Batteries Float charging

A schematic circuit for an emergency power supply is shown in Fig. 1.5. The load is fed directly from the mains supply (which also float charges the battery), but in the event of mains failure, the load is automatically switched to the inverter by the mains monitoring system. 

Nominal 3.6 V batteries have been designed for direct mounting on printed circuit boards for CMOS and NMOS memory support applications. Such cells are normally float charged from the main microcomputer DC power supply and constitute a form of uninterruptible power supply (UPS). 

It must be emphasized that the most appropriate charging regime is very dependent on the cell system under consideration. Some are tolerant to a considerable amount of overcharging (e.g. nickel-cadmium batteries), while for others, such as zinc-silver oxide and most lithium secondary cells, overcharging can result in permanent damage to the cell. Sealed battery systems require special care float charging should not be used and trickle charge rates should be strictly limited to the manufacturer s recommended values, since otherwise excessive cell temperatures or thermal runaway can result. 

The majority of VRLA batteries produced today are used in standby applications to provide a reliable source of power in the event of failure of the mains supply. Discharges are infrequent and the batteries are maintained by float charging at a preset voltage. For example, European practice is to use parallel strings (usually 48 V) across a 54.5 V supply (2.27 V per cell). Adequately designed new cells, after conditioning and free of impurities, have a float current of < 1 mA per Ah at 20°C. 

The above data on float charging of VRLA batteries raise several significant points ... 

To complicate the situation even more, many of the properties of an AGM battery are already exhibited by modern, totally maintenance-free batteries with liquid electrolytes. This is particularly true for starter batteries as they are only expected to have a service-life of about 2000 h, which is considerably less than one-tenth of that which industrial battery manufacturers claim their products achieve when used in constant-voltage, float-charging applications. As a result, a VRLA battery will be used in vehicles with conventional electric power systems only if its advantages over a flooded battery are really needed. These include the following. 

The best way to avoid dry out and runaway is to remember that, during float charging, the battery produces some heat and, therefore, avoid packing batteries closely together. It is much better to provide a spacing of at least 10 mm between adjacent batteries. Keeping batteries at such a distance normally allows an effective... 

Papers published over the past few years have described the benefits of gas recombination catalysts in VRLA cells [34-36]. The insertion of a catalyst into the headspace of a cell can assist the combination of a significant amount of hydrogen that would otherwise escape and constitute water loss. It is claimed that such catalysts not only reduce water loss but can also balance float charging of VRLA batteries. There remains some uncertainty, however, about the reliability and service-life of such catalysts. 

Sealed lead-acid batteries are in both cylindrical and prismatic shapes. The cyclindrical ones (usually designed as SLA batteries) have excellent high-rate characteristics. Other than in portable devices, sealed batteries can be used in standby applications, e.g. telephone exchange stations, were they are kept in float charge. In this case too, oxygen recombination is possible. 

In recent years there has been a tendency to prefer Pb cells instead of NiCd cells. This has been due to the development of what has become known as maintenance free or sealed type lead-acid batteries. The basic concept is one of retaining the gases evolved during the charging process and to allow the oxygen to recombine as float charging takes place, see Reference 4. If the operating and ambient conditions are not subject to excessive variation then the concept is satisfactory in practice and the life expectancy of the battery can be as much as 10 years. 

If too much gas is evolved and is released through a special safety valve than the life expectancy will be reduced. The amount of gas evolved is a function of the float charging current level and the ambient temperature. The temperature of the electrolyte will be a function of the ambient temperature of the air surrounding the battery. Therefore a high float charging current and a high ambient temperature will cause the life expectancy to fall. If the ambient temperature has an average value of 30°C then the life expectancy will be halved, and at 40°C reduced to a quarter, i.e. 2 to 3 years instead of 10. 

The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. If removed from the charger, the SLA retains the charge four times longer than the NiCd. Based on the per ampere-hour cost, the SLA is the lowest priced battery, but in terms of cost-per-cycle, it is more expensive than the NiCd. 

FLOAT CHARGE CURRENT COMPENSATES FOR THE SELF-DISCHARGE AFTER THE BATTERY IS FULLY CHARGED. 

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