intended to provide long life span and high reliability to VRLA batteries.