Float/floating voltage

Figure 8. Structure of the corrosion layer at the grid surface. Penetration and corrosion rates are approximated for room temperature and normal float voltage (2.23-2.25 V/cell) (see text).

To ensure reliability over long periods of service, good purity standards are essential. With proper control of temperature and float voltage, service-lives in excess of 10 years are normal and positive grid corrosion is the usual failure mode. At high... 

Table 9.2. Float voltage and gassing characteristics in a 24-V/5.0-Ah cell string floated at 2.35V/cell.

The picture is somewhat more complicated at the positive electrode. Again, the float voltage must be sufficiently positive to drive the PbS04/Pb02 reaction to completion and maintain it, but this must be balanced against the secondary... 

During stand-by service, the battery should be maintained in a completely charged state by applying a float voltage that is slightly higher than the battery open-circuit... 

The fact that, with the same float voltage, quite dissimilar currents can flow through an AGM cell, as determined by the oxygen recombination efficiency, has been discussed previously [6], The situation in a gel battery is different at the beginning of life when the recombination efficiency is always rather low. After some water loss, however, the recombination efficiency in these batteries will increase also. If too much water is lost (dry out), the recombination efficiency of gel batteries can be quite high. 

Gel batteries are also produced in 6-V and 12-V monoblocs with capacities between 5 and 180 Ah. These are also mainly used for telecommunications applications. They have flat positive plates, and provide a service-life of more than 12 years. The results of an accelerated life test of 12-V, 16-Ah batteries at 40°C and a float voltage of 2.22-V per cell are shown in Fig. 13.9. It can be seen that the capacity decreased below the 80% level after about 35 months and reached 50% after about 40 months. This equates to an expected life of more than 12 years at 20°C. The influence of discharge rate and temperature on the available capacity is shown in Fig. 13.10. Most of the applications of this battery-type require discharge currents between 1 and 4 times the 10-h discharge rate. Therefore, this battery-type has been optimized to a high energy density for discharge times between 10 and 2h. 

C of gel batteries (A400-type) with a float voltage of... 

An accelerated life-time test, at a temperature of 55°C and a float-voltage of 2.27 V per cell on 12-V, 50-Ah, front-terminal AGM batteries, confirms the expected life of 12 years at 20°C. At intervals of about two months, capacity was checked at the 10-h discharge rate, and the test results are shown in Fig. 13.15. During more than 300 days the capacity remained at a high level and rather stable. Afterwards, the capacity decreased significantly, and the test was stopped after 380 days when the capacity had dropped to 70% of the nominal value. The float life at 55°C is, therefore, 1 year, and this equates to a life of 12 years for a float operation at 20°C. 

Fig. 13.15. Accelerated lifetime test of 12-V, 50-Ah front-terminal batteries (Marathon-type) at 55°C with a float voltage of 2.27 V per cell.

Application of batteries in other imcontroUed environments, such as outdoor telecommunications cabinets, photovoltaic systems, and marine uses also reduces battery life. When batteries are operated in hot envirorunents, manufacturers often recommend reducing the charging current or float voltage to prevent the battery from overheating. 

Recharge to float voltage after heavy discharge of the battery packs... 

When the battery approaches the state of full charge, the charging reaction diminishes, and finally the secondary reactions are the only ones that remain when the battery is overcharged or kept at a comparatively low float voltage in standby operation. 

At the lower float voltage of 2.27 V/cell, polarization of 4-73 mV and — 77 mV of the positive and the negative electrodes, respectively, fulfills the required... 

Another important parameter is the dependence of the float current on float voltage and the temperature. Both parameters markedly influence the float current and thereby the water loss by electrolysis. Furthermore, both parameters also influence corrosion of the grid and all conducting elements that are connected to the positive plate. 

As a consequence the accuracy of the float voltage has strictly to be observed, especially with devices that employ valve-regulated lead-acid batteries, since this type contains no surplus of electrolyte and water cannot be refilled. Therefore most of the battery manufacturers give directions (tables and curves) for float charging of their products. 

But not only the float voltage, also the cell capacity depends on the electrolyte temperature, as shown in Figure 7.18. 

Figure 7.17 Float current versus float voltage of an aged OPzS battery at various temperatures referred to 100 Ah of nominal capacity.

The accuracy of the float voltage is very important. Figure 7.17 indicates the strong increase of the float current with cell voltage. For this reason all relevant standards give a tolerance of not more than +/ — 1% for the charger output voltage. 

The principle of the circuit is based on the fact that at mains operation the consumers are switched parallel only with a part of the battery, with 27 cells with a 60-V power supply. The main rectiher is adjusted to an output voltage that corresponds to the float voltage of this part battery. The other cells get the charge voltage from the additional rectifier. In the case of mains failure consumers are switched with achievement of a corresponding voltage border to the total battery while SI closes meanwhile S2 opens. The decoupling diode makes possible the interruption-free switch. 

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